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1

Vortex wake investigation behind a wing-flap model with jet simulations  

Microsoft Academic Search

To get a better insight in the effect of jets on vortex development and decay, stereo-PIV measurements were performed in a towing tank behind a flapped aircraft model. The experimental data set yields the wake vortex behavior in a range that extends from the vortex formation stage up to the mid-field (approximately t* =2 corresponding to 100 wingspans for a

L. L. M. Veldhuis; R. De Kat

2008-01-01

2

Vortex methods and their application to trailing wake vortex simulations  

NASA Astrophysics Data System (ADS)

Vortex methods are competitive for simulating incompressible unsteady flows, because they have negligible dispersion error and good energy conservation. The various methods are presented, including the recent developments: particle redistribution, diffusion, relaxation (by projection), efficient solvers (fast multipole method, vortex-in-cell method, hybrid method) and parallel computer implementations. Examples relating to wing/aircraft trailing wake vortices are presented: 2-D and 3-D, inviscid and viscous, direct numerical simulation and large eddy simulation. We consider wake roll-ups, vortex tube dynamics, 3-D instabilities and the complexity/turbulence they produce. A vortex system in ground effects is also presented. To cite this article: G. Winckelmans et al., C. R. Physique 6 (2005).

Winckelmans, Grégoire; Cocle, Roger; Dufresne, Louis; Capart, Raphaël

2005-05-01

3

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.

4

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

Microsoft Academic Search

A series of flight tests over the ocean of a four engine tur- boprop airplane in the cruise configuration have provided a data set for improved understanding of wake vortex physics and atmospheric interaction. An integrated data- base has been compiled for wake-characterization and validation of wake-vortex computational models. This paper describes the wake-vortex flight tests, the data pro- cessing,

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

5

Experimental investigation of wake vortex in a water towing tank  

NASA Astrophysics Data System (ADS)

Wake vortex behind an aircraft would introduce great hazard to the following aircraft and threaten to the flight safety. Generic model using two rectangular airfoils was employed for generating wake vortex system to investigate the method of wake vortex alliviation. The investigation was carried out in a water towing tank equipt with Particle Image Velocimetry system. Characteristics of double-vortex flow were analyzed for selected cases, proving that the intensity of the vortex is reduced with respect to the interaction between the wake vortices. The study exhibited that the R-L instability was most effectively triggered with parameter combinations of ?1=10°, ?2=8°and b=50mm respectively. As a result, the circulation of the wake vortices was alleviated by nearly 40% accordingly.

Liu, Yue; Wang, Junwei; Liu, Zhirong; Bao, Feng

2012-10-01

6

Vortex shedding in the wake of a step cylinder  

NASA Astrophysics Data System (ADS)

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

Morton, Chris; Yarusevych, Serhiy

2010-08-01

7

Identification of vortex pairs in aircraft wakes from sectional velocity data  

NASA Astrophysics Data System (ADS)

The dynamics of multiple-vortex wake systems behind aircraft endangering air traffic can be assessed also from physical modelling. Large-scale laboratory investigations of multiple-vortex systems have been performed in a free-flight laboratory and in a water towing tank. Specialized PIV measurements provide time-resolved flow velocity fields normal to the wake axis. The applicability of various ? u-based vortex identification schemes to planar velocity data is addressed and demonstrated for unequal-strength co- and counter-rotating vortex pairs. Large vortices shed off the wing tips and flaps are identified employing a ? u-based criterion. Their cooperative mechanisms of generation and decay are evidenced from iso-surfaces of squared swirling strength and from further characteristic vortex parameters.

Carmer, Carl F. V.; Konrath, Robert; Schröder, Andreas; Monnier, Jean-Claude

2008-03-01

8

Wake Vortex Tracking Using a 35 GHz Pulsed Doppler Radar  

Microsoft Academic Search

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

Robert T. Neecea; Charles L. Brittb; Joseph H. Whitec; Ashok Mudukutored; Chi Nguyene; Bill Hooperf

9

Flight safety, aircraft vortex wake and airport operation capacity  

Microsoft Academic Search

One of the major problems that challenge today's aeronautics is the problem of improving flight safety. A zone of increased hazard is the aerospace in the vicinity of an airport. Here, one of aircraft accidents' causes is wake turbulence generated by aircraft. The encountering of an aircraft on take-off or landing with the vortex wake of a preceding aircraft can

Victor V. Vyshinsky

2001-01-01

10

Traversing field of view and AR-PIV for mid-field wake vortex investigation in a towing tank  

NASA Astrophysics Data System (ADS)

Wake vortex flow experiments are performed in a water tank where a 1:48 scaled model of a large transport aircraft A340-300 is towed at the speed of 3 and 5 ms-1 with values of the angle of attack ?={2°, 4°, 8°}. Particle image velocimetry (PIV) measurements are performed in a plane perpendicular to the towing direction describing the streamwise component of the wake vorticity. The instantaneous field of view (I-FOV) is traversed vertically with an underwater moving-camera device tracking the vortex core during the downward motion. An adaptive resolution (AR) image-processing technique is introduced that enhances the PIV interrogation in terms of spatial resolution and accuracy. The main objectives of the investigation are to demonstrate the applicability of PIV diagnostics in wake vortex research with towing-tank facilities. The specific implementation of the traversing field-of-view (T-FOV) technique and the AR image processing are driven by the need to characterize the vortex wake global properties as well as the vortex decay phenomenon in the mid- and far-field. Relevant aerodynamic information is obtained in the mid-field where the time evolution of the vortex structure (core radius and tangential velocity) and of the overall vortex wake (vortex trajectory, descent velocity, circulation) are discussed.

Scarano, F.; van Wijk, C.; Veldhuis, L. L. M.

2002-08-01

11

Vortex dynamics in a wire-disturbed cylinder wake  

NASA Astrophysics Data System (ADS)

The effect of a thin control wire on the wake properties of the flow around a circular cylinder has been investigated numerically. The governing equations are solved using a spectral element method for a Reynolds number of ReD=100. The diameter ratio of the main cylinder and the wire equals D/d=50 so no vortex shedding is expected to occur for the wire. However, the vorticity introduced by the wire in the vicinity of the upper shear layer of the cylinder still affects the vortex dynamics in the wake of the main cylinder. The primary effect of the wire is the reduction of the velocity fluctuations in the vortex formation region of the main cylinder. The maximum decrement occurs at a wire position of yw/D=0.875. The secondary effect of the wire is observed in the kinematics of the vortices, leading to a modified vortex arrangement and strength difference between the upper and lower vortices. Due to these effects, for yw/D<=0.875, a downward wake deflection is observed, while for larger values of yw/D>0.875, an upward deflection is found. The maximum downward deflection occurs at wire position yw/D=0.75 where the maximum positive mean lift coefficient, minimum drag coefficient, and minimum fluctuating lift coefficient are seen. Based on the observations, it is concluded that the deflection of the wake is primarily caused by a modification of the vortex arrangement in the wake. This modified vortex arrangement is caused by different formation times of the upper and lower vortices, by different vortex strengths, or by both.

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

2010-09-01

12

Analysis of rotor wake aerodynamics during maneuvering flight using a free-vortex wake methodology  

NASA Astrophysics Data System (ADS)

The problem of helicopter rotor wake aerodynamics during maneuvering flight conditions was analyzed using a time-accurate, free-vortex wake methodology. The free-vortex method consists of a Lagrangian representation of the rotor flow field using vortex elements, where the evolution of the flow field is simulated by tracking the free motion of these vortex elements and calculating their induced velocity field. Traditionally, free-vortex methods are inviscid, incompressible models, but in the present approach the viscous effects are incorporated using a viscous splitting method where the viscous and inviscid terms are modeled as successive sub-processes. The rotor aerodynamics and rigid blade flapping dynamics are closely coupled with the wake model and solved for in a consistent manner using the same numerical scheme. Validations of the methodology with experimental data were performed to study the wake response to perturbations in collective and cyclic pitch inputs. The numerical simulations captured all the essential wake dynamics observed in flow visualization. The predictions of the transient inflow and airloads response were found to be in excellent agreement with the available experimental measurements. It was observed that the rotor wake was extremely sensitive to perturbations in collective and cyclic blade pitch inputs. The characteristic wake response was found to be the bundling of the wake vorticity into a vortex ring structure. The evolution, convection and subsequent breakdown of this bundled ring of tip-vortices was found to be highly nonlinear, and occurs with a temporal lag. The nonlinear induced velocity field associated with unsteady wake evolution can cause considerable fluctuations in the rotor airloads time-history if the bundled tip-vortex structure comes into close proximity to the rotor blades. Furthermore, the interaction of these tip-vortices with the blades results in steep gradients in the rotor airloads across the rotor disk, thereby contributing to impulsive rotor noise. Several free-flight maneuver simulations were analyzed to gain better insight into the unsteady, nonlinear wake development under high-rate, large-amplitude maneuvers such as roll to starboard or port, roll reversals, and the quickstop maneuver. It is shown that the rotor wake response in almost all maneuvering flight conditions is highly nonlinear and emphasizes the need to accurately predict the transient wake aerodynamics to obtain accurate estimates of the unsteady rotor airloads and the resulting rotor acoustics.

Ananthan, Shreyas

13

Wake Vortex Alleviation Using Rapidly Actuated Segmented Gurney Flaps  

NASA Astrophysics Data System (ADS)

A study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as Miniature Trailing Edge Effectors (MiTEs), for active wake vortex alleviation is conducted using a half-span model wing with NACA 0012 shape and an aspect ratio of 4.1. All tests are performed with the wing at an 8.9 degree angle of attack and chord based Reynolds number around 350,000. The wing is equipped with an array of 13 MiTE pairs. Each MiTE has a flap that in the neutral position rests behind the blunt trailing edge of the wing, and in the down position extends 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing. Dynamic PIV is used to measure the time dependent response of the vortex in the intermediate wake to various MiTE actuation schemes that deflect the vortex in both the spanwise and liftwise directions. A maximum spanwise deflection of 0.041 chord lengths is possible while nearly conserving lift. These intermediate wake results as well as pressure profile, five-hole probe, and static PIV measurements are used to form complete, experimentally-based initial conditions for vortex filament computations that are used to compute the far wake evolution. Results from these computations show that the perturbations created by MiTEs can be used to excite vortex instability.

Matalanis, Claude; Eaton, John

2006-11-01

14

Wake vortex alleviation using rapidly actuated segmented Gurney flaps  

NASA Astrophysics Data System (ADS)

All bodies that generate lift also generate circulation. The circulation generated by large commercial aircraft remains in their wake in the form of trailing vortices. These vortices can be hazardous to following aircraft due to their strength and persistence. To account for this, airports abide by spacing rules which govern the frequency with which aircraft can use their runways when operating in instrument flight rules. These spacing rules are the limiting factor on increasing airport capacity. We conducted an experimental and computational study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as Miniature Trailing Edge Effectors (MiTEs), for active wake vortex alleviation. Wind tunnel tests were performed on a half-span model NACA 0012 wing equipped with an array of 13 independent MITE pairs. The chord-based Reynolds number was around 350,000. Each MiTE could extend 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing. Pressure profiles and a five-hole probe survey in the near wake were used to examine the influence that the MiTEs had upon the wing aerodynamics and the vortex rollup process. Particle image velocimetry was used to measure the static and time-dependent response of the vortex in the intermediate wake to various MiTE actuation schemes. These results were used to form complete initial conditions for vortex filament computations of the far wake evolution. Results from these computations showed that the perturbations created by MiTEs could be used to excite a variety of three-dimensional inviscid vortex instabilities. Finally, the research performed on MiTEs led to the invention of a more practical wake alleviation device: the spanwise actuating Gurney flap. Prototype tests showed that this device could produce similar perturbations to the MiTEs.

Matalanis, Claude G.

15

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

16

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

17

Experimental Study of Rotor Vortex Wakes in Descent  

NASA Astrophysics Data System (ADS)

An experimental study is performed on a three-bladed rotor model in a water towing tank. The blade pitch and rotational velocity, the rotor plane angle of attack, and the carriage speed are all varied in order to simulate a wide range of rotorcraft operating states. Circulation Reynolds numbers are of order 105 and blade Reynolds numbers are of order 104. Flow visualization is done using air bubbles or dye injected from the blade tips to mark the vortex core, showing the development of an instability on the helical vortices in the wake. PIV data provide quantitative measures of the flow field as the wake develops. Strain gages are also used to record transient load measurements, allowing a correlation to be made between the rotor performance and the development of the vortex wake. The data so far indicate that as the instability develops, the adjacent vortices merge and form thick vortex rings, especially during descent. The vorticity spreads and is periodically shed from the wake, resulting in significant fluctuations in the rotor loading.

Stack, James; Carradonna, Frank; Savas, Omer

2002-11-01

18

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

National Technical Information Service (NTIS)

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 wa...

D. D. Vicroy P. M. Vijgen H. M. Reimer J. L. Gallegos P. R. Spalart

1998-01-01

19

The Wake Vortex Prediction and Monitoring System WSVBS  

NASA Astrophysics Data System (ADS)

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

Gerz, T.; Holzäpfel, F.

2009-09-01

20

Kinetic Simulations of Unsteady Vortex Structures in the Wake of a Cylinder  

Microsoft Academic Search

The development of an unsteady vortex street in the wake of a cylinder has been studied in computational experiments using recently developed Unified Flow Solver (UFS). A major objective was to investigate a spontaneous breakdown of flow symmetry in the wake and effects of gas compressibility and rarefaction on the laminar vortex shedding. The boundary of the wake instability on

Robert R. Arslanbekov; Vladimir I. Kolobov; Anna A. Frolova

2008-01-01

21

Mean flow structure in the near wake of a turbulent junction vortex  

Microsoft Academic Search

The mean flow structure in the near wake of turbulent junction or horseshow vortex is reported for an incompressible, subsonic flow. Measurements of the primitive variables of velocity and pressure are reported on all surfaces bounding a region of the wake of the vortex extending from the trailing edge of the body generating the vortex flow to one full chord

F. J. Pierce; C. M. Kim; S. Nath; J. Shin

1987-01-01

22

Wake Vortex Control using Segmented Rapidly Actuated Gurney Flaps  

Microsoft Academic Search

Gurney flaps are small flaps oriented perpendicular to the freestream at the trailing edge of a wing, which can increase the lift considerably with little drag penalty. Meso-scale trailing edge effectors (MiTEs) are segmented, rapidly actuated, independent Gurney flaps that have an analogous effect local to their spanwise position. MiTEs show great potential in helping to alleviate the wake vortex

Claude Matalanis; John Eaton

2004-01-01

23

Wake Vortex Alleviation Using Rapidly Actuated Segmented Gurney Flaps  

Microsoft Academic Search

A study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as Miniature Trailing Edge Effectors (MiTEs), for active wake vortex alleviation is conducted using a half-span model wing with NACA 0012 shape and an aspect ratio of 4.1. All tests are performed with the wing at an 8.9 degree angle of attack and chord based

Claude Matalanis; John Eaton

2006-01-01

24

Implementation of a Free-Vortex Wake Model in Real-Time Simulation of Rotorcraft  

Microsoft Academic Search

Free-vortex wake models are capable of providing an accurate and physically detailed representation of the main rotor wake for flight dynamics simulation. Recent advances in computingpowerandefficientalgorithmshavemadeitfeasibletousefreewakesforreal-time simulation. The CHARM free-vortex wake model was integrated with the GENHEL flight dynamics simulation of the UH-60A helicopter. A high fidelity wake model was defined by increasing the spatial and temporal resolution of the

Joseph F. Horn; Derek O. Bridges; Sarma Rani; Daniel Wachspress

2006-01-01

25

Wake Vortex Control using Segmented Rapidly Actuated Gurney Flaps  

NASA Astrophysics Data System (ADS)

Gurney flaps are small flaps oriented perpendicular to the freestream at the trailing edge of a wing, which can increase the lift considerably with little drag penalty. Meso-scale trailing edge effectors (MiTEs) are segmented, rapidly actuated, independent Gurney flaps that have an analogous effect local to their spanwise position. MiTEs show great potential in helping to alleviate the wake vortex hazard. By periodically varying the loading distribution across the span of a wing, it may be possible to excite natural instabilities that accelerate vortex destruction. The problem is to introduce large enough disturbances while holding the total lift of the wing nearly constant. The purpose of this work is to assess how different MiTE actuation patterns can alter the strength and position of the trailing vortex. Our experimental apparatus consists of an untapered NACA 0012 wing with a 30 cm chord length and an aspect ratio of 2 mounted in a wind tunnel. Reynolds numbers based on the chord are of order 105. The wing is equipped with an array of 14 MiTEs. PIV is used to measure tangential velocities of the trailing vortex roughly five chord lengths behind the wing. Data from static MiTE configurations show that the vortex core can be displaced by at least 0.01 chord lengths.

Matalanis, Claude; Eaton, John

2004-11-01

26

An experimental investigation of the vortex wakes of hovering and descending rotors  

NASA Astrophysics Data System (ADS)

Two sets of experiments are performed on a three-bladed rotor model, one in a towing tank to simulate descent, and the other in a stationary tank to simulate hover. The rotor's rotational speed, collective pitch angle, descent angle, and descent speed are all varied, simulating a wide range of rotorcraft operating states. For this work, circulation-based Reynolds numbers are of order 105 and chord-based Reynolds numbers are of order 104. The focus of the first set of experiments is on descent speeds and angles where the rotor is operating in or near vortex ring state. Simultaneous flow visualization and thrust measurement allow the rotor's performance to be correlated to the development of the vortex wake. Periodic shedding of vorticity from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 20--50 rotor rotations---with the oscillation period exhibiting a power law dependence on the advance speed and varying inversely with the collective angle. The hover experiments use particle image velocimetry (PIV) and flow visualization to analyze the development of the wakes from either rectangular planform rotor blades or blades with triangular flaps attached near the blade tips. The dynamics of the two wakes differ substantially, with short-wave instabilities observed developing on the tip vortices of both wakes, while a long-wave instability develops only in the wake of the rectangular blades. The rectangular blade wake also decays somewhat more rapidly than the triangular-flap blade wake. While the wake can be altered significantly through modifications to the blade planform geometries, the trajectories of the tip vortices are not significantly affected by varying the rotor speed or collective angle. The effects of the experimental techniques employed here on the flow are also explored, with the injection of fluid from the blade tips in flow visualization experiments having a particularly strong effect on the strengths of the tip vortices.

Stack, James Anthony

27

Numerical simulations of wake structure generated by rotating blades using a time marching, free vortex blob method  

Microsoft Academic Search

Vorticity fields in the wake generated by rotating blades are calculated using a time accurate, free vortex blob method without a non physical model of the far wake. The computed free wake geometry of single rotor in hover is represented by the three wake regions: well defined tip vortex region, intermediate entangled region, and initially generated wake bundle. The air

Duck-Joo Lee; Seon-Uk Na

1997-01-01

28

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

Microsoft Academic Search

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

Frédéric Barbaresco; Uwe Meier

2010-01-01

29

Vortex formation in the wake of an oscillating cylinder  

NASA Astrophysics Data System (ADS)

The effects of cross-flow oscillation of a cylinder on the formation of vortices in its wake are investigated experimentally by means of flow-visualization studies in a 4.57 x 1.07 x 1.07-m X-Y towing tank. The results are presented in extensive graphs, diagrams, and photographs and discussed in detail, with a focus on the evolution of several synchronization regions, in which the frequency of vortex formation is locked to the oscillation frequency. These findings are then used to explain the sudden shift observed by Bishop and Hassan (1964) in the character of the cylinder forces.

Williamson, C. H. K.; Roshko, A.

1988-07-01

30

Analysis of the radar reflectivity of aircraft vortex wakes  

NASA Astrophysics Data System (ADS)

Radar has been proposed as a way of tracking wake vortices to reduce aircraft spacing and tests have revealed radar echoes from aircraft wakes in clear air. The mechanism causing refractive index gradients in these tests is thought to be the same as that for homogeneous and isotropic atmospheric turbulence in the Kolmogorov inertial range, for which there is a scattering analysis due to Tatarski. In reality, however, the structure of aircraft wakes has a significant coherent part superimposed with turbulence, about whose structure very little is known. This work adopts a picture of a coherent (in fact two-dimensional) wake to perform a scattering analysis and calculate the reflected power. In particular, two simple mechanisms causing refractive index gradients are considered: (A) radial pressure (and therefore density) gradient in a columnar vortex arising from the rotational flow; (B) adiabatic transport of atmospheric fluid within a descending oval surrounding a vortex pair. In the scattering analysis, 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, an approximate analysis 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. Reflectivities calculated for the oval (mechanism B) are within 2 13 dB m2 of the measurements ([approximate][minus sign]70 dB m2) of MIT Lincoln Laboratory at Kwajalein atoll. However, the present predictions have a cut-off away from normal incidence which is not present in the measurements. This implies that the two-dimensional picture is not entirely complete. Estimates suggest that the thin layer of vorticity which is baroclinically generated at the boundary of the oval is turbulent and this may account for reflectivity away from normal incidence. The reflectivity of a vortex (mechanism A) is comparable to that of the oval (mechanism B) but occurs at a frequency (about 50 MHz) that is lower than those considered in all the experiments to date. This result may be useful because: (i) existing atmospheric radars (known as ST radars) already operate at this frequency and so the present prediction could be verified; (ii) rain clutter is not a problem at this frequency; (iii) mechanism A is more robust because it is independent of atmospheric stratification.

Shariff, Karim; Wray, Alan

2002-07-01

31

Interaction of a Strong Vortex with Decaying Turbulence.  

National Technical Information Service (NTIS)

The evolution of a localized, axially symmetric vortex under the action of shear stresses associated with decaying two-dimensional turbulent vorticity which is inhomogeneous in the presence of the vortex is studied analytically. For a vortex which is suff...

P. W. Terry

1988-01-01

32

Stability characteristics of counter-rotating vortex pairs in the wakes of triangular-flapped airfoils  

Microsoft Academic Search

A rapidly growing instability is observed to develop between unequal strength, counter-rotating vortex pairs in the wakes of airfoils with outboard triangular flaps. To investigate the physical mechanisms for this instability, a linear stability analysis is performed on a single vortex pair. This analytical model reveals that the instability is driven by the strain rate field from one vortex acting

Jason Marc Ortega

2001-01-01

33

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

Microsoft Academic Search

A study was conducted using strip theory to systematically investigatethe effects of progressively more complete descriptions of theinteraction of an airplane with a wake vortex system. The emphasiswas in roll-dominant, parallel, vortex encounters. That is, the simulatedairplane's longitudinal axis was nearly parallel to the rotationaxis of the vortex system for most of the results presented. The studybegan with a drag-less

Eric C. Stewart

1999-01-01

34

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

NASA Astrophysics Data System (ADS)

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

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

35

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

National Technical Information Service (NTIS)

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 ...

A. C. Trujillo D. M. Williams G. W. Lohr

2008-01-01

36

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

NASA Astrophysics Data System (ADS)

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

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

2010-01-01

37

A Comparison of Wake-Vortex Models for Use in Probabilistic Aviation Safety Analysis  

Microsoft Academic Search

The goal of this paper is to address part of a larger safety issue in aviation: What is the probability that a landing aircraft flies through a wake vortex generated by the aircraft in front of it and subsequently crashes? An important element needed to address this overall research question is a model to predict the evolution of wake vortices

J. F. Shortle

38

A prediction model for the vortex shedding noise from the wake of an airfoil or axial flow fan blades  

NASA Astrophysics Data System (ADS)

An analytical model is presented for predicting the vortex shedding noise generated from the wake of axial flow fan blades. The downstream wake of a fan blade is assumed to be dominated by the von Karman vortex street, and the strength and the shedding frequency of the wake vortex are determined from the wake structure model. The fluctuating pressure and lift on the blade surface, which are induced from the vortices in the wake, are analyzed by incorporating the wake model for the von Karman vortex street with thin airfoil theory. The predicted vortex shedding frequency and the overall sound pressure level compare favorably with the measured results for the vortex shedding noise from axial flow fans.

Lee, C.; Chung, M. K.; Kim, Y.-H.

1993-06-01

39

Identification of vortex pairs in aircraft wakes from sectional velocity data  

Microsoft Academic Search

The dynamics of multiple-vortex wake systems behind aircraft endangering air traffic can be assessed also from physical modelling.\\u000a Large-scale laboratory investigations of multiple-vortex systems have been performed in a free-flight laboratory and in a\\u000a water towing tank. Specialized PIV measurements provide time-resolved flow velocity fields normal to the wake axis. The applicability\\u000a of various ?u-based vortex identification schemes to planar

Carl F. v. Carmer; Robert Konrath; Andreas Schröder; Jean-Claude Monnier

2008-01-01

40

Hot-wire and vorticity meter wake vortex surveys  

Microsoft Academic Search

The vorticity meter used in the experiments consists of a paddle-wheel sensor mounted on a rotating shaft and fitted with a jeweled bearing. Vorticity data for a trailing vortex obtained with the vorticity meter are presented in a graph, taking into account an injected and a basic vortex. The tangential and axial velocity profile of a trailing vortex, as determined

A. D. Zalay

1976-01-01

41

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

PubMed Central

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

Tytell, Eric D; Ellington, Charles P

2003-01-01

42

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

43

Stability characteristics of counter-rotating vortex pairs in the wakes of triangular-flapped airfoils  

NASA Astrophysics Data System (ADS)

A rapidly growing instability is observed to develop between unequal strength, counter-rotating vortex pairs in the wakes of airfoils with outboard triangular flaps. To investigate the physical mechanisms for this instability, a linear stability analysis is performed on a single vortex pair. This analytical model reveals that the instability is driven by the strain rate field from one vortex acting on the perturbations of its neighboring vortex. Another linear stability analysis is conducted to include the effects of the other counter-rotating vortex pair. The qualitative features of the instability, such as its wavelength and non-linear evolution, are examined by flow visualization measurements that are made in a towing tank facility at a chord-based Reynolds number of O(105). From these observations, a sinuous instability is seen to develop on the weaker flap vortices and have a wavelength of order one wingspan. The instability wavelengths that are observed in the flow visualization data compare favorably with those predicted by the two- and four-vortex linear stability analyses, demonstrating that the analytical models capture the essential physics of the instability growth. Quantitative measurements of the vortex wakes are made with a PIV technique, allowing the vortex structure, trajectories, kinetic energy, and distribution to be assessed up to several hundred wingspans downstream of the airfoils. Additionally, the circulation-based Reynolds number is seen to be of O(105). The PIV data indicate that the wake's two-dimensional kinetic energy decreases substantially as the instability transforms the two-dimensional nature of the wake into a three- dimensional one. Finally, the wake alleviation properties of this instability are measured by computing the maximum rolling moment and downwash that a following wing might experience if it were placed in the wakes of these airfoils. These calculations show that by 75 wingspans, the wakes of the triangular-flapped airfoils have rolling moments and downwash that are always less than those of a conventional rectangular airfoil. This rapid reduction in the rolling moment and downwash leads to the conclusion that this instability between unequal strength, counter- rotating vortex pairs has the potential to solve the wake hazard problem.

Ortega, Jason Marc

44

Vortex-shedding characteristics in the wake of an oscillating airfoil at low Reynolds number  

Microsoft Academic Search

An experimental investigation was carried out to study the unsteady characteristics of vortex shedding in the near wake of an oscillating airfoil. The airfoil was given a harmonic pitching motion about the quarter-chord axis at four reduced frequencies: 0.1, 0.2, 0.3, and 0.4. The mean incidence and the oscillating amplitude were 0° and 3°, respectively. The velocity in the wake

Y. W. Jung; S. O. Park

2005-01-01

45

Analysis of the radar reflectivity of aircraft vortex wakes  

Microsoft Academic Search

Radar has been proposed as a way of tracking wake vortices to reduce aircraft spacing and tests have revealed radar echoes from aircraft wakes in clear air. The mechanism causing refractive index gradients in these tests is thought to be the same as that for homogeneous and isotropic atmospheric turbulence in the Kolmogorov inertial range, for which there is a

Karim Shariff; Alan Wray

2002-01-01

46

Wing Wake Vortices and Temporal Vortex Pair Instabilities  

Microsoft Academic Search

In this presentation we include selected results which have originated from vortex dynamics studies conducted at Cornell, in collaboration with IRPHE, Marseille. These studies concern, in particular, the spatial development of delta wing trailing vortices, and the temporal development of counter-rotating vortex pairs. There are, as might be expected, similarities in the instabilities of both of these basic flows, as

C. H. K. Williamson; T. Leweke; G. D. Miller

2001-01-01

47

Vortex street and turbulent wakes behind a circular cylinder placed in a rotating rectangular channel  

Microsoft Academic Search

A rotating wind tunnel was designed to study the stability and secondary flow effects of the Coriolis force on a rotating shear layer. A vortex street and turbulent wakes behind a circular cylinder placed in a rotating rectangular channel of low aspect ratio are described in this paper; the axis of the cylinder was taken parallel or normal to the

H. S. Koyama; T. Saito; M. Ohuchi

1989-01-01

48

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

2013-10-30

49

Wind tunnel studies of a ship model using vortex generators to improve wake velocities  

Microsoft Academic Search

Severe vibration during trials of a 13,000 ton displacement cargo ship was attributed to the propeller working in a strongly non-uniform velocity field. This report gives the results of a series of wind tunnel experiments performed on a reflex model fitted with vortex generators which substantially improved the wake velocity distribution. It was recommended that these generators be geometrically scaled

N. Matheson

1974-01-01

50

Airport radar monitoring of wake vortex in all weather conditions  

Microsoft Academic Search

To assess maturity and capability of X-band radars to monitor wake roll-ups in all weather conditions, Radar data were collected on airports, near runway at ORLY airport and just under its ILS Interception Area. Additional trials took place on Paris-CDG Airport to benchmark Lidar & Radar Technologies. Continuous Detection, characterization and profiling capabilities of wake vortices, up to a range

Frédéric Barbaresco

2010-01-01

51

Wing Wake Vortices and Temporal Vortex Pair Instabilities  

Microsoft Academic Search

In this presentation we include selected results which have originated from vortex dynamics studies conducted at Cornell,\\u000a in collaboration with IRPHE, Marseille. These studies concern, in particular, the spatial development of delta wing trailing\\u000a vortices, and the temporal development of counter-rotating vortex pairs. There are, as might be expected, similarities in\\u000a the instabilities of both of these basic flows, as

C. H. K. Williamson; T. Leweke; G. D. Miller

52

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

PubMed

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

2012-09-12

53

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

54

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

55

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

56

Windmill wake turbulence decay: a preliminary theoretical model  

Microsoft Academic Search

The results are given of initial theoretical attempts to predict dynamic wake characteristics, particularly turbulence decay, downstream of wind turbine generators in order to assess the potential for acoustic noise generation in clusters or arrays of turbines. These results must be considered preliminary, because the model described is at least partially based on the assumption of isotropy in the turbine

Bossanyi

1983-01-01

57

Fluid Transport by Pulsed-Jet Swimmers: Entrainment and Added Mass in Vortex Wakes  

NASA Astrophysics Data System (ADS)

Pulsed-jet swimmers share a common vortex ring wake motif that has been identified in laboratory studies and field observations. Many of these animals rely on the generated vortex wakes for both propulsion and feeding. Hence, control and optimization of fluid transport is critical to the overall success of these locomotor systems, and for manmade vehicles whose designs are inspired by them. We have previously demonstrated the ability to quantitatively track the growth of vortex ring structures due to ambient fluid entrainment. In the present work, we extend those results to consider fluid particle drift induced by vortex rings. It is demonstrated that the effect of this induced drift is analogous to the added mass of a solid body translating in an inviscid fluid. These results provide physical insight to support previous global measurements of unsteady force generation during vortex ring formation, which showed increased impulse generation relative to a steady jet flow. In addition, consequences for optimal vortex ring formation are suggested.

Dabiri, John O.; Gharib, Morteza

2004-11-01

58

Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent  

NASA Astrophysics Data System (ADS)

An experimental study is performed on a three-bladed rotor model in a water tow tank. The blade pitch and rotational velocity, the rotor plane angle of attack (descent angle), and the carriage speed are all varied in order to simulate a wide range of rotorcraft operating states, with the focus being on descent speeds and angles where the rotor is operating in or near vortex ring state an area in which there is currently very little available data. Circulation and blade Reynolds numbers are of order 10^5. Flow visualization is done by injecting air bubbles and fluorescent dye tangentially from the blade tips to mark the vortex core, showing the development of both short-wave (sinuous) and long-wave (leapfrogging) instabilities on the helical vortices in the wake. Strain gages are used to record transient loads, allowing a correlation between the rotor thrust performance and the development of the vortex wake. Test runs are performed for extended periods up to 500 rotor revolutions demonstrating the repeatability of the patterns of thrust variation. The data indicate that as the instabilities develop, adjacent vortices merge and form thick vortex rings, especially during descent. Periodic shedding of these rings from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 2050 rotor revolutions, depending on flow parameters. Preliminary particle image velocimetry (PIV) data provide a quantitative measure of the entire rotor flow field for the case of a hovering rotor. The data yield additional information on the vortex filament instability, in particular the axial flow in the vortex cores.

Stack, James; Caradonna, Frank; Savas, Omer

2003-11-01

59

Active and passive vortex wake mitigation using control surfaces  

Microsoft Academic Search

Aircraft trailing vortices constitute a hazard to following aircraft and are therefore one of the main concerns for airport capacity constraints. At the Institute of Aerospace Engineering (ILR) experiments on wake vortices are conducted in a towing tank using particle image velocimetry. The motivation behind the presented investigations is the alleviation of the rolling moment induced on following aircraft using

S. Haverkamp; G. Neuwerth; D. Jacob

2005-01-01

60

Vortex Formation in the Wake of Dark Matter Propulsion  

Microsoft Academic Search

Future spaceflight will require a new theory of propulsion; specifically one that does not require mass ejection. A new theory is proposed that uses the general view that closed currents pervade the entire universe and, in particular, there is a cosmic mechanism to expel matter to large astronomical distances involving vortex currents as seen with blazars and blackholes. At the

G. A. Robertson; M. J. Pinheiro

2011-01-01

61

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

62

Evaluation of Large-Scale Wing Vortex Wakes from Multi-Camera PIV Measurements in Free-Flight Laboratory  

NASA Astrophysics Data System (ADS)

Multiple-vortex systems of aircraft wakes have been investigated experimentally in a unique large-scale laboratory facility, the free-flight B20 catapult bench, ONERA Lille. 2D/2C PIV measurements have been performed in a translating reference frame, which provided time-resolved crossvelocity observations of the vortex systems in a Lagrangian frame normal to the wake axis. A PIV setup using a moving multiple-camera array and a variable double-frame time delay has been employed successfully. The large-scale quasi-2D structures of the wake-vortex system have been identified using the QW criterion based on the 2D velocity gradient tensor ?H u, thus illustrating the temporal development of unequal-strength corotating vortex pairs in aircraft wakes for nondimensional times tU0/b?45.

Carmer, Carl F. v.; Heider, André; Schröder, Andreas; Konrath, Robert; Agocs, Janos; Gilliot, Anne; Monnier, Jean-Claude

63

Phase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence  

Microsoft Academic Search

The vortex formation and shedding process in the near wake region of a 2D square-section cylinder at incidence has been investigated by means of particle image velocimetry (PIV). Proper orthogonal decomposition (POD) is used to characterize the coherent large-scale flow unsteadiness that is associated with the wake vortex shedding process. A particular application of the POD analysis is to extract

B. W. van Oudheusden; F. Scarano; N. P. van Hinsberg; D. W. Watt

2005-01-01

64

Unsteady Vortex Structures in the Wake of a Piezoelectric Flapping Wing  

Microsoft Academic Search

An experimental study was conducted to characterize the behavior of Unsteady Vortex Structures in the Wake of a piezoelectric flapping wing with miniaturized size (about 10mm in chord length), large flapping amplitude (up to 2.0 times of chord length) and high flapping frequency (60Hz) to explore the potential application of piezofans as the compact, gearless flapping-wings for the development of

Lucas Clemons; Hirofumi Igarashi; Hui Hu

2009-01-01

65

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

66

Unsteady Vortex Structures in the Wake of a Piezoelectric Flapping Wing  

NASA Astrophysics Data System (ADS)

An experimental study was conducted to characterize the behavior of Unsteady Vortex Structures in the Wake of a piezoelectric flapping wing with miniaturized size (about 10mm in chord length), large flapping amplitude (up to 2.0 times of chord length) and high flapping frequency (60Hz) to explore the potential application of piezofans as the compact, gearless flapping-wings for the development of novel piezoelectric-flapping-wing-based Nano-Air-Vehicles (NAVs). The experimental investigation was performed in a low-speed wind tunnel. A digital particle image velocimetry (PIV) system was used to achieve phased-locked flow field measurements to quantify the transient behavior of the unsteady vortex structures in wake of the piezoelectric flapping wing. The effects of important parameters such as incoming flow velocity (i.e., forward flight speed), the flapping amplitude, and the incline angle of the flapping wing in relation to the incoming flow direction (i.e. the angle of attack) on the wake vortex shedding processes were examined to elucidate underlying physics in order to explore/optimize design paradigms for the development of novel piezoelectric-flapping-wing-based NAVs.

Clemons, Lucas; Igarashi, Hirofumi; Hu, Hui

2009-11-01

67

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

NASA Astrophysics Data System (ADS)

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

Currin, Hugh D.

68

Flow Visualization and Particle Image Velocimetry Analysis of Rotor Vortex Wakes  

NASA Astrophysics Data System (ADS)

An experimental study is performed on a three-bladed rotor model in a stationary water tank, simulating a helicopter rotor operating in hover at Reynolds numbers of order 10^5. Flow visualization is done by injecting air bubbles and fluorescent dye tangentially from the blade tips to mark the vortices, showing the development of both short-wave (sinuous) and long-wave (leapfrogging) instabilities on the helical vortex filaments in the wake. The flow images show that as the instabilities develop, adjacent vortices merge and form thick vortex rings as quickly as half a rotor diameter downstream of the rotor disk. Particle image velocimetry data demonstrate the existence of the short-wave instability, evidenced by oscillations in the adjacent vortex core separation distance. At the same time, the long-wave instability is also apparent, as the vortices can be seen orbiting each other prior to merger. The circular axis and the moments of vorticity distribution are used to analyze the kinematics of vortex filament triads during formation, instability, and merger phases. Comparison of flow visualization and vorticity movies, however, shows interesting differences in the vortex merger process, possibly as a result of axial flow in the vortex cores.

Stack, James; Caradonna, Francis; Savas, Omer

2004-11-01

69

Exploration of the vortex wake behind of wind turbine rotor  

NASA Astrophysics Data System (ADS)

The present paper describes a wind tunnel study of flow downstream a small horizontal axis wind turbine (HAWT). The experimental investigations were carried out with the use of particle image velocimetry (PIV). To obtain the flow field in the rotating frame of reference, the phase-locked technique was applied. Explorations were carried out in azimuth planes with different angles. The 3D velocity field was reconstituted by processing the images resulting from the explored azimuth planes. In addition to PIV investigations, hot-wire measurements were also carried out immediately behind the wind turbine rotor at different radial and axial distances. The obtained results are very useful to analyze wind turbine wake and to constitute a reference for CFD computation.

Massouh, F.; Dobrev, I.

2007-07-01

70

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

71

Wake Similarity and Vortex Formation for Two-Dimensional Bluff Bodies  

NASA Astrophysics Data System (ADS)

An experimental study of the flow around a cylinder with a single straight perturbation was conducted in a wind tunnel. With this bluff body, positioned in a uniform crossflow, the vortex shedding frequency and other flow characteristics could be manipulated. The Strouhal number has been shown to be a function of the perturbation angular position, theta _{rm p}, as well as the perturbation size and Reynolds number. As much as a 50% change in Strouhal number could be achieved, simply by changing theta _{rm p} by 1^ circ. The perturbation size compared to the boundary-layer thickness, delta, was varied from approximately 1delta to about 20delta. The Reynolds number was varied from 10,000 to 40,000. A detailed investigation of the characteristic Strouhal number variation has shown that varying theta_{rm p} had a significant influence on the boundary -layer separation and transition to turbulence. These significant changes occurring in the boundary-layer have been shown to cause variations in the spacing between the shear layers, base pressure, vortex formation length, drag, lift, and the longitudinal spacing between the vortices in the vortex street. The unique ability of the cylinder with a single straight perturbation to control the Strouhal number and other flow characteristics, was used to evaluate several previously proposed wake similarity concepts by Fage and Johansen(1927), Roshko(1955), Bearman(1967) and Griffin(1981). It was shown that these wake similarity concepts did not satisfactorily apply to the bluff body which was used in this study. The experimental results have shown that a wake similarity parameter, S_{rm M} = kf_{rm v}d*/U _infty has smaller variations from its mean values S_{rm Mo } = 0.39, when compared to previously proposed wake similarity parameters. The quantity, k, is the base pressure parameter, f_{rm v} , is the vortex shedding frequency, d*, is the spacing between the shear layers and U_infty , is the free stream velocity. The similarity parameter S_{rm M}, when applied to Fage and Johansen's measurements on a wide range of bluff bodies showed less variance and resulted in numbers near 0.39. The parameter, S_{rm M}, when used to evaluate the lateral-to-longitudinal stability of vortices in the vortex street was shown to favor von Karman's over Kronauer's wake stability criterion.

Nebres, Jose Luis Villafranca

72

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

73

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

PubMed

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

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

2008-03-01

74

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

NASA Astrophysics Data System (ADS)

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

Hu, Hui; Yang, Zifeng; Sarkar, Partha

2010-11-01

75

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

NASA Astrophysics Data System (ADS)

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 ? ranging in 1˜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 ? . 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 ? was given and the flow characteristics relevant to the critical spacing values and the hysteretic mode transitions were investigated.

Wang, Si-Ying; Tian, Fang-Bao; Jia, Lai-Bing; Lu, Xi-Yun; Yin, Xie-Zhen

2010-03-01

76

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

Microsoft Academic Search

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

Yuan-dong Huang; Wen-quan Wu

2006-01-01

77

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

PubMed

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

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

2011-06-15

78

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

PubMed Central

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

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

2012-01-01

79

An experimental and analytical study of the stability of counter-rotating vortex pairs with applications for aircraft wake turbulence control  

Microsoft Academic Search

Aircraft trailing vortex wakes are commonly referred to as `wake turbulence' and may pose a flight safety hazard to other aircraft that may encounter the wake. This hazard is of critical interest during the take-off and landing stages of flight, where aircraft are in the closest proximity to one another. During these flight stages, it is common for transport aircraft

Brian Matthew Babie

2008-01-01

80

Helical vortex-based model of deterministic stresses for Large-Eddy-Simulation of a wind turbine wake  

Microsoft Academic Search

When representing a wind turbine in LES using a drag disk (e.g. A. Jimenez et al. 2007), the periodic effects due to the turbine's rotating elements remain unresolved. The periodic effects on the mean flow can be represented in a simulation using deterministic stresses in the wake. In this work, based on the Biot-Savart law with a helical vortex street

Marc Bracons; Charles Meneveau; Marc Parlange

2008-01-01

81

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

82

Interfacing comprehensive rotorcraft analysis with advanced aeromechanics and vortex wake models  

NASA Astrophysics Data System (ADS)

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

Liu, Haiying

83

On the increased decay of swirl after vortex breakdown  

NASA Astrophysics Data System (ADS)

In this study, vortex breakdown in swirling flows and critical swirl rate for its occurrence was experimentally and numerically investigated. In order to understand and control this interesting phenomenon, a special pipe flow test facility with a rotating honeycomb type swirl generator was constructed. Measurements of all velocity components were carried out by using LDV combined with refractive index matching technique. The maximum Reynolds number and swirl intensity (ratio of angular momentum flux to axial momentum flux) of the flow were ReD = 30,000 and So = 11, respectively. Measurements at a few diameters downstream of the honeycomb revealed that, beyond a critical swirl intensity setting, the swirl component decayed faster as the swirl intensity was further increased. It is also measured that the axial flow attained reduced or even negative velocities around the centreline after this critical swirl intensity was exceeded. It is argued that rapid decay of swirl component due to vortex breakdown causes the change of tendencies in the flow. Critical swirl intensity was hereby proposed to be So 0.95, which is important for design and prediction of swirling flows. In order to complement these experimental results CFD analyses were carried out.

Genc, Balkan; Ertunc, Ozgur; Vaidya, Haresh; Koeksoy, Cagatay; Delgado, Antonio

2010-11-01

84

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

Microsoft Academic Search

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

Hui Hu; Lucas Clemons; Hirofumi Igarashi

2011-01-01

85

Traversing field of view and AR-PIV for mid-field wake vortex investigation in a towing tank  

Microsoft Academic Search

Wake vortex flow experiments are performed in a water tank where a 1:48 scaled model of a large transport aircraft A340-300 is towed at the speed of 3 and 5 ms-1 with values of the angle of attack alpha={2°, 4°, 8°}. Particle image velocimetry (PIV) measurements are performed in a plane perpendicular to the towing direction describing the streamwise component

F. Scarano; C. van Wijk; L. L. M. Veldhuis

2002-01-01

86

Traversing field of view and AR-PIV for mid-field wake vortex investigation in a towing tank  

Microsoft Academic Search

Wake vortex flow experiments are performed in a water tank where a 1:48 scaled model of a large transport aircraft A340-300 is towed at the speed of 3 and 5 ms-1 with values of the angle of attack !={2°, 4°, 8°}. Particle image velocimetry (PIV) measurements are performed in a plane perpendicular to the towing direction describing the streamwise component

F. Scarano; C. van Wijk; L. Veldhuis

2002-01-01

87

Helical vortex-based model of deterministic stresses for Large-Eddy-Simulation of a wind turbine wake  

NASA Astrophysics Data System (ADS)

When representing a wind turbine in LES using a drag disk (e.g. A. Jimenez et al. 2007), the periodic effects due to the turbine's rotating elements remain unresolved. The periodic effects on the mean flow can be represented in a simulation using deterministic stresses in the wake. In this work, based on the Biot-Savart law with a helical vortex street and various simplifications, we develop an analytical expression for the deterministic, periodic velocity fluctuations in the wake. Then, the deterministic stress tensor is obtained by the product of the approximated fluctuating components of velocity, and integration over a helical period. The resulting model is implemented within a Large Eddy Simulation of an array of wind turbines, using the scale-dependent Lagrangian dynamic model (Bou-Zeid et al. 2005). The importance of the deterministic stresses on the computed wake structure is examined by varying the strength of the helical vortices.

Bracons, Marc; Meneveau, Charles; Parlange, Marc

2008-11-01

88

3D Structure of the Inverse Karman Vortex Street in the Wake of a Flapping Foil  

NASA Astrophysics Data System (ADS)

Flapping foils are being considered for lift generation and/or propulsion in Micro Aerial Vehicles (MAVs) and Autonomous Underwater Vehicles (AUVs). In the present study, a DNS/LES solver that is capable of simulating these flows in all their complexity will be used. The flow around a NACA 0012 foil undergoing pitch oscillation at a chord Reynolds number of 12600 has been investigated and the comparison of mean thrust coefficient results with the experiment has indicated significant under-prediction of the thrust although good match is observed with a 2D RANS calculation. This discrepancy could be related to the absence of 3D effects in both numerical simulations. Although this conclusion has also been reached in other studies, the details of the physical mechanism that lead to inaccurate prediction of surface pressure and ultimately to thrust force for pitching and heaving flapping foils have not been clarified yet. In this study, the streamwise (secondary) vortical structures in the inverse Karman Vortex Street generated in the wake of a thrust producing flapping foil will be studied.

Bozkurttas, Meliha; Mittal, Rajat; Dong, Haibo

2004-11-01

89

Transition and Turbulence Decay in the Taylor-Green Vortex  

NASA Astrophysics Data System (ADS)

The Taylor-Green Vortex (TGV) is a fundamental case that has been traditionally used as prototype of vortex stretching and consequent production of small-scale eddies, to investigate the basic dynamics of transition to turbulence. As such, it is also a very convenient case in which to test the ability of explicit and implicit subgrid scale (SGS) modeling to simulate the basic laws of turbulence. We report on the performance of Monotone Integrated LES (MILES) in emulating the space/time development of transition to turbulence and self-similar decay in the TGV without resorting to an explicit SGS model. MILES based on various limiting algorithms, including Flux Corrected Transport, characteristics-based Godunov, Lagrange-Remap, and several other hybrid methods is tested and compared with a conventional (mixed) LES method combining one-equation eddy-viscosity and scale-similarity models. The agreement between MILES, mixed-model LES, and the previous DNS by Brachet et al. (1983) is quite good in estimating the time and height of the dissipation peak associated with the TGV inviscid instability.

Grinstein, Fernando; Drikakis, Dimitris; Fureby, Christer; Youngs, David

2005-11-01

90

Coherent structure in the turbulent wake behind a circular cylinder 2. Numerical simulation using the vortex filament model  

NASA Astrophysics Data System (ADS)

In the previous paper the authors reported observing the formation of a spoon-shaped vortex chain in a wake behind a circular cylinder as a coherent structure in turbulence. In this report numerical simulation is carried out based on the assumption that the structure is formed by deformation of the Kánnán vortices. The basic equation is the localized induction equation for a single vortex filament with an influence of the background mean flow. The vortex filament is given an initial deformation within a plane at an angle ? to the x-z plane (x is the mean flow direct and z the spanwise direction) with the width Zw, and the further deformation process of the filament is numerically traced. The first calculation is made with fixed Zw and various values of ?. The result shows that the vortex filament finally reaches a structure lying on a plane with a constant angle of 30° ~ 45° to the x-z plane irrespective of the initial values of ?. The second calculation is made with fixed ? and various values of Zw. In this case the final spanwise scale of the deformed region of the filament has almost constant values of about 4d-6d (d is diameter of the cylinder). These results indicate that the final structure of the vortex filament is stable and definite irrespective of the initial disturbances. Translated from Nagare, Journal of Japan Society of Fluid Mechanics 6 (1987) 124-131

Okubo, Masaaki; Yamane, Ryuichiro; Oshima, Shuzo

1988-07-01

91

Near-Wake Decaying Turbulence Behind a Cross-bar  

Microsoft Academic Search

The present paper reports on a direct numerical simulation of a turbulent wake behind a cross- bar made of two perpendicular square cylinders in a bi-plane arrangement. The motivation for the study is that the cross-bar arrangement can be seen as the \\

Lyazid Djenidi; Philippe Lavoie

2007-01-01

92

An experimental and analytical study of the stability of counter-rotating vortex pairs with applications for aircraft wake turbulence control  

NASA Astrophysics Data System (ADS)

Aircraft trailing vortex wakes are commonly referred to as `wake turbulence' and may pose a flight safety hazard to other aircraft that may encounter the wake. This hazard is of critical interest during the take-off and landing stages of flight, where aircraft are in the closest proximity to one another. During these flight stages, it is common for transport aircraft to be in a high-lift, or flaps down, configuration. In an effort to study these wakes a generic four-vortex wake is generated experimentally, such that the results are independent of a specific wing loading condition. Three principle objectives served to focus the research project that is presented in this dissertation. The first two objectives were to develop an improved understanding of the wake configurations that were conducive to large instability growth rates and to subsequently use quantitative methods to identify the instability modes that dominate the far-field wake dynamic. With a clear understanding of the physics of an unstable aircraft wake, the third objective of the research project was to use this newly attained information to recommend methods for a reliable wake control strategy. A compilation of flow visualization results shows a design space of counter-rotating wake configurations, defined by the circulation and span ratios, where rapidly amplifying instabilities are consistently seen to exist. This design space is also seen to encompass rigidly-translating wake systems. A combination of quantitative flow visualization estimates, hot-wire anemometry and an analytical stability analysis was successful in identifying two forms of bending wave instability, namely the long and short-wavelength modes. Having identified two bending instability modes in the experimental wake, it was possible to suggest a strategy by which these modes could be exploited for the control of aircraft wakes.

Babie, Brian Matthew

93

Waking.  

PubMed

An indubitable aspect of laboring in the realm of hospice care is the "everydayness" of human loss or the stark encounter of death in the human experience. This can pose as opportunity to adopt each day in a particular manner. As such, the focus of my reflection is on transposing certain dynamics of a (funeral) wake to broader professional and personal socioexistential processes. PMID:22811212

Moon, Paul J

2012-07-18

94

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

95

Numerical study of particle dispersion in the wake of gas-particle flows past a circular cylinder using discrete vortex method  

Microsoft Academic Search

A numerical investigation on the particle dispersion in the wake of particle-laden gas flows past a circular cylinder at Reynolds number of 105 is presented. In the numerical method, the Discrete Vortex Method with the diffusion velocity model is employed to calculate the unsteady gas flow fields and a Lagrangian approach is applied to track individual particles. A dispersion function

Yuandong Huang; Wenquan Wu; Hongwu Zhang

2006-01-01

96

Vortex Formation from an Oscillating Cylinder: Three-Dimensional Features of the Near-Wake  

Microsoft Academic Search

Three-dimensional features of the near-wake of a stationary cylinder have been extensively investigated; the corresponding wake structure from an oscillating cylinder, however, has received relatively little attention. A cylinder is subjected to controlled motion in the cross-stream, streamwise, and orbital modes. An orthogonal-plane scanning technique of high-image-density particle image velocimetry, in conjunction with lift and drag measurements, provides the space-time

O. Cetiner; J.-C. Lin; D. Rockwell

1998-01-01

97

Studies on the influence of outboard flaps on the vortex wake of a rectangular wing  

Microsoft Academic Search

Aircraft trailing vortices constitute a hazard to following aircraft, and are therefore one of the main concerns for airport capacity constraints. At the Institute of Aerospace Engineering (ILR) experiments on wake vortices up to a distance of 60 spans behind the model of a rectangular wing are conducted in a towing tank. The motivation behind the presented experiments is the

S Haverkamp; G Neuwerth; D Jacob

2003-01-01

98

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 ...

99

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

100

Vortex Ring Formation in the Wake of Biologically Inspired Flapping Foils  

Microsoft Academic Search

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

M. B. Read

2005-01-01

101

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

102

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

103

Numerical analysis of the tip and root vortex position in the wake of a wind turbine  

NASA Astrophysics Data System (ADS)

The stability of tip and root vortices are studied numerically in order to analyse the basic mechanism behind the break down of tip and root vortices. The simulations are performed using the CFD program "EllipSys3D". In the computations the so-called Actuator Line Method is used, where the blades are represented by lines of body forces representing the loading. The forces on the lines are implemented using tabulated aerodynamic aerofoil data. In this way, computer resources are used more efficiently since the number of mesh points locally around the blade is decreased, and they are instead concentrated in the wake behind the blades. We here present results of computed flow fields and evaluate the flow behaviour in the wake. In particular we compare the position of the root vortices as to the azimuthal position of the tip votices.

Ivanell, S.; Sørensen, J. N.; Mikkelsen, R.; Henningson, D.

2007-07-01

104

Passive Control of the Vortex Wake Past a Flat Plate at Incidence  

Microsoft Academic Search

.    A passive control, based on wall suction acting at the leading edge, is proposed to stabilize the vortex shedding from a flat\\u000a plate at incidence. The correct suction amount is determined by a potential flow model where the large-scale vortical structures\\u000a formed near the plate edges are represented by point vortices of variable intensity, and the wall suction by an adequately

Luca Zannetti; Angelo Iollo

2003-01-01

105

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

NASA Astrophysics Data System (ADS)

Potential flow and 2-D Navier-Stokes calculations are used to investigate the role of vortex shedding in the non-resonant flow-induced vibration of periodic tube arrays. This dual approach untangles the effects of potential and vortical flow. The negative damping theory is shown to be inconsistent with the Navier-Stokes simulations, and allowing only a single degree of freedom in tube motion significantly overestimates the critical velocity. In contrast, Navier-Stokes simulations which allow all tubes to move in both the transverse and streamwise directions give results in good agreement with experiment. Somewhat surprisingly, potential flow calculations including an artificial phase lag between fluid force and tube motion give reasonably accurate results for a wide range of phase lags. This may be due to the fact that the most unstable mode at onset appears to be streamwise anti-phase (not whirling), as observed in the potential flow case.

Kevlahan, N. K.-R.

2011-07-01

106

Effect of swirl decay on vortex breakdown in a confined steady axisymmetric flow  

NASA Astrophysics Data System (ADS)

This numerical study of the steady axisymmetric motion of a viscous incompressible fluid in a sealed cylindrical container with one end wall rotating reveals that swirl decay, induced by friction at the sidewall, plays an important role in the development of vortex breakdown (VB). When the flow is slow, it is multi-cellular. As the flow strength increases (i) meridional circulation becomes global, (ii) flow convergence toward the axis focuses near the still end wall, (iii) a few local minima of pressure appear, (iv) a few flow reversals occur near the axis, and (v) circulation regions merge and an elongated double counterflow develops. Stages (i)-(v) are common for a number of vortex devices. If the swirl decay is diminished by additional rotation of the sidewall, VB disappears.

Shtern, Vladimir N.; del Mar Torregrosa, María; Herrada, Miguel A.

2012-04-01

107

The Human Aerodynamic Wake  

NASA Astrophysics Data System (ADS)

The wake that trails behind a walking person in still air is, in effect, that of an irregular 3-D cylinder. At a brisk walking speed of 1.3 m/s (3 mph), the human wake is characterized by a Reynolds number of about 50,000. It is thus turbulent with underlying large-scale vortex motion. We show that buoyancy plays no role at this Reynolds number, even though it is dominant in the plume of a standing person. Computational Navier-Stokes solutions and laser-light-sheet experiments with a human subject reveal a large recirculation zone behind the torso and flow between the legs. The decay of a passive scalar introduced on the human body is found to be exponential with downstream distance. The volume flux in the human wake is roughly constant with downstream distance until the recirculation closes, whence it grows due to turbulent entrainment. Further experiments reveal the development of the wake from the human thermal plume as the Reynolds number (proportional to walking speed) is increased from zero to 50,000. These results pertain to the sensing of chemical traces in the wakes of walking persons for aviation security. Supported by FAA Grant 99-G-040.

Settles, Gary; Moyer, Zachary; Paterson, Eric; Edge, Brian

2003-11-01

108

Vortex Bursting.  

National Technical Information Service (NTIS)

Vortex bursting is studied by means of visualization. The physical behavior of the phenomenon is emphasized, and its similarity with boundary layer separation or wake bursting becomes apparent. The essential influence of an increasing pressure gradient on...

H. Werle

1984-01-01

109

Numerical study of decay of vortex tangles in superfluid helium at zero temperature  

NASA Astrophysics Data System (ADS)

We submit the results of the numerical experiment on the decay of the quantum turbulence in the absence of the normal component of the superfluid helium. Computations were fulfilled inside a fixed domain with the use of the vortex filament method. The purpose of this study was to ascertain the role of the various factors arising in the numerical procedure, such as change in length in the reconnection processes, the procedures regulating the amount of points on the lines, eliminations of very small loops below the space resolution as well as the evaporation of the loops from the volume. We would like to stress that the widely accepted mechanism—a cascadelike transfer of the energy by nonlinear Kelvin waves (and radiation of sound)—was not considered. One of the reasons is that the space resolution along the lines did not allow to detect generation of high harmonics, moreover, particularly to get harmonics, which really radiate sound. In addition, the use of the method assumes that the fluid is incompressible. Numerical simulations have been performed for the cubic domain with transparent walls embedded in an unbounded space, and for a cube with solid smooth walls. Calculations showed that in the case of unlimited space the decay of quantum turbulence caused by the evaporation of vortex loops, which is implemented in a diffusion-like manner. The rate of the attenuation of the vortex line density agrees with the one, predicted by the theory of diffusion of nonuniform vortex tangles. In the case of a cube with solid walls, the main decay is also due to the diffusion of the vortex loops to boundaries. The vortex loops, whose ends glide on a smooth wall, execute the sophisticated motion (especially when they jump from the one face to the other) with many subsequent reconnections. As a result, there appear smaller and smaller loops with a size of few spatial resolutions, which were removed from the calculation. Indirect comparison with the experiments shows that the time of decay agrees with the measured data.

Kondaurova, Luiza; Nemirovskii, Sergey K.

2012-10-01

110

Short-wavelength instability and decay of a vortex pair in a stratified fluid  

NASA Astrophysics Data System (ADS)

The evolution of a counter-rotating vortex pair in a stably stratified fluid is investigated using direct numerical simulations. The study focuses on the short-wavelength elliptic instability occurring in this flow and the subsequent decay of the vortices. Depending on the level of stratification, as characterized by the Froude number which indicates the time scale of buoyancy to that of the instability, and the stage of evolution, stratification effects may significantly alter the behaviour of the flow. In the case of weak to moderate stratification, the elliptic instability develops qualitatively in the same manner as in unstratified fluid. The primary effect of stratification is to reduce the vortex separation distance which enhances the mutually induced strain. Consequently, the instability has an earlier onset and higher growth rate with increasing stratification. The behaviour is essentially described by linear stability theory for unstratified flow if the varying separation distance is taken into account. On the other hand, the final breakdown and decay of the flow may be greatly modified by stratification since buoyancy effects eventually emerge after sufficient time has elapsed. The decay is enhanced owing to additional mechanisms not present in unstratified flow. Secondary vertical vortex structures form between the primary vortices promoting fluid exchange in the transverse direction. Detrainment of fluid from the primary vortices by the generated baroclinic torque also contributes to the more rapid breakdown of the flow. In the case of strong stratification, in which the time scale of buoyancy is comparable to that of the instability, the flow is significantly altered. As a result of strong baroclinic torque, the primary vortices are brought together and detrainment occurs earlier. The associated reduction in radii of the vortices results in a higher axial wave mode and a more complex radial structure of the instability. Detrainment and mixing accelerate their decay. Late time evolution is dominated by the successive generation of alternate signed baroclinic torque which results in an oscillation of the total flow circulation at the buoyancy frequency.

Nomura, Keiko K.; Tsutsui, Hideaki; Mahoney, Daniel; Rottman, James W.

2006-04-01

111

Vortex  

NSDL National Science Digital Library

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

Exploratorium, The

2012-06-26

112

An experimental and numerical study of the vortex filaments in the wake of an operational, horizontal-axis, wind turbine  

Microsoft Academic Search

The paper describes a wind-tunnel study of the wake dynamics of an operational, horizontal-axis wind turbine. The behaviour of the vorticity trailing from the turbine blade tips and the effect of was interference on wake development were considered. Laser sheet visualisation (LSV) techniques were used to measure the trajectories of the trailing vorticity under various conditions of turbine yaw and

I. Grant; M. Mo; X. Pan; P. Parkin; J. Powell; H. Reinecke; K. Shuang; F. Coton; D. Lee

2000-01-01

113

Controlling Wake Turbulence  

Microsoft Academic Search

This Letter introduces a control strategy for taming the wake turbulence behind a cylinder. An angular momentum injection scheme is proposed to synchronize the vertical velocity field. We show that the base suction, wake formation length, absolute instability, and the Kármán vortex street are effectively controlled by the angular momentum injection. A control equation is designed to implement the injection.

B. S. V. Patnaik; G. W. Wei

2002-01-01

114

Numerical renormalization group of vortex aggregation in two-dimensional decaying turbulence: The role of three-body interactions  

Microsoft Academic Search

We introduce a numerical renormalization group procedure which permits long-time simulations of vortex dynamics and coalescence in a two-dimensional turbulent decaying fluid. The number of vortices decreases as N~t-xi, with xi~1 instead of the value xi=4\\/3 predicted by a naïve kinetic theory. For short time, we find an effective exponent xi~0.7 consistent with previous simulations and experiments. We show that

Clément Sire; Pierre-Henri Chavanis

2000-01-01

115

Numerical renormalization group of vortex aggregation in 2D decaying turbulence: the role of three-body interactions  

Microsoft Academic Search

In this paper, we introduce a numerical renormalization group procedure which\\u000apermits long-time simulations of vortex dynamics and coalescence in a 2D\\u000aturbulent decaying fluid. The number of vortices decreases as $N\\\\sim t^{-\\\\xi}$,\\u000awith $\\\\xi\\\\approx 1$ instead of the value $\\\\xi=4\\/3$ predicted by a na\\\\\\

Clément Sire; Pierre-Henri Chavanis

1999-01-01

116

Wake Turbulence Mitigation for Arrivals (WTMA).  

National Technical Information Service (NTIS)

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 ...

A. C. Trujillo D. M. Williams G. W. Lohr

2008-01-01

117

Onset of the second wake: Dependence on the Reynolds number  

Microsoft Academic Search

The second wake transition occurs in the far wake of a bluff body. This transition destroys the Benard-von Karman vortex street originating in the near wake and produces a secondary vortex street with a lower characteristic frequency. We characterize the onset of the second wake for Reynolds numbers 50-1\\/2 power law. Our two-dimensional far-wake numerical simulations are in good agreement

Peter Vorobieff; Daniel Georgiev; Marc S. Ingber

2002-01-01

118

Preliminary study of the three-dimensional deformation of the vortex in Karman vortex street  

NASA Astrophysics Data System (ADS)

The mechanism for 3D evolution of the isolated Karman vortex and the thin-vortex filament in a circular cylinder wake is studied numerically using the LIA method. The results show that the vortex motion is unstable for small 3D disturbances in the separated wake of a circular cylinder. Karman vortex in the time-averaged wake flowfield wolves into a horseshoe-spoon-like 3D structure. The thin vortex filament deforms three-dimensionally in the braid and generates streamwise vortex structures which incline to the region maximum-deformation direction of the flowfield.

Ling, Guocan; Guo, Liang; Wu, Zuobin; Ma, Huiyang

1992-03-01

119

Decaying vortex and wave turbulence in rotating shallow water model, as follows from high-resolution direct numerical simulations  

NASA Astrophysics Data System (ADS)

We report results of direct numerical simulations of decaying turbulence in an inviscid rotating shallow water model. We use a new-generation high-resolution well-balanced shock-capturing finite-volume scheme with several types of initializations: ``classical'' ones with random velocity and/or height fields, or an initialization with randomly oriented coherent vortex dipoles. Together with ``full'' turbulence simulations we also perform pure wave-turbulence ones, starting from an initial random wave field of small amplitude with zero potential vorticity anomaly and a given initial spectrum. Statistical properties of the rotating shallow water turbulence, as well as the development of coherent structures and their interactions are studied in detail. For all ``full'' turbulence simulations we find a tendency to form coherent structures with clear cyclone-anticyclone asymmetry and very steep energy spectra, with exponents close to -6. We also observe a decorrelation of the vortex and wave fields in time, even at significant Rossby numbers. However, we do not observe a universal power law in the evolution of coherent vortices, predicted by the ``universal decay'' theory for the 2D turbulence. A clear sensitivity to the initial conditions is thus established. For wave-turbulence simulations we observe a tendency to form very steep spectra different from the predictions of the so-called weak turbulence, and of both the turbulence of cusped nonlinear waves and the shock turbulence.

Lahaye, Noé; Zeitlin, Vladimir

2012-11-01

120

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

121

Vortex safety in aviation  

NASA Astrophysics Data System (ADS)

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

Turchak, L. I.

2012-10-01

122

Controlling wake turbulence.  

PubMed

This Letter introduces a control strategy for taming the wake turbulence behind a cylinder. An angular momentum injection scheme is proposed to synchronize the vertical velocity field. We show that the base suction, wake formation length, absolute instability, and the Kármán vortex street are effectively controlled by the angular momentum injection. A control equation is designed to implement the injection. The Navier-Stokes equations, along with the control equation, are solved. The occurrence of a new recirculation free zone is identified. PMID:11863732

Patnaik, B S V; Wei, G W

2002-01-17

123

Assimilation Experiment of Lidar Measurements for Wake Turbulence  

Microsoft Academic Search

Numerical simulation of wake turbulence was performed by integrating the lidar measurements using four-dimensional variational method. A bogus vortex technique was adopted to ensure the existence of wake vortices in the flow field. The validation of the method was performed by an idealized test case using virtual lidar measurement which was produced by the reference simulation of a vortex pair.

Takashi Misaka; Takeshi Ogasawara; Shigeru Obayashi; Izumi Yamada; Yoshinori Okuno

2008-01-01

124

Detrainment from a Vortex Pair in a Nonstratified Fluid  

NASA Astrophysics Data System (ADS)

Laboratory experiments of detrainment from a vortex pair are reported to better understand and numerically model the evolution of aircraft trailing wake vortices. The vortex pair was generated by towing a model wing down a water-filled, unstratified towing tank at an angle of attack. Nearly neutrally buoyant particles were introduced into the vortex cell or into the vortex core. The detrainment of fluid from the vortex cell is shown to be faster than the detrainment from the vortex core.

Delisi, Donald P.; Lai, David Y.

2011-09-01

125

Video Images of Smoke Dispersion in the Near Wake of a Model Building. Part 1. Temporal and Spatial Scales of Vortex Shedding.  

National Technical Information Service (NTIS)

In a wind-tunnel study, recorded video images of smoke dispersion in the wake of a rectangular-shaped building were analyzed. A continuous source of smoke was emitted at floor level, midway along the leeward side of the building. Smoke was observed to bui...

A. H. Huber

1988-01-01

126

Periodic Wake Effects on Turbulent Juncture Flows  

Microsoft Academic Search

The horseshoe vortex (HV) that develops in juncture geometries with a turbulent approach flow has been shown to exhibit a periodic behavior that correlates with the bursting frequency of the impinging turbulent boundary layer. To examine the additional complication of impinging blade wakes on such juncture flows, as encountered in turbomachinery environments, periodic wakes were systematically introduced upstream of a

Daniel Sabatino; Charles Smith

2000-01-01

127

The interaction of a wing-tip vortex and free-stream turbulence  

NASA Astrophysics Data System (ADS)

The formation and development of a vortex generated by a wing with a NACA0012 airfoil section in the presence of grid-generated turbulence was studied at Reynolds numbers 29000 in a water tunnel and 240000 in a wind tunnel. Flow visualization indicated that the free-stream turbulence did not significantly affect the large-scale features of the vortex during its formation, however, it resulted in an increase in the turbulence within the vortex as well as an increase in the wandering motion of the vortex. Extensive single-point and two-point, three-component hot-wire anemometry.measurements were conducted for the no-grid case and four cases using two different grids. Time-averaged statistics were found to be significantly influenced by wandering of the vortex position. Quantitative analysis of vortex wandering showed a significant increase in its amplitude with increasing free-stream turbulence. The radial profile of the circumferential velocity with respect to the wandering vortex axis was reconstructed using two-point measurements. The rate of decay of the peak circumferential velocity was found to increase with increasing free-stream turbulence, but the radial location at which it occurred did not change significantly. Analysis of the velocity signals indicated that the turbulent eddies from the wing's wake and the grid turbulence tend to wrap around the vortex core.

Bailey, Sean C. C.

128

Passive control of unsteady-wing tip vortex via a slender half-delta wing in both reverse and regular configurations  

NASA Astrophysics Data System (ADS)

The passive control of the tip vortex generated by an oscillating NACA 0012 wing via a tip-mounted half-delta wing, in both regular and reverse configurations, was investigated experimentally at Re = 2.45 × 105. Velocity and vorticity measurements, performed using a miniature triple hot-wire probe, show that vortex breakdown occurred when the regular half-delta wing (HDW) was mounted, but not for the reverse half-delta wing (RHDW) configuration. The HDW vortex breakdown led to a rapidly diffused tip vortex, suggesting an enhanced wake-vortex decay. For the RHDW wing configuration, the tip vortex remained concentrated but had a smaller size and also a weaker strength and rotation compared to the oscillating baseline wing. In addition, the vortex center of the oscillating RHDW wing-generated tip vortex was also found to be greatly displaced, especially in the transverse direction, which could translate into an increased blade-vortex-impingement miss distance and, as a result, an alleviated blade-vortex interaction.

Lee, T.; Pereira, J.

2013-07-01

129

Interactions in the far wake behind a pair of cylinders  

Microsoft Academic Search

We present an experimental study of far wakes behind a pair of cylinders (diameter D) separated by a cross-flow axis-to-axis distance S in a quasi-two-dimensional gravity-driven soap-film flow. A secondary vortex street forms in the far wake of each cylinder. As we decrease S, we observe coupling between the structures in the far wake. Visualization of the far wake behind

Tanveer Shakeel; Daniel Georgiev; Jesse Vigil; Peter Vorobieff

2002-01-01

130

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

NASA Astrophysics Data System (ADS)

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

Tang, Zhan-Qi; Jiang, Nan

2011-05-01

131

Flight Test Investigation of Rotorcraft Wake Vortices in Forward Flight.  

National Technical Information Service (NTIS)

This report presents the results of helicopter flight tests and wake vortex measurements which were designed to provide data necessary for the assessment of hazards to following aircraft. The tests described in this report were conducted using small probe...

S. A. Teager K. J. Biehl L. J. Garodz J. J. Tymczyszym D. C. Burnham

1996-01-01

132

Cylinder wakes in flowing soap films  

Microsoft Academic Search

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

Peter Vorobieff; Robert E. Ecke

1999-01-01

133

Wake Instabilities Behind Bluff Bodies  

Microsoft Academic Search

\\u000a The observation by Bénard of a vortex street in the wake of a circular cylinder has been commonly associated with the stability\\u000a analysis of the double alternate street proposed by von Kármán. After a short historical review of these studies, we present\\u000a the main progress in understanding this instability during the last decade. New experiments and the control of two-dimensional

Michel Provansal

134

Analysis of Fan Noise Spectrum Based onWake Characteristics of an Arc Blade  

Microsoft Academic Search

In this study, the wake characteristics of an arc blade were measured by means of a wind tunnel experiment; the characteristics were defined as the width of the wake, diameter of the vortex, ratio of the vortex structure, and local lift. The influence of the angle of attack on the aerodynamic noise of the blade was quantitatively predicted by using

Souichi SASAKI; Hidechito HAYASHI; Daisuke SATOH; Shutaro NASU

135

Application of Wake Characteristics to Prediction of Broadband Noise of a Multiblade Fan  

Microsoft Academic Search

In this study, the wake characteristics of an arc blade were measured by the wind tunnel experiment; the characteristics were defined as the width of the wake, diameter of the vortex, ratio of the vortex scale, and the local lift. The influence of the angle of attack on the aerodynamic noise of the blade was quantitatively predicted by using these

Soichi Sasaki; Hidechito Hayashi

2008-01-01

136

A quantitative comparison of bird and bat wakes.  

PubMed

Qualitative comparison of bird and bat wakes has demonstrated significant differences in the structure of the far wake. Birds have been found to have a unified vortex wake of the two wings, while bats have a more complex wake with gradients in the circulation along the wingspan, and with each wing generating its own vortex structure. Here, we compare quantitative measures of the circulation in the far wake of three bird and one bat species. We find that bats have a significantly stronger normalized circulation of the start vortex than birds. We also find differences in how the circulation develops during the wingbeat as demonstrated by the ratio of the circulation of the dominant start vortex and the total circulation of the same sense. Birds show a more prominent change with changing flight speed and a relatively weaker start vortex at minimum power speed than bats. We also find that bats have a higher normalized wake loading based on the start vortex, indicating higher relative induced drag and therefore less efficient lift generation than birds. Our results thus indicate fundamental differences in the aerodynamics of bird and bat flight that will further our understanding of the evolution of vertebrate flight. PMID:19324669

Johansson, L Christoffer; Wolf, Marta; Hedenström, Anders

2009-03-25

137

An Analytical Model of Wake Deflection Due to Shear Flow  

Microsoft Academic Search

The main motivation behind this work is to create a purely analytical engineering model for wind turbine wake upward deflection due to shear flow, by developing a closed form solution of the velocity field due to an oblique vortex ring. The effectiveness of the model is evaluated by comparing the results with those of a free-wake model. The solution of

D. Micallef; C. J. Simao Ferreira; T. Sant; G. J. W. Van Bussel

2010-01-01

138

A bypass wake induced laminar\\/turbulent transition  

Microsoft Academic Search

The process of laminar to turbulent transition induced by a von Karman vortex street wake, was studied for the case of a flat plate boundary layer. The boundary layer developed under zero pressure gradient conditions. The vortex street was generated by a cylinder positioned in the free stream. An X-type hot-wire probe located in the boundary layer, measured the streamwise

N. K. Kyriakides; E. G. Kastrinakis; S. G. Nychas; A. Goulas

1999-01-01

139

Aircraft Wake Vortices: From Fundamental Research to Operational Application  

NASA Astrophysics Data System (ADS)

Aircraft trailing vortices constitute both a kaleidoscope of instructive fluid dynamics phenomena and a challenge for the sustained development of the safety and capacity of the air-transportation system. This section gives an overview of the wake vortex issue commencing at its historical roots, proceeding with a sketch of the nature and characteristics of wake vortices resulting from field measurement and numerical simulation, and concluding with a depiction of the design and performance of wake vortex simulation systems established for the prediction of dynamic aircraft separations in different flight phases and for sensitivity and risk analysis.

Holzäpfel, Frank; Gerz, Thomas

140

A free wake method for performance prediction of VAWT  

NASA Astrophysics Data System (ADS)

Based on the lifting line theory and a free vortex wake model, a method including dynamic stall effects is presented for predicting the performance of a three-dimensional vertical-axis wind turbine (VAWT). A vortex model is used in which the wake is composed of trailing streamwise and shedding spanwise vortices, whose strengths are equal to the change in the bound vortex strength as dictated by Helmholtz and Kelvin's theorems. Performance parameters are calculated by application of the Biot-Savart law along with the Kutta-Joukowski theorem and a semi-empirical dynamic stall model. Predictions are shown to compare favorably with existing experimental data.

Ilin, S.; Dumitrescu, H.; Cardos, V.; Dumitrache, A.

2012-09-01

141

Experimental investigations on wake vortices and their alleviation  

NASA Astrophysics Data System (ADS)

Recent wake vortex research in the laboratory has benefited considerably from concurrent analytical and numerical research on the instability of vortex systems. Tow tank, with dye flow visualization and particle image velocimetry is the most effective combination for laboratory research. Passive and active wake alleviation schemes have been successfully demonstrated in the laboratory. The passive alleviation systems exploit the natural evolution of vortex instabilities while the active systems rely on hastening selected instabilities by forcing the vortices individually or as a system. Their practical applicability, however, will have to meet further criteria beyond those dictated by fluid dynamics. To cite this article: Ö. Sava?, C. R. Physique 6 (2005).

Sava?, Ömer

2005-05-01

142

SHORT WAVE INSTABILITIES OF COUNTER-ROTATING BATCHELOR VORTEX PAIRS  

Microsoft Academic Search

Recently, investigations have considered the cooperative elliptic instability which forms by the mutual co-existence of a counter-rotating Batchelor-type vortex pair. Such vortex pairs are observed in the wake of an aircraft and pose a significant danger to trailing aircraft. An aircraft flying through the wake of a lead aircraft can exhibit significant loss of lift and control forces; several accidents

Kris RYAN; Gregory J. SHEARD; Mark C. THOMPSON

143

Direct numerical simulation of a turbulent vortex ring  

NASA Astrophysics Data System (ADS)

Engineers have been fascinated by vortex rings for over a hundred years, due to their numerous engineering and biological applications and their presence as a constituent of fully turbulent flow. Although the laminar ring has received much attention, the turbulent vortex ring is less well understood, due to the difficulty in its visualisation and measurement. Glezer and Coles [1] used ensemble averaging of experimental data to show that the radial expansion, circulation decay and slowing of the turbulent ring occur in a self-similar fashion. Circulation decreases in a staircase-like fashion [2] as the ring sheds hairpin vortices [3] into a wake. The radial growth of the ring is due to a slight excess in the amount of entrainment over detrainment[1]. The movement of dye within the ring suggests the existence of secondary vortices that wrap around the core, influencing the local entrainment, detrainment and production of turbulence [1]. In previous work [4], we investigated the laminar evolution of the ring and focused on the development of the Tsai-Widnall-Moore-Saffman (TWMS) instability [5, 6], and transition to turbulence. Here, we examine the temporal development of the turbulent vortex ring.

Archer, P. J.; Thomas, T. G.; Coleman, G. N.

144

Experiment on the characteristics of 3-D vortex ring behind a flexible oscillating caudal fin  

Microsoft Academic Search

A test for the wake vortex of a flexible oscillating caudal fin is carried out with Digital Particle Image Velocimetry (DPIV), and the variation of vortex distance and the vorticity in the range of oscillating frequency from 0.704 Hz to 1.17 Hz are analyzed. It is found that with the increase of the oscillating frequency, the vortex distance decreases and

Li-jun LI; Wen-chao CONG

2010-01-01

145

Self-excited oscillations in the wake of two-dimensional bluff bodies and their control  

NASA Astrophysics Data System (ADS)

The onset of Karman-vortex shedding is studied experimentally in the wake of different two-dimensional bluff bodies, namely an oblong cylinder, circular cylinders, and plates of rectangular cross section. Different control measures, such as wake heating, transverse body oscillations, and base bleed are investigated. As the steady-periodic Karman shedding has previously been identified as a limit-cycle, i.e. as self-excited oscillations, the experiments are interpreted in the framework of the Stuart-Landau model. The coefficients of the Stuart-Landau equation for the characteristic vortex shedding amplitude, i.e. the linear temporal growth rate, linear frequency, and the Landau constant, are fully determined for the two cylinders and in part for the plate. For this purpose transients are generated by suddenly switching transverse body oscillations or base bleed on or off. The analysis of these transients by a refined method based on complex demodulation provides reliable estimates of the model coefficients and yields an experimental validation of the concept that a global instability mode grows or decays as a whole. Also, it is demonstrated that the coefficients of the Stuart-Landau equation are independent of the experimental technique used to produce the transients.

Schumm, Michael; Berger, Eberhard; Monkewitz, Peter A.

1994-07-01

146

Wake Behind a Sphere Second Bifurcation  

NASA Astrophysics Data System (ADS)

The objective was to study second transition phenomenon and to make transition between second and third regimes better known. We present in this communication very controlled experiments concerning second instabilities in the wake of the sphere. We discuss the first transition from homogenous flow to a stationary instability and we present original results about peristaltic instability preceeding the hairpins shedding. From our results we proposed now a new insight on the generation of hairpins-vortex shedding phenomena. We obtained a new scenario ``precursor'' of the hairpin vortex shedding, with a peristaltic instability of oscillations of the two parallel counter-rotating vortices behind a sphere.

Gumowski, Konrad; Miedzik, Jan; Goujon-Durand, Sophie; Jenffer, Patrice; Bouchet, Gilles; Wesfreid, Jose-Euardo

2007-11-01

147

Vortex interactions with flapping wings and fins can be unpredictable  

PubMed Central

As they fly or swim, many animals generate a wake of vortices with their flapping fins and wings that reveals the dynamics of their locomotion. Previous studies have shown that the dynamic interaction of vortices in the wake with fins and wings can increase propulsive force. Here, we explore whether the dynamics of the vortex interactions could affect the predictability of propulsive forces. We studied the dynamics of the interactions between a symmetrically and periodically pitching and heaving foil and the vortices in its wake, in a soap-film tunnel. The phase-locked movie sequences reveal that abundant chaotic vortex-wake interactions occur at high Strouhal numbers. These high numbers are representative for the fins and wings of near-hovering animals. The chaotic wake limits the forecast horizon of the corresponding force and moment integrals. By contrast, we find periodic vortex wakes with an unlimited forecast horizon for the lower Strouhal numbers (0.2–0.4) at which many animals cruise. These findings suggest that swimming and flying animals could control the predictability of vortex-wake interactions, and the corresponding propulsive forces with their fins and wings.

Lentink, David; Van Heijst, GertJan F.; Muijres, Florian T.; Van Leeuwen, Johan L.

2010-01-01

148

Flow visualizations of perpendicular blade vortex interactions  

NASA Astrophysics Data System (ADS)

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

Rife, Michael C.; Davenport, William J.

1992-10-01

149

The Effects of Aircraft Wake Dynamics on Contrail Development  

Microsoft Academic Search

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

D. C. Lewellen; W. S. Lewellen

2001-01-01

150

Thrust Production and Wake Structure of an Actuated Lamprey Model  

NASA Astrophysics Data System (ADS)

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

Buchholz, James; Smits, Alexander

2004-11-01

151

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

152

Axisymmetric Turbulent Wakes with New Nonequilibrium Similarity Scalings  

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

153

Wake fields and wake field acceleration  

SciTech Connect

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

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

1984-12-01

154

Identifying optimal vortex spacing for swimming and flying animals  

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

155

Vortex structure in strongly stratified flows  

NASA Astrophysics Data System (ADS)

Turbulence decaying experiments have been performed, with the aim of focusing in the middle of a strongly stratified density interface. The experiments have been done under different external conditions[1]: Non-Rotating Decaying 2D Turbulence experiments , Rotating Decaying 2D Turbulence experiments, And steady rotating stratified experiments. Non-Rotating experiments were performed in a 1mx1m tank, while the Rotating experiments were performed in a rectangular tank of 4mx 2m; this rectangular tank was placed in the middle of the Coriolis Rotating platform at the Trondheim Marine Systems Research Infrastructure supported by the European Union TMR Project HydraLab. The set of stirred experiments is a compilation of several series of traversing grid mixing experiments, dependent on the initial interface Richardson number [2]. PIV was used to map the velocity and vorticity plots in time. The density of the brine used in the experiments to create a sharp density interface. The boundary conditions for all the rotating experiment are related to initial Reynolds Rer, Rossby Ro, Ekman Ek and Richardson gradient Rig numbers, the results are summarized and presented in a 3D parameter map using power relationships. The experimental results of the strongly non-homogeneous turbulent dynamics shows the different decay of the strongest vortices as a function of the local Richardson number and the interaction mechanisms between inertial and internal waves. A study of vortex decay number indicates a strong non linear relationship with a slower decay due to the internal wave activity at intermediate Richardson number experiments. The intermittency of the flow is studied using a generalized intermitency parameter family that depends on the order.[3,4] [1] Matulka A.M. PhD Thesis UPC, Barcelona 2010. [2]Matulka A.M., Redondo J.M. and Carrillo A. Experiments in rotating decaying 2D flows Il nuovo cimento C, 31, 5-6, 757-770. 2008. [3]Ben-Mahjoub O., Babiano A. y Redondo J.M. Velocity structure and Extended Self Similarity in nonhomogeneous Turbulent Jets and Wakes. Journal of flow turbulence and combustion. 59 , 299-313. 1998. [4]Ben-Mahjoub O., Redondo J.M., and R. Alami. Turbulent Structure Functions in Geophysical Flows, Rapp. Comm. int. Mer Medit., 35, 126-127. 1998 [3]Babiano, A., Dubrulle, B., Frick, P. Some properties of two-dimensional inverse energy cascade dynamics, Phys. Rev. E. 55, 2693, 1997. [4]Vindel J.M., Yague C. and J.M. Redondo, Structure function analysis and intermittency in the ABL, NonLin. Proc. Geophys. 15, 6. 915-929. 2009.

Magdalena Matulka, Anna

2010-05-01

156

Spatial perturbation of a wing-tip vortex using pulsed span-wise jets  

NASA Astrophysics Data System (ADS)

The separation distance required between transport aircraft to avoid wake vortices remains a limiting factor on airport capacity. The dissipation of the wake can be accelerated by perturbing co-operative instabilities between multiple pairs of vortices. This paper presents the results of a preliminary experimental investigation into the use of pulsed span-wise air jets in the wing tip to perturb a single tip vortex in the very near field. Velocity measurements were made using PIV and hot-wire anemometry. The results demonstrate that the vortex position can be modulated at frequencies up to 50 Hz and, as such, the method shows promise for forcing instability in multiple vortex wakes.

Heyes, A. L.; Smith, D. A. R.

157

Simplified Wake Model of a Flapping Wing  

NASA Astrophysics Data System (ADS)

A vortex wake model consisting of two parts, a strong leading-edge vortex that is shed during the flapping cycle and a continuously attached vortex line determined by quasi-steady lifting line theory was developed. The leading edge vortex is essentially an expression of the Magnus effect, while the strength of the wing-tip vortex is determined by unsteady lifting line theory. Combined, these produce the ``ladder vortex" pattern seen downstream of root-flapping wings with fixed span, such as insects and most man-made flapping wing vehicles. A small flapping wing experimental setup in still air was used to provide experimental comparison to the model. Measurements include flow visualization and velocity obtained using a stereo PIV system. The flapping mechanism was mounted on a two-component force balance to obtain time-resolved lift and thrust. Data were ensemble averaged with the flapping phase cycle and used to calculate vorticity. These were then reconstructed to show the space-time development of vorticity shed from the wing during the flapping motion to compare to the model predictions.

Apker, Thomas; Corke, Thomas

2006-11-01

158

Vortex Methods with Spatially Varying Cores  

NASA Astrophysics Data System (ADS)

The accuracy of vortex methods employing smooth vortex particles/blobs is determined by the blob size, which can be viewed as a mollifier of the vorticity field. For computational efficiency, this core size needs to be spatially variable as particles are used to discretize different parts of the flow field, such as the boundary layer and the wake in bluff body flows. We derive here a consistent approximation for the viscous Navier-Stokes equations using variable size vortex particles. This derivation is based on the implementation of mappings that allow the consistent formulation of the diffusion and convection operators of the Navier-Stokes equations in the context of vortex methods. Several local mappings can be combined giving the capability of ``mesh-embedding'' to vortex methods. It is shown that the proposed variable method offers a significant improvement on the computational efficiency of constant core size methods while maintaining the adaptive character of the method. The method is ideally suited to flows such as wakes and shear layers and the validity of the approach is illustrated by showing results from cylinder flows and wall-vortex interactions. Using this scheme, previously unattainable simulations of cylinders undergoing rotary oscillations at high Reynolds numbers reveal an interesting mechanism for drastic drag reduction.

Cottet, Georges-Henri; Koumoutsakos, Petros; Salihi, Mohamed Lemine Ould

2000-07-01

159

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

Microsoft Academic Search

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

Helge Aagaard Madsen; Flemming Rasmussen

2004-01-01

160

Wind dependent concepts for wake avoidance: a comparative analysis of capacity benefits and implementation risks  

Microsoft Academic Search

The FAA and NASA are jointly embarking on a multiphased research and development program to develop and implement wake vortex avoidance solutions that can safely reduce separations and improve capacity at airports in the NAS. Many options have been proposed by the wake turbulence community and it is necessary to focus research efforts on the most promising solutions. As part

Clark Lunsford; Anand Mundra; Laurence Audenaerd; Jillian Cheng; Chris Devlin; Amy Gross; Ralf Mayer; J. Sherry; W. Bryant; E. Johnson; B. McKissick

2005-01-01

161

Effects of surface roughness and freestream turbulence on the wake turbulence structure of a symmetric airfoil  

Microsoft Academic Search

The effects of surface roughness on the wake characteristics of a simulated turbine airfoil, operating in a compressible, high-speed environment, are studied at different freestream turbulence levels. The effects of these parameters on wake distributions of mean velocity, turbulence intensity, and turbulence length scale, as well as on power spectral density profiles and vortex shedding frequencies are quantified one chord

Qiang Zhang; Sang Woo Lee; Phillip M. Ligrani

2004-01-01

162

Vortex methods.  

National Technical Information Service (NTIS)

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...

A. J. Chorin

1993-01-01

163

Linear analysis of the cylinder wake mean flow  

NASA Astrophysics Data System (ADS)

A highly accurate 2D linear stability analysis is performed on the mean flow of laminar vortex shedding from a circular cylinder for Reynolds numbers between 46 and 180. Consistent with past studies of mean profiles, the analysis shows that the eigenfrequency of the mean flow tracks almost exactly the Strouhal number of vortex shedding. The linear growth rate reveals that the wake mean flow is a marginally stable state over the whole range of Reynolds numbers for stable 2D vortex shedding. This is contrasted with 2D stability analysis about the unstable steady base flow. The relevance to nonlinear saturation and frequency selection are discussed.

Barkley, D.

2006-09-01

164

Vortex methods and vortex statistics.  

National Technical Information Service (NTIS)

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...

A. J. Chorin

1993-01-01

165

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

166

Vortex-induced vibrations of a sphere  

NASA Astrophysics Data System (ADS)

There are many studies on the vortex-induced vibrations of a cylindrical body, but almost none concerned with such vibrations for a sphere, despite the fact that tethered bodies are a common configuration. In this paper, we study the dynamics of an elastically mounted or tethered sphere in a steady flow, employing displacement, force and vorticity measurements. Within a particular range of flow speeds, where the oscillation frequency (f) is of the order of the static-body vortex shedding frequency (f_{vo}), there exist two modes of periodic large-amplitude oscillation, defined as modes I and II, separated by a transition regime exhibiting non-periodic vibration. The dominant wake structure for both modes is a chain of streamwise vortex loops on alternating sides of the wake. Further downstream, the heads of the vortex loops pinch off to form a sequence of vortex rings. We employ an analogy with the lift on an aircraft that is associated with its trailing vortex pair (of strength Gamma(*) and spacing b(*) ), and thereby compute the rate of change of impulse for the streamwise vortex pair, yielding the vortex force coefficient (cvortex): [ cvortex = {8}/{pi} {U^*_{v}}b^*( - Gamma^*). ] This calculation yields predicted forces in reasonable agreement with direct measurements on the sphere. This is significant because it indicates that the principal vorticity dynamics giving rise to vortex-induced vibration for a sphere are the motions of these streamwise vortex pairs. The Griffin plot, showing peak amplitudes as a function of the mass damping (m(*zeta) ), exhibits a good collapse of data, indicating a maximum response of around 0.9 diameters. Following recent studies of cylinder vortex-induced vibration, we deduce the existence of a critical mass ratio, m(*_{crit}) {?} 0.6, below which large-amplitude vibrations are predicted to persist to infinite normalized velocities. An unexpected large-amplitude and highly periodic mode (mode III) is found at distinctly higher flow velocities where the frequency of vibration (f) is far below the frequency of vortex shedding for a static body. We find that the low-frequency streamwise vortex pairs are able to impart lift (or transverse force) to the body, yielding a positive energy transfer per cycle.

Govardhan, R. N.; Williamson, C. H. K.

2005-05-01

167

Active Control of a Cylinder Wake Using Surface Plasma  

NASA Astrophysics Data System (ADS)

An experimental investigation has been undertaken using high-speed Particle Image Velocimetry to study the possibility of controlling the global flow field in the near wake of a circular cylinder at Re = 6,500.Surface plasma actuators were mounted at strategic locations around the cylinder (both fore and aft of the separation point) and used for flow control by producing a body force close to the wall.It was found that the plasma can significantly alter the vortex shedding in the wake of the cylinder, with effectiveness depending upon the actuator location and forcing frequency.The most dramatic effects were observed when the plasma was located very close to the natural laminar separation point.Here, amplification of the shedding was observed when the plasma was excited at the natural vortex shedding frequency (St f ? 0.2; St K = 0.206).This was accompanied by periodic flow reattachment to at least the rearward stagnation point.At higher forcing frequency (St f ? 0.8), the plasma completely suppressed the vortex shedding process which lead to a short and narrow wake, reduced turbulence intensity, and 60% reduction in the wake momentum thickness.At still higher frequency (St f ? 2.0; St SL = 1.7), only the shear layers were excited and the vortex street remained unaltered.

Jukes, T.; Choi, K.-S.

168

Modeling the Dielectric Constant Distribution of Wake Vortices  

Microsoft Academic Search

The study of scattering characteristics of a wake vortex is very important for hazard avoidance, especially near airports. Obtaining the dielectric constant distribution is one of the basic issues in the scattering study for radar detection. The present work proposes a modeling method for the dielectric constant distribution. In the method the dielectric constant variation (also the radar cross section

Jianbing Li; Xuesong Wang; Tao Wang

2011-01-01

169

A Numerical Study of Aircraft Wake Induced Ice Cloud Formation  

Microsoft Academic Search

Numerical simulations of ice cloud formation in the wake of an aircraft flying at cruise altitude have been performed. The engine exhaust has been excluded from the simulations in order to study cloud formation due solely to aerodynamic effects. The ice is formed via homogeneous freezing nucleation of ambient haze droplets in the upwelling limbs of the vortex pair behind

K. M. Gierens; J. Ström

1998-01-01

170

Predicted low frequency structures in the wake of elliptical cylinders  

Microsoft Academic Search

The vortex structures in the wake of 2D elliptical cylinders at low Reynolds numbers are investigated for a Reynolds numbers range of 75 to 175 using direct numerical simulation. By varying the aspect ratio of an elliptical cylinder, the geometry is varied between the extremes of a circular cylinder and a flat plate normal to the flow. The power spectrum

Shaun A. Johnson; Mark C. Thompson; Kerry Hourigan

2004-01-01

171

Joint US/UK Vortex Tracking Program at Heathrow International Airport. Volume II. Data Analysis.  

National Technical Information Service (NTIS)

From May 1974 through June 1975, the approach region to runway 28R at Heathrow International Airport was equipped with aircraft wake vortex tracking equipment. The vortices from approximately 13,000 aircraft were monitored along with the attendant meteoro...

J. N. Hallock B. P. Winston D. C. Burnham T. E. Sullivan I. G. McWilliams

1977-01-01

172

Joint US/UK Vortex Tracking Program at Heathrow International Airport. Volume I. Executive Summary.  

National Technical Information Service (NTIS)

From May 1974 through June 1975 the approach region to Runway 28R at Heathrow International Airport was equipped with aircraft wake vortex tracking equipment. The vortices from approximately 13000 aircraft were monitored along with the attendant meteorolo...

J. N. Hallock W. D. Wood

1976-01-01

173

Wake Turbulence Training Aid.  

National Technical Information Service (NTIS)

The goal of the Wake Turbulence Training Aid is to reduce the number of wake-turbulence related accidents and incidents by improving the pilot's and air traffic controller's decision making and situational awareness through increased and shared understand...

G. C. Hay R. H. Passman

1995-01-01

174

Nonuniqueness in wakes and boundary layers  

NASA Astrophysics Data System (ADS)

Instreamlined flow past a flat plate aligned with a uniform stream, it is shown that the Goldstein near wake and the Blasius boundary layer are nonunique solutions locally for the classical boundary layer equations, whereas the Rott-Hakkien very near wake appears to be unique. Concerning non-streamlined flow, new similarity forms are described for the pressure free vicous symmetric closure of a predominantly slender long wake beyond a large-scale separation. Features arising include nonuniqueness, singularities and algebraic behavior, consistent with non-entraining shear layers with algebraic decay. Nonuniqueness also seems possible in reattachment onto a solid surface and for nonsymmetric or pressure controlled flows including the wake of a symmetric cascade.

Smith, F. T.

1983-05-01

175

Wake Oscillation of Column Wall Jet in Uniform Flow  

NASA Astrophysics Data System (ADS)

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

Yoshida, Yohei; Sato, Kotaro; Ono, Yoichi

176

Point vortex dynamics: Recent results and open problems  

SciTech Connect

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

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

1987-01-01

177

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

178

Characterizing cylinder and hydrofoil wake dynamics  

NASA Astrophysics Data System (ADS)

A number of high speed PIV measurements of wakes trailing a NACA 0015, c=0.081m, and a cylinder D= 0.0127m, in the speed range 2 through 9 m/s have been made in the high speed water tunnel at SAFL- UMN. The cylinder vortex shedding follows closely St=0.2, while that off the hydrofoil is more irregular. Although the hydrofoil shows a more irregular nature a measure for both shedding frequencies and vortex strength of is of great interest also for drag analysis. The direct approach mapping individual structures, e.g. vorticity based analysis, can be obscured by the quality of the measurements made, hence other methods to reveal frequency and strength are in demand. A study were the mapping of time variation of the main flow direction impulse flux integrated over the wake at, minimum two, downstream positions has been made. A cross-correlation analysis of the impulse flux can reveal structure transport speeds, the frequency spectrum will reflect the shedding frequency, while the temporal variation represents the strength. For the hydrofoil it's shown that a significant cross- correlation is present. In terms of spectra even the more structured shedding from cylinders are hard to capture, and finally the vortex strength found using the cited algorithm seems somewhat unreliable. A thorough comparison between the suggested measure and traditional measures is given.

Kjeldsen, Morten; Seim, Bjarte G.; Arndt, Roger E. A.

2009-11-01

179

Vortex Sheets of Aircraft in Takeoff and Landing  

Microsoft Academic Search

\\u000a In the present paper the development of vortex wake starting from the vortex sheet at the trailing edge of a transport aircraft\\u000a wing up to the far field over 60 spans downstream is investigated. Different configurations of a half model were investigated\\u000a in wind and water tunnels as well as in a towing tank by hot-wire anemometry and particle image

Robert Schöll; Rolf Henke; Günther Neuwerth

180

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

181

Verification of Vortex Workflows  

Microsoft Academic Search

Vortex is a workflow language to support decision making activities. It centers around gathering and computing attributes of in- put objects. The semantics of Vortex is declarative, and the dependency graphs of Vortex programs are acyclic. This paper discusses the appli- cation of symbolic model checking techniques to verification of Vortex programs. As a case study we used a Vortex

Xiang Fu; Tevfik Bultan; Richard Hull; Jianwen Su

2001-01-01

182

Experimental study of steady concentration fields in turbulent wakes  

NASA Astrophysics Data System (ADS)

The pollutant transportation process in turbulent wakes is studied experimentally using planar laser-induced fluorescence (PLIF). The concentration fields in the very near wake region behind typical bluff bodies are measured for steady flow. The characteristics of the mean and instantaneous concentration fields behind circular and sinusoidal islands and peninsulas are investigated. The results indicate that the pollutant distribution is closely related with the unsteady vortex shedding in the flow field. Compared with that of the circular island, more pollutants enter into the wake generated by the sinusoidal-shaped island. The time needed for pollutants to accumulate in or drain out of the wake region after the peninsula before reaching a relatively constant value is longer than that for the islands, regardless of the island or peninsula shape. The results will facilitate pollutant control behind sea islands and other natural or man-made structures in water. Also the results provide some fundamental data for checking numerical models.

Jiang, C. B.; Li, Y. L.; Li, Y. S.; Liang, D. F.; Yu, C. Z.

183

Brownian vortexes  

NASA Astrophysics Data System (ADS)

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

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

2009-07-01

184

Response of the Sphere Wake to Freestream Fluctuations  

NASA Astrophysics Data System (ADS)

Direct numerical simulations have been used to investigate the response of the wake of a sphere to freestream fluctuations. This study has been motivated by the need to understand particle-induced turbulence enhancement in particulate flows. A sequence of simulations of flow past a sphere have been carried out where the frequency and amplitude of the freestream fluctuations and the flow Reynolds number has been varied systematically. It has been suggested that turbulence enhancement is primarily caused by vortex shedding from particles (Gore and Crowe, 1989; Hetsroni, 1989). Our simulations of the forced wake indicate that turbulence enhancement may be attributed to natural vortex shedding only when the freestream fluctuation level is low and the Reynolds number is greater than about 300. In addition to natural vortex shedding, the current simulations also suggest another mechanism for turbulence enhancement. It is found that in the presence of freestream fluctuations, the wake behaves like an oscillator and returns large amounts of kinetic energy to the surrounding fluid at resonance. This mechanism is not associated with natural vortex shedding and is therefore capable of enhancing freestream turbulence even at Reynolds numbers less than 300. Simulations also indicate that when the turbulence intensity of the carrier fluid is high, this resonance mechanism might be solely responsible for turbulence enhancement. Finally, our simulations also suggest a possible explanation for the correlation between turbulence enhancement and the ratio of the particle size to the size of energy containing eddies of turbulence found by Gore and Crowe (1989).

Mittal, R.

185

Dynamic analysis of marine risers with vortex excitation  

SciTech Connect

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

Nordgren, R.P.

1982-03-01

186

Building wake diffusion  

SciTech Connect

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

Ramsdell, J.V.

1989-01-01

187

Wake transition in flow past a circular cylinder  

NASA Astrophysics Data System (ADS)

The transition of the wake of a circular cylinder is investigated numerically via a stabilized finite element method for 150<=Re<=350. Both the flow and aerodynamic coefficients are studied. The onset of the three-dimensionality of the flow takes place via the mode-A instability at Re=200. At this Re, the flow exhibits pure mode-A type flow structures for t<1800. At larger times, the vortex dislocations appear spontaneously and destroy the spanwise periodicity in the flow. This confirms the hypothesis that the fully developed mode-A flow structures cannot exist without vortex dislocations. The appearance of dislocations leads to time variation in the vortex shedding frequency. They also lead to a reduction in the global aerodynamic parameters such as drag coefficient, rms value of lift coefficient, and dominant vortex shedding frequency. The vortex dislocations repetitively appear and disappear from the flow. The aerodynamic coefficients achieve a relatively lower value at the time instant when vortex dislocations appear in the flow. This leads to a low frequency modulation in the time variation of aerodynamic coefficients. The onset of mode-A is hysteretic. This is demonstrated in the present work via computations perhaps for the first time for increasing and decreasing Re. The transition from mode-A to mode-B vortex structures is gradual and not hysteretic. Mode-B is devoid of vortex dislocations and, therefore, the aerodynamic coefficients achieve a relatively larger value. The discontinuity in the variation of aerodynamic coefficients with Re is captured very well by the present computations. Unlike mode-A, the flow structures of the mode-B instability are restricted to the near wake.

Behara, Suresh; Mittal, Sanjay

2010-11-01

188

The stability of a family of vortex rings  

NASA Astrophysics Data System (ADS)

Jetting swimmers, such as squid or jellyfish, propel themselves by forming axisymmetric vortex rings. In order to evaluate the performance of these swimmers, we must asses the optimality of the vortex wakes they produce, which requires an understanding of their stability. We consider the Norbury family of vorticesootnotetextJ. Norbury, J. Fluid Mech., 57, 417-431, 1973. as a model for the vortex rings produced by jetting swimmers. PozrikidisootnotetextC. Pozrikidis, J. Fluid Mech., 168, 337-367, 1986. has studied the stability of Hill's spherical vortex under axisymmetric prolate and oblate shape perturbations. However, the stability of other members of the Norbury family to axisymmetric perturbations of the type that might occur during the vortex formation process in jetting swimmers is unknown. In order to asses the stability of different members of the family, we introduce physically pertinent shape perturbations and simulate their development in a manner akin to Pozrikidis' analysis.

O'Farrell, Clara; Dabiri, John O.

2010-11-01

189

Interaction of exhaust jets and aircraft wake vortices: small-scale dynamics and potential microphysical-chemical transformations  

NASA Astrophysics Data System (ADS)

This paper summarizes recent progress made in the understanding of the interaction between exhaust jets and trailing vortices, in the near field of an aircraft wake. Emphasis is placed on the effects of the jet on the wake vortex dynamics and the effects of the wake on the exhaust dispersion, as well as their potential microphysical and chemical transformations. We discuss in detail results of high-resolution numerical simulations of jet/vortex interaction that include microscale turbulent mixing, gas-phase chemistry and contrail formation. To cite this article: R. Paoli, F. Garnier, C. R. Physique 6 (2005).

Paoli, Roberto; Garnier, François

2005-05-01

190

A model for pattern selection in wake flows  

NASA Astrophysics Data System (ADS)

A model for the selection of vortex roll-up patterns in finite-aspect-ratio, two-dimensional wakes at low to moderate Reynolds number is studied. Two primary patterns are found: a chevron pattern encompassing the entire spanwise domain and one with regions adjacent to the side-wall boundaries, which are divorced from the central region by means of dislocation layers. These patterns can be understood in terms of spanwise-propagating phase waves, wave-number shocks, and the criterion for sustained global modes. Selection criteria for the frequency and scale of internal cells in the wake behind tapered cylinders are also discussed.

Park, D. S.; Redekopp, L. G.

1992-08-01

191

Fixed-wake analysis of the Darrieus rotor  

SciTech Connect

Development and validation of a Darrieus wind turbine aerodynamic performance prediction model is described. Using a fixed-wake approach, the model combines some of the more desirable features of vortex/lifting line and conservation of momentum/streamtube approaches. The model thus accounts for up- and downwind differences that are predicted by vortex approaches while retaining the short computer run times found with streamtube models. The model treats the effects of stall, curved blades, blade pitch, and blade attachment location. Results agree with those obtained with Sandia National Laboratories' 17-m-diameter Darrieus VAWT.

Wilson, R.E.; Walker, S.N.

1981-07-01

192

Vortex shedding from struts in an annular exhaust diffuser  

SciTech Connect

Results from scale-model experiments and industrial gas turbine tests show that strut vortex shedding in an annular exhaust diffuser can effectively be modified by adding tapered chord to the struts. The struts are bluff bodies at full-speed, no-load conditions, when inlet swirl is close to 60 deg. Data from wind tunnel tests show that wake Strouhal number is 0.47, larger than that expected for an isolated cylinder wake. This value of Strouhal number agrees with those measured in full-scale exhaust diffusers. Wind tunnel tests showed that a strut with tapered chord most effectively reduced wake amplitudes and shifted shedding frequency. The tapered strut was also effective in reducing shedding amplitude in a scale-model diffuser. Finally, gas turbine tests employing a tapered strut showed significant reductions in unsteady pressure and noise. A major benefit of strut taper is a reduction of noise by uncoupling of vortex shedding from acoustic resonant response.

Fric, T.F.; Villarreal, R.; Auer, R.O. [General Electric Co., Niskayuna, NY (United States). Corporate Research and Development; James, M.L.; Ozgur, D.; Staley, T.K. [General Electric Co., Schenectady, NY (United States)

1998-01-01

193

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.

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

2009-01-01

194

Evolving Structure of Tip-Vortex Generated by Helicopter Rotor Blade in a Hover  

NASA Astrophysics Data System (ADS)

In a hover the vortex trail forms a helical trace with respect to the vertical rotor hub axis, and the slip stream becomes contracted rather drastically within one revolution of rotor blade, making the angular momentum of the tip vortex grow quickly. And, as wake age grows, tip vortex experiences diffusion, distortion and stretching during its evolving process. Vortex diffusion proceeds continuously just after matured until disappeared, but distortion and stretching eventuates far downstream with both being correlated. At this stage the helical trace cannot be maintained. Two bladed rotor provides information about vortex distortion as well as diffusion within one revolution of both blades with simplicity. This system makes it possible to observe the change of vortex structure before and after 180 degrees of wake age within one revolution of blade. It is naturally expected that tip vortex affected by the second blade may experience the distortion including the vortex diffusion. This paper aimed primarily to investigate change of vortex structures without and with the second blade effect by the use of experimental devices. It was resulted that tip vortices generated by the first blade satisfy Landgrebe's model of their locations even after they were accelerated by the second blade in downstream. Swirl velocity components follow Vatistas' n=2 model on both regions without loss of vortex circulation.

Oun Han, Yong; Park, Byung Ho; Son, Yong Joon

2009-11-01

195

Wakes of Self-propelled Bodies in Stratified Fluids  

NASA Astrophysics Data System (ADS)

Using high Reynolds number (Re=10^4-10^5) experiments, the dynamics of stratified momentum wakes of self-propelled underwater and surface bodies were studied in (i) deep linearly stratified (deep ocean pycnocline), (ii) two layer (shallow pycnocline), and (iii) surface stratified (turbocline) fluids, and theoretical models wee advanced to explain the flow behavior. These models: (i) predict conditions under which submerged wakes signatures penetrate to the water surface, as expressed by the Confinement and Contrast numbers, and (ii) describe IR (infra-red) surface wakes signatures, as expressed by the Contrast and modified Froude numbers. If decaying turbulence is present surrounding the wake, the penetration of wake signature to the surface is still possible. Estimates for typical oceanic cases are given. PIV, LIF and high sensitivity Infrared Imaging cameras were employed for flow diagnostics.

Voropayev, S. I.; Fernando, H. J. S.

2008-11-01

196

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

2013-10-02

197

Impact of wake on downstream adjacent rotor in low-speed axial compressor  

Microsoft Academic Search

This paper investigates the effect of upstream wake on unsteady separated flow field of downstream adjacent vanes by solving\\u000a 2D unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme. The results indicated that the maximum\\u000a relative reduction of loss coefficient was 27.2% when the relative passing frequency (the ratio of wake passing frequency\\u000a to the characteristic frequency of trailing-vortex shedding

Xinqian Zheng; Sheng Zhou

2004-01-01

198

Experimental estimation of a D-shaped cylinder wake using body-mounted sensors  

Microsoft Academic Search

The effectiveness of a small array of body-mounted sensors, for estimation and eventually feedback flow control of a D-shaped\\u000a cylinder wake is investigated experimentally. The research is aimed at suppressing unsteady loads resulting from the von-Kármán\\u000a vortex shedding in the wake of bluff-bodies at a Reynolds number range of 100–1,000. A low-dimensional proper orthogonal decomposition\\u000a (POD) procedure was applied to

Oksana Stalnov; Vitali Palei; Ilan Fono; Kelly Cohen; Avi Seifert

2007-01-01

199

A new unstable mode in the wake of a circular cylinder  

NASA Astrophysics Data System (ADS)

The flow past a circular cylinder looses stability at Re ~ 47, via the primary wake (PW) mode. Linear stability analysis of the steady base flow, in two dimensions, is conducted using a stabilized finite element formulation. A new mode, referred to as the secondary wake (SW) mode, is discovered which is found to be unstable for Re >= 110.8. The relative roles of the PW and SW mode in the development of Karman vortex shedding are also investigated.

Verma, Abhishek; Mittal, Sanjay

2011-12-01

200

Self-excited oscillations in the wake of two-dimensional bluff bodies and their control  

Microsoft Academic Search

The onset of Karman-vortex shedding is studied experimentally in the wake of different two-dimensional bluff bodies, namely an oblong cylinder, circular cylinders, and plates of rectangular cross section. Different control measures, such as wake heating, transverse body oscillations, and base bleed are investigated. As the steady-periodic Karman shedding has previously been identified as a limit-cycle, i.e. as self-excited oscillations, the

Michael Schumm; Eberhard Berger; Peter A. Monkewitz

1994-01-01

201

Experimental study of low precessing frequencies in the wake of a turbulent annular jet  

Microsoft Academic Search

This paper investigates the flow structure in the wake behind the centrebody of an annular jet using time-resolved stereoscopic\\u000a PIV measurements. Although the time-averaged flow field is symmetric, the instantaneous wake is asymmetric. It consists of\\u000a a central toroidal vortex (CTV), which closes downstream at the stagnation point. This stagnation point lies off-axis and\\u000a hence the axis of the CTV

Maarten Vanierschot; Eric Van den Bulck

2011-01-01

202

An experimental and numerical study of 2D cylinder wakes  

Microsoft Academic Search

We study the flow structure in two-dimensional cylinder wakes using experiment (soap film flow) and numerical simulations (finite-element and vorticity methods). We investigate the range of Reynolds numbers Re from 50 to 1000. In this range, we measure the Strouhal number St (vortex-shedding frequency). Unlike the three-dimensional case, where the Re -- St graph has discontinuities due to spanwise instabilities,

Peter Vorobieff; Daniel Georgiev; Marc Ingber; Erin Rericha; Robert Ecke

2000-01-01

203

Development of a coupled fluid\\/structure aeroelastic solver with applications to vortex breakdown-induced twin tail buffeting  

Microsoft Academic Search

Simulation of tail buffet is studied for several delta wing-vertical tail configurations. Flow conditions are chosen such that the wing primary-vortex cores experience vortex breakdown and the resulting turbulent wake flow impinges on the vertical tail. The dimensions and material properties of the vertical tails are chosen such that the deflections are large enough to insure interaction with the flow,

Steven James Massey

1997-01-01

204

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

SciTech Connect

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

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

1998-11-13

205

The ultra-low Reynolds number airfoil wake  

NASA Astrophysics Data System (ADS)

Lift force and the near wake of an NACA 0012 airfoil were measured over the angle (?) of attack of 0°-90° and the chord Reynolds number ( Re c ), 5.3 × 103-5.1 × 104, with a view to understand thoroughly the near wake of the airfoil at low- to ultra-low Re c . While the lift force is measured using a load cell, the detailed flow structure is captured using laser-Doppler anemometry, particle image velocimetry, and laser-induced fluorescence flow visualization. It has been found that the stall of an airfoil, characterized by a drop in the lift force, occurs at Re c ? 1.05 × 104 but is absent at Re c = 5.3 × 103. The observation is connected to the presence of the separation bubble at high Re c but absence of the bubble at ultra-low Re c , as evidenced in our wake measurements. The near-wake characteristics are examined and discussed in detail, including the vortex formation length, wake width, spanwise vorticity, wake bubble size, wavelength of K-H vortices, Strouhal numbers, and their dependence on ? and Re c .

Alam, Md. Mahbub; Zhou, Y.; Yang, H. X.; Guo, H.; Mi, J.

2010-01-01

206

Experimental study of the instability of unequal-strength counter-rotating vortex pairs  

NASA Astrophysics Data System (ADS)

A rapidly growing instability is observed to develop between unequal-strength counter- rotating vortex pairs. The vortex pairs are generated in a towing tank in the wakes of wings with outboard triangular flaps. The vortices from the wing tip and the inboard tip of the flap form the counter-rotating vortex pair on each side of the wing. The flow fields are studied using flow visualization and particle image velocimetry. Both chord- based and circulation-based Reynolds numbers are of O(105). The circulation strength ratios of the flap- to tip-vortex pairs range from [minus sign]0.4 to [minus sign]0.7. The initial sinuous stage of the instability of the weaker flap vortex has a wavelength of order one wing span and becomes observable in about 15 wing spans downstream of the wing. The nearly straight vortex filaments first form loops around the stronger wing-tip vortices. The loops soon detach and form rings and move in the wake under self-induction. These vortex rings can move to the other side of the wake. The subsequent development of the instability makes the nearly quasi-steady and two-dimensional wakes unsteady and three-dimensional over a distance of 50 to 100 wing spans. A rectangular wing is also used to generate the classical wake vortex pair with the circulation ratio of [minus sign]1.0, which serves as a reference flow. This counter-rotating vortex pair, under similar experimental conditions, takes over 200 spans to develop visible deformations. Velocity, vorticity and enstrophy measurements in a fixed plane, in conjuction with the flow observations, are used to quantify the behaviour of the vortex pairs. The vortices in a pair initially orbit around their vorticity centroid, which takes the pair out of the path of the wing. Once the three-dimensional interactions develop, two-dimensional kinetic energy and enstrophy drop, and enstrophy dispersion radius increases sharply. This rapid transformation of the wake into a highly three-dimensional one offers a possible way of alleviating the hazard posed by the vortex wake of transport aircraft.

Ortega, J. M.; Bristol, R. L.; Savas, Ö.

2003-01-01

207

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

208

On the role of subharmonic perturbations in the far wake  

NASA Astrophysics Data System (ADS)

The principal considerations of the present numerical investigations of an excitation of the individual subharmonic perturbations in each of the shear layers forming the far wake allow for the existence of two equivalent subharmonic modes which can by opposite routes lead to a doubling of the wake's wavelength. Two-dimensional numerical simulations assuming incompressible flow and nearly inviscid dynamics show the opposite developments of regions dominated by the two different modes, and confirm the possibility of a resulting group structure. It is demonstrated that, while vortex-pairing plays an important role in the growth of the far-wake structure, it need not be related to the excitation of the subharmonic peak in the frequency spectrum.

Meiburg, E.

1987-04-01

209

Coherent structure in the turbulent wake behind a circular cylinder  

NASA Astrophysics Data System (ADS)

A wake behind a circular cylinder at Reynolds number 850-1700 was visualized by the smoke-wire method. The observations of the How together with the results of quantitative measurements, such as various velocity correlation coefficients, illustrated the formation process of spoon-shaped large eddies in the region 90 <= x/d <= 230 attained through the deformation and rearrangement of the regular Karman vortices. A spoon vortex was likely to pair with the counterpart on the opposite side of the wake. The large-scale bulges of the turbulent and non-turbulent interface of the wake were shown to correspond to these spoon vortices. These results indicate that some coherent structures are organized by rearrangement and deformation of initially regular vortices in turbulent flow. Translated from Nagare, Journal of Japan Society of Fluid Mechanics 3 (1984) 128-138

Shirakashi, Masataka; Yamaguchi, Shuichi; Mochimaru, Yoshihiro; Yamane, Ryuichiro

1988-07-01

210

Numerical investigation of wake structures of slow-flying bats  

NASA Astrophysics Data System (ADS)

Recently, some unique features of wake structure in bat flight have been revealed by experiments. It is found that the flow structure of bat flight is more complex than that of bird. A conceptual wake model of bat flight has been "rebuilt" using 2D DPIV images, but there is some risk of missing the details regarding dynamics of 3D vortex structures. Detailed flow information is still needed to understand the unsteady flow in bat flying. In this work, we perform 3D simulation of bat flying at the Reynolds number of 1000 (based on upstream flow and mean chord length) using the immersed boundary method. The geometry and wing-beat kinematics of bat are taken from the work of Watts et al (2001). The topology and evolution of the wake structures are described. The variation of topology in wake structures with the flapping Strouhal number is investigated. Moreover, the link between the generation of high lift and leading edge vortex is also studied.

Wang, Shizhao; Zhang, Xing; He, Guowei

2010-11-01

211

Waking with the hypothalamus  

Microsoft Academic Search

An essential component of the whole-body homoeostasis provided by the hypothalamus is the management of available energy.\\u000a This includes the regulation of sleeping and waking, feeding and drinking, body temperature and activity, as well as the endocrinium.\\u000a The waking brain, in particular the cerebral cortex, needs to be activated through neuronal pathways ascending from the brainstem\\u000a reticular formation (ascending reticular

Helmut L. Haas; Jian-Sheng Lin

212

Vortex Flow Aerodynamics, volume 1  

SciTech Connect

Vortex modeling techniques and experimental studies of research configurations utilizing vortex flows are discussed. Also discussed are vortex flap investigations using generic and airplane research models and vortex flap theoretical analysis and design studies.

Campbell, J.F.; Osborn, R.F.; Foughner, J.T. Jr.

1986-07-01

213

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

NASA Astrophysics Data System (ADS)

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

Lee, T.

2011-02-01

214

Generation of Vortex Dipoles in Superfluid Fermi Gas in BCS Limit  

NASA Astrophysics Data System (ADS)

We theoretically investigate the generation of the vortex dipoles in superfluid Fermi gas in the BCS limit. The vortex dipoles are generated in superfluid either by moving an obstacle above a critical speed or due to the decay of the shock waves obtained on the sudden mixing of two superfluid fragments. We observe that in pancake-shaped traps, the shock waves can lead to the formation of density ripples, which decay into vortex dipoles due to the onset of snake instability.

Gautam, S.

2013-04-01

215

Stability of parallel wake flows in quasigeostrophic and frontal regimes  

NASA Astrophysics Data System (ADS)

Recent laboratory experiments [G. Perret, A. Stegner, M. Farge, and T. Pichon, Phys. Fluids 18, 036603 (2006)] have shown that the vortex-street formed in the wake of a towed cylinder in a rotating shallow-water layer could present a strong cyclone-anticyclone asymmetry. In extreme cases, only large-scale anticyclones were observed in the far wake. This asymmetry occurs in the so-called frontal regime when the Rossby number is small and the surface deviation is large. This asymmetry may have various origins and in particular may be attributed to the asymmetry of the flow around the cylinder, to the linear stability property of the wake, or to its nonlinear evolution. To discriminate between these mechanisms, we study the stability of two idealized parallel flows in the quasigeostrophic and in the frontal regimes. These parallel flows correspond to two velocity profiles measured just behind the cylinder in a region where the perturbations are negligible. According to our linear stability analysis, the most unstable mode, in the frontal regime, is localized in the anticyclonic shear region whether the base flow profile is symmetric or not. On a linear basis, it is thus more the instability that imposes the asymmetry than the base flow. Direct numerical simulations of the synthetic parallel wake flows show that nonlinearity exacerbates the dominance of the anticyclonic mode linearly selected. By numerically studying the spatio-temporal evolution of a small perturbation localized in space, we show that, unlike incompressible two-dimensional wake flows and the symmetric wake in the quasigeostrophic regime, the parallel asymmetric wake is strongly convectively unstable in the frontal regime, and not absolutely unstable. When the surface deformation becomes large, the wake instability changes from the absolute instability in the quasi-geostrophic regime to the strongly convective instability of the frontal regime. This explains well the changes.

Perret, G.; Stegner, A.; Dubos, T.; Chomaz, J. M.; Farge, M.

2006-12-01

216

Near-field tip vortex behind a swept wing model  

NASA Astrophysics Data System (ADS)

The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared with a rectangular wing at the same lift force and Re=1.81×105. The tangential velocity decreased with the downstream distance while increased with the airfoil incidence. The core radius was about 3% of the root chord c r, regardless of the downstream distance and ? for ?<8°. The core axial velocity was always wake-like. The core ?c and total ?o circulation of the tip vortex remained nearly constant up to x/ c r=3.5 and had a ?c/?o ratio of 0.63. The total circulation of the tip vortex accounted for only about 40% of the bound root circulation ?b. For a rectangular wing, the axial flow exhibited islands of wake- and jet-like velocity distributions with ?c/?o=0.75 and ?o/?b=0.70. For the sweptback and tapered wing tested, the inner region of the tip vortex flow exhibited a self-similar behavior for x/ c r?1.0. The lift force computed from the spanwise circulation distributions agreed well with the force-balance data. A large difference in the lift-induced drag was, however, observed between the wake integral method and the inviscid lifting-line theory.

Gerontakos, P.; Lee, T.

2006-01-01

217

Wingtip Vortex Propeller.  

National Technical Information Service (NTIS)

A device which increases the energy efficiency and aerodynamic properties of aircraft was developed. A wingtip pusher propeller is positioned aft of the wingtip to rotate in the crossflow of the wingtip vortex. The propeller rotates against the vortex swi...

J. C. Patterson

1984-01-01

218

The Swirling Vortex  

Microsoft Academic Search

We consider an infinite vortex line in a viscous fluid interacting with a plane boundary surface at right angles to the line. If the boundary surface were absent, the vortex would impart to the fluid a circular motion about the vortex line with speed inversely proportional to the distance to the line. The presence of the boundary surface, however, leads

J. Serrin

1972-01-01

219

Turbulent Elliptic Wakes  

NASA Astrophysics Data System (ADS)

This paper describes results of an experimental study on turbulent wake of an elliptic disk set normal to the main flow, whose major diameter is 2.0 or 3.0 minor diameters, Reynolds number being 2.0x104on the basis of the minor diameter D. Two periodic components of velocity fluctuations were found in the wake. One is centred around the minor plane, being due to the alternate shedding of rolled-up, hairpin-like vortices. The other is centred around the major plane, which is likely to be due to a meandering motion of the wake. The axis switching, which is a cross-over of half-widths in the major and minor planes plotted against the streamwise distance, occurred at approximately 4.0 D downstream of the disk. The mechanism of the axis switching is different from that in elliptic jets, and it is proposed that it is due to a difference in the growth rate of the fundamental Fourier modes in the minor and major planes. The structure of the wake is studied by flow visualization and a survey of the time-mean velocity, turbulence intensities and Reynolds shear stresses. Wavelet analysis of the velocity fluctuations disclosed a low-frequency unsteadiness in the wake. This unsteadiness has different representative frequencies in the major and minor planes, being approximately one-fifth of the frequency of the corresponding periodic component in both planes.

Kiya, M.; Abe, Y.

1999-10-01

220

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.

221

Streamline bifurcations and scaling theory for a multiple-wake model  

Microsoft Academic Search

We investigate the interaction between multiple arrays of (reverse) von Kármán streets as a model for the mid-wake regions produced by schooling fish. There exist configurations where an infinite array of vortex streets is in relative equilibrium, that is, the streets move together with the same translational velocity. We examine the topology of the streamline patterns in a frame moving

Babak G. Oskouei; Eva Kanso; Paul K. Newton

2011-01-01

222

Numerical Simulation of Ultrafine Particle-Laden Cylinder Wake Flow with Coherent Structures  

Microsoft Academic Search

The phenomena of the wake flows containing ultrafine particles such as dust and pollutant in the atmosphere are usually observed downwind of the mountains and buildings. Different from the usually heavy particles, the ultrafine particles suspended in fluid undergo the processes of nucleation, growth and coagulation. Coherent structure in typical occurrence of the stretching of the Karman vortex street can

Changbin Wang; Shujie Zhi; Zhanhong Wan; Zhilin Sun; Hai Ding

2009-01-01

223

Dynamics of a vertical riser with weak structural nonlinearity excited by wakes  

Microsoft Academic Search

In this paper we investigate the effect of a weak structural nonlinearity on the dynamical behaviour of a vertical offshore riser subjected to vortex-induced vibration (VIV). Coupling of the riser dynamics with the flow of the surrounding fluid is achieved by attaching a wake oscillator to a reduced model of the structure, which is obtained through the application of the

Marko Keber; Marian Wiercigroch

2008-01-01

224

Interactions between nonsymmetrical wake structure and turbulent diffusion flames behind a rear-facing semicircular cylinder  

Microsoft Academic Search

Effects of the nonsymmetrical wake structure on turbulent diffusion flames, stabilized behind a rear-facing semicircular cylinder between two divided, vertically upward flowing airstreams, are experimentally investigated. Interactions between two vortex sheets in cold flows are also examined. Detailed optical observations and measurements of velocity and temperature are made on non flames, while varying the ratio of the lower airstream velocity,

Norio Ohiwa; Yojiro Ishino; Shigeki Yamaguchi

1994-01-01

225

Evidence of vortex jamming in Abrikosov vortex flux flow regime  

NASA Astrophysics Data System (ADS)

We report on dynamics of nonlocal Abrikosov vortex flow in mesoscopic superconducting Nb channels. Magnetic field dependence of the nonlocal voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly moderates vortex dynamics in mesoscopic geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with engineered pinning centers.

Karapetrov, G.; Yefremenko, V.; Mihajlovi?, G.; Pearson, J. E.; Iavarone, M.; Novosad, V.; Bader, S. D.

2012-08-01

226

Evidence of Vortex Jamming in Abrikosov Vortex Flux Flow Regime  

NASA Astrophysics Data System (ADS)

We report on dynamics of non-local Abrikosov vortex flow in mesoscopic superconducting Nb channels. Magnetic field dependence of the non-local voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly moderates vortex dynamics in mesoscopic geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with engineered pinning centers.

Karapetrov, Goran; Yefremenko, V.; Mihajlovic, G.; Pearson, J. E.; Iavarone, M.; Novosad, V.; Bader, S. D.

2012-02-01

227

The Dynamic Lunar Wake  

NASA Astrophysics Data System (ADS)

Bodies that lack a significant atmosphere and internal magnetic fields, such as the Moon, are obstacles to the solar wind. The solar wind ions and electrons directly impact the surface of the Moon due to the lack of atmosphere, and the interplanetary magnetic field passes through the obstacle relatively undisturbed. Since the solar wind is absorbed by the bodies, a wake is created behind the object. This wake is gradually filled by solar wind plasma downstream of the body, through thermal expansion and the resulting ambipolar electric field, along the magnetic field lines. Here we investigate the global Moon-solar wind interaction using a hybrid model (particle ions, fluid electrons). We focus in particular on the morphology of the wake region, and how it responds to changing solar wind conditions. The model predictions are compared to plasma observations by NASA's ARTEMIS spacecrafts.

Holmstrom, M.; Halekas, J. S.

2011-12-01

228

Vortex-induced noise and vibration in flow past several flat plates  

NASA Astrophysics Data System (ADS)

A model formulation for the problem of vortex impingement on a pair of elastic tandem cylinders is presented. The parametric relations among a variety of fluid dynamic and structural dynamic properties are illustrated. The noise field essentially responds at the cylinder response frequency and the sound pressure level is generally increased by the vibration of the cylinders. The interaction of an array of vortices is also considered and the results indicate that four vortices are required for the possibility of the chaotic motion and the broadband noise. The vortex shedding off an inclined flat plane is modelled using the discrete vortex method along with the Lamb vortex model. The model tested for the rolling-up of a vortex sheet behind the elliptically loaded wing demonstrates that the smooth rollup is achieved inside the core region of the vortex sheet. The subsequent application of the model to the vortex shedding problem shows that the computed results such as the kinematics of the wake development, the fluid loading and the Strouhal number are in fair agreement with previous experimental measurements. The noise field exhibits a broadband character with the peak occurring at the vortex shedding frequency. A numerical conformal mapping technique is developed to transform multiple flat plates into the same number of circular cylinders. Employing the multiple body mapping method, the flow past a series of flat plates is investigated. The calculated results indicate that the presence of a downstream body in the wake of another body produces a feedback effect upstream which, in turn, has a significant effect on the upstream flow. In the case of vortex shedding, the presence of a downstream splitter plate in the center of the wake of an inclined plate appears to suppress the regular, periodic vortex shedding process. The addition of more inclined plates appears to reduce the Strouhal frequency which is the frequency at which the noise field responds.

Kim, Chan Mun

229

Turbulent Wake Profiles of an Aeroelastic Wind Energy Converter  

NASA Astrophysics Data System (ADS)

Ahmadi (1979), used the principle of torsional aeroelastic instability and created the Aeroelastic Wind Energy Converter. This Converter is an H-section prism with a pendulum to increase the mass and lowering the natural frequency of the system. By placing a rectifier on the system a small amount of electrical power can be created to power remote equipment i.e., remote sensors. This device has the ability to create power at very low wind speeds when ordinary wind turbines are unproductive. By modulating the torsional frequency, the frequency of the vortex shedding and amplitude of the system can be changed to maximize the power output as the air speed changes. To improve the design, a Particle Image Velocimetry (PIV) system was used to study the turbulent wake profile of the converter. In the first case that was studied, the H- section’s steady wake profile was examined, i.e., no torsional vibration occured, but the angle of attack was changed from -30 deg. to 30 deg. at 5 deg intervals. In the second case, the H section’s unsteady wake profile was examined, i.e., a torsional vibration was produced with an angle of attack ranging from -30 deg. to 30 deg. A comparison between the turbulent wake profiles, at the same angle of attack, was made to understand the difference between theses cases. Also a comparison between the angle of attack and direction of travel was made to understand the unsteady wake better.

Schmit, Ryan; Glauser, Mark; Ahmadi, Goodarz

1999-11-01

230

Exploration of the vortex wake behind of wind turbine rotor  

Microsoft Academic Search

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

F Massouh; I Dobrev

2007-01-01

231

The Pressure Field of a Vortex Wake in Ground Effect.  

National Technical Information Service (NTIS)

The ground pressures under a descending pair of vortices, such as might be generated by aircraft either landing or taking off are computed. It is found that as the vortices first approach the ground, only positive pressures are produced; however as the de...

C. E. Brown

1975-01-01

232

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

233

Dual leading-edge vortex structure for flow over a simplified butterfly model  

Microsoft Academic Search

The dye visualization experiments show that a dual leading-edge vortex (LEV) structure exists on the suction side of a simplified\\u000a butterfly model of Papilio ulysses at ? = 8°?12°. Furthermore, the results of particle image velocimetry (PIV) measurement indicate that the axial velocity of the\\u000a primary (outer) vortex core reaches the lower extreme value while a transition from a “wake-like” to a

Y. Hu; J. J. Wang

2011-01-01

234

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

235

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

236

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

237

Vorticity Measurements in the Wake of an Inclined Prolate Spheroid  

NASA Astrophysics Data System (ADS)

The generation and evolution of axial vorticity in the wake of an inclined 6:1 prolate spheroid is studied experimentally, with comparison to Computational Fluid Dynamics (CFD) results. 2D Particle Image Velocimetry (PIV) measurements were obtained in planes normal to the flow at several stations along the body and at downstream distances up to one body length, at angles of attack of 5, 10, and 20 degrees and body Reynolds numbers (ReL=UL/?) of 13.7, 27.3, 45.6 x 10^4. As an extension of previous numerical and experimental studies on the vortex roll-up on the body of a 6:1 Prolate Spheroid [for example, Fu et al (1994), Tsai and Whitney (1999)], this study is focused on characterizing the downstream vorticity distribution as a function of the angle of attack and body Reynolds number. Long time average measurements of the circulation, core size, and core location are presented as a function of the angle of attack and the free stream velocity. In addition, measurements of turbulence characteristics of the wake are presented. Vortex migration velocities are found to be less than those estimated from inviscid vortex dipole theory. Experimental results for the 10-degree case are compared. Reynolds Average Navier-Stokes (RANS) CFD calculations show significant differences in the vorticity distribution near the stern, but with good agreement at one body length downstream.

Keller, Kurt; Brant, Alan; Kalumuck, Ken; Schemm, Charles; Scorpio, Steve

2009-11-01

238

Numerical studies on the propulsion and wake structures of finite-span flapping wings with different aspect ratios  

Microsoft Academic Search

An immersed-boundary method is used to investigate the flapping wings with different aspect ratios ranging from 1 to 5. The numerical results on wake structures and the performance of the propulsion are given. Unlike the case of the two-dimensional flapping foil, the wing-tip vortices appear for the flow past a three-dimensional flapping wing, which makes the wake vortex structures much

Xue-ming SHAO; Ding-yi PAN; Jian DENG; Zhao-sheng YU

2010-01-01

239

Ballistic Wake of Turbulence in a Plasma Shock Wave  

Microsoft Academic Search

The role of ballistic term in the turbulence that occurs in plasma shock waves is discussed. For electrostatic turbulence these terms are shown to give rise to a wake behind a shock wave in which the energy density in the fluctuating fields decays spatially as x?3 for a class of distribution functions including resonance functions. The importance of the ballistic

Nicholas A. Krall; Derek A. Tidman

1969-01-01

240

From vortex layers to vortex sheets  

NASA Astrophysics Data System (ADS)

This paper shows that the solution of the Birkhoff-Rott equation for the vortex sheet can be approximated, for short times, by the solutions of the Euler equation for a thin vortex layer of vorticity, when its thickness vanishes and its vorticity intensity diverges suitably. The result is obtained in an analytical setup, and an example seems to indicate that this is indeed necessary.

Benedetto, D.; Pulvirenti, M.

1992-08-01

241

Suppression of Kelvon-induced decay of quantized vortices in oblate Bose-Einstein condensates  

SciTech Connect

We study the Kelvin mode excitations on a vortex line in a three-dimensional trapped Bose-Einstein condensate at finite temperature. Our stochastic Gross-Pitaevskii simulations show that the activation of these modes can be suppressed by tightening the confinement along the direction of the vortex line, leading to a strong suppression in the vortex decay rate as the system enters a regime of two-dimensional vortex dynamics. As the system approaches the condensation transition temperature, we find that the vortex decay rate is strongly sensitive to dimensionality and temperature, observing a large enhancement for quasi-two-dimensional traps. Three-dimensional simulations of the recent vortex dipole decay experiment of Neely et al.[Phys. Rev. Lett. 104, 160401 (2010)] confirm two-dimensional vortex dynamics and predict a dipole lifetime consistent with experimental observations and suppression of Kelvon-induced vortex decay in highly oblate condensates.

Rooney, S. J.; Blakie, P. B.; Bradley, A. S. [Jack Dodd Center for Quantum Technology, Department of Physics, University of Otago, Dunedin (New Zealand); Anderson, B. P. [College of Optical Sciences, University of Arizona, Tucson, Arizona 85721 (United States)

2011-08-15

242

Brain Wake-Ups  

NSDL National Science Digital Library

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

1997-01-01

243

Wake field acceleration experiments  

SciTech Connect

Where and how will wake field acceleration devices find use for other than, possibly, accelerators for high energy physics. I don't know that this can be responsibly answered at this time. What I can do is describe some recent results from an ongoing experimental program at Argonne which support the idea that wake field techniques and devices are potentially important for future accelerators. Perhaps this will spawn expanded interest and even new ideas for the use of this new technology. The Argonne program, and in particular the Advanced Accelerator Test Facility (AATF), has been reported in several fairly recent papers and reports. But because this is a substantially new audience for the subject, I will include a brief review of the program and the facility before describing experiments. 10 refs., 7 figs.

Simpson, J.D.

1988-01-01

244

Fractional vortex in asymmetric 0-? long Josephson junctions  

NASA Astrophysics Data System (ADS)

We consider an infinitely long 0-? Josephson junction consisting of 0 and ? regions having different critical current densities jc,0 and jc,?. The ground state of such a junction corresponds to a spontaneously formed asymmetric semifluxon with tails decaying on different length scales. We calculate the depinning current of such a fractional vortex and show that it is different for positive and negative bias polarity. We also show that upon application of a bias current, the fractional flux (topological charge) associated with the vortex changes. We calculate the range of fractional flux associated with the vortex when the bias changes from negative to positive critical (depinning) values.

Goldobin, E.; Kleiner, R.; Koelle, D.

2013-06-01

245

The role of body stiffness in wake production for anguilliform swimmers  

NASA Astrophysics Data System (ADS)

We compare wake structures shed by the undulatory motion of physical and computational models of an anguilliform swimmer, the lamprey. The physical model is a robotic lamprey-like swimmer with an actively flexing tail, and with passively flexible tails of different stiffnesses. The computational model is a two-dimensional computational fluid dynamic (CFD) model that captures fluid-structure interaction using the immersed boundary framework. The CFD model included both actively flexing and passively flexible tail regions. Both models produced wakes with two or more same-sign vortices shed each time the tail changed direction (a ``2P'' or higher- order wake). In general, wakes became less coherent as tail flexibility increased. We compare the pressure distribution near the tail tip and the timing of vortex formation in both cases and find good agreement. Differences between self-propelled and tethered cases are detailed. Finally, we examine the effects of material resonance on force production.

Tytell, Eric; Leftwich, Megan; Hsu, Chia-Yu; Cohen, Aves; Fauci, Lisa; Smits, Alexander

2011-11-01

246

Impact of wake on downstream adjacent rotor in low-speed axial compressor  

NASA Astrophysics Data System (ADS)

This paper investigates the effect of upstream wake on unsteady separated flow field of downstream adjacent vanes by solving 2D unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme. The results indicated that the maximum relative reduction of loss coefficient was 27.2% when the relative passing frequency (the ratio of wake passing frequency to the characteristic frequency of trailing-vortex shedding of downstream adjacent rotor) is less than 2.5. The amplitude of wake defect exists a threshold value and the aerodynamic performance is enhanced monotonically with the amplitude of wake defect basically. The effective range of incidence must be greater than 8°, which is near the stall boundary.

Zheng, Xinqian; Zhou, Sheng

2004-05-01

247

Wing Kinematics and Wake Velocity Characteristics of Bat Flight  

NASA Astrophysics Data System (ADS)

Bats demonstrate unequalled flight characteristics and are capable of highly efficient flight as well as extreme maneuverability at high speeds. They have morphological properties that are unique in the animal world including jointed wings skeletons, elastic wing membranes and very complex wing motions. We report on a series of experiments on bats flying in a flight cage along both a straight path and through a 90-degree turn. Measurements of their kinematic wing motion (using high speed photography) and wake velocity structures (using stereo PIV) are reported. The live animal measurements are also interpreted with the help of a series of companion wind tunnel experiments using model structures that mimic some key features of bat flight mechanics. The results reveal a complex vortex wake structure which is compared and contrasted to that found in bird and insect flight.

Swartz, Sharon

2005-11-01

248

Simultaneous Measurement of Velocity and Pressure in a Wing-Tip Vortex  

NASA Astrophysics Data System (ADS)

Simultaneous measurements of fluctuating velocity and pressure have been conducted in the near field of a wing-tip vortex trailing from a NACA0012 half wing. In the vortex core, the time-averaged streamwise velocity shows a wake-like profile which exhibits a deficit by approximately 25% of free stream velocity. The meandering of the vortex is evident from both velocity and pressure measurements: The power spectral density (PSD) of the transverse velocity fluctuation increases in the lower frequency range, and the distribution of the pressure fluctuation exhibits an elongated shape in the z direction as well as two peaks slightly off from the vortex center. The distribution of the measured velocity-pressure correlation varies significantly near the center of the vortex especially in downstream locations.

Naka, Yoshitsugu; Azegami, Suguru; Kawata, Takuya; Fukagata, Koji; Obi, Shinnosuke

249

A wake-based correlate of swimming performance in seven jellyfish species  

NASA Astrophysics Data System (ADS)

Animal-fluid interactions have been hypothesized as a principal selective pressure on the evolution of aquatic and aerial animals. However, attempts to discover the fluid dynamic mechanisms that dictate the fitness of an animal---or even to quantify `fitness'---have been limited by an inability to measure the fluid interactions of freely moving animals (i.e., in the absence of tethers or artificial water/wind currents) in comparative studies of multiple species with similar evolutionary histories. We used digital particle image velocimetry (DPIV) measurements to calculate wake kinetic energy, drag, and swimming speed of the seven co-occurring species of free-swimming jellyfish. Using this new data, we demonstrate that the swimming and foraging behavior are related to a robust fluid dynamic threshold between two distinct configurations of the wake vortices. The transition between the two wake vortex configurations is known as optimal vortex formation, because it maximizes the fluid dynamic thrust generated for a given energy input (Krueger and Gharib, Phys. Fluids 2003). By comparing the observed wake structures created by each jellyfish species with the optimal vortex configuration, we are able to predict their relative swimming efficiencies and proficiencies and to deduce their corresponding ecological niches.

Dabiri, John; Colin, Sean; Katija, Kakani; Costello, John

2009-11-01

250

The role of blade elasticity in the prediction of blade-vortex interaction noise  

Microsoft Academic Search

An analytical study of the role of a main rotor blade's flap, chord and torsional stiffnesses on vibratory airloads and sound pressures has been carried out. A rotor analysis code typically applied to blade dynamics and performance was modified to capture the airload due to blade-vortex and blade-wake interaction by using a finer azimuthal computation grid. The blade elasticity of

Robert C. Derham; Byung K. Oh

1991-01-01

251

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

252

Improved vortex reactor system  

DOEpatents

An improved vortex reactor system for affecting fast pyrolysis of biomass and Refuse Derived Fuel (RDF) feed materials comprising: a vortex reactor having its axis vertically disposed in relation to a jet of a horizontally disposed steam ejector that impels feed materials from a feeder and solids from a recycle loop along with a motive gas into a top part of said reactor.

Diebold, James P. (Lakewood, CO); Scahill, John W. (Evergreen, CO)

1995-01-01

253

Vortex diode jet  

DOEpatents

A fluid transfer system that combines a vortex diode with a jet ejector to transfer liquid from one tank to a second tank by a gas pressurization method having no moving mechanical parts in the fluid system. The vortex diode is a device that has a high resistance to flow in one direction and a low resistance to flow in the other.

Houck, Edward D. (Idaho Falls, ID)

1994-01-01

254

Vortex diode jet.  

National Technical Information Service (NTIS)

A fluid transfer system that combines a vortex diode with a jet ejector to transfer liquid from one tank to a second tank by a gas pressurization method having no moving mechanical parts in the fluid system. The vortex diode is a device that has a high re...

E. D. Houck

1993-01-01

255

Kelvin-Tkachenko waves of few-vortex arrays in trapped Bose-Einstein condensates  

SciTech Connect

We have calculated the low-lying elementary excitations of three-dimensional few-vortex arrays in trapped Bose-Einstein condensates. The number of different Kelvin-Tkachenko vortex wave branches found matches the number of vortices in the condensate. The lowest odd-parity modes exhibit superfluid gyroscopic vortex motion. Experimentally, these modes could be excited and observed individually or in connection with the formation and decay of quantum turbulence.

Simula, T. P. [School of Physics, Monash University, Victoria 3800 (Australia); Department of Physics, Okayama University, Okayama 700-8530 (Japan); Machida, K. [Department of Physics, Okayama University, Okayama 700-8530 (Japan)

2010-12-15

256

Directional Landau damping of wake-field potentials  

SciTech Connect

The wake-field potentials produced by charged particles moving in a plasma are investigated in the presence of finite Landau-damping effects in the wave dispersion relation. In the frame of reference moving with the particle, the phase velocities of the spectrum of waves excited by the moving charge depend on the angle of propagation of the wave vector giving rise to a directional wave-particle resonance condition. As a consequence, the wake structure is seen to decay both along and transverse to the direction of motion of the charged particle.

Bose, Anirban; Janaki, M.S. [Saha Institute of Nuclear Physics 1/AF Bidhannagar, Kolkata 700 064 (India)

2005-10-01

257

On the effects of microbubbles on Taylor Green vortex flow  

NASA Astrophysics Data System (ADS)

The paper describes a numerical study of the effects of microbubbles on the vorticity dynamics in a Taylor Green vortex flow (TGV) using the two-fluid approach. The results show that bubbles with a volume fraction {˜}10(-2) enhance the decay rate of the vorticity at the centre of the vortex. Analysis of the vorticity equation of the bubble-laden flow shows that the local positive velocity divergence of the fluid velocity, {bm nabla} {bm *} {bm U}, created in the vortex core by bubble clustering, is responsible for the vorticity decay. At the centre of the vortex, the vorticity omega_c(t) decreases nearly linearly with the bubble concentration C_m(t). Similarly, the enstrophy in the core of the vortex, omega(2(t)) , decays nearly linearly with C(2(t)) . The approximate mean-enstrophy equation shows that bubble accumulation in the high-enstrophy core regions produces a positive correlation between omega(2) and {bm nabla} {bm *} {bm U}, which enhances the decay rate of the mean enstrophy.

Ferrante, Antonino; Elghobashi, Said E.

258

Effect of forcing on the vorticity field in a confined wake  

NASA Astrophysics Data System (ADS)

Several recent studies have found that when a low Reynolds number, plane wake is forced with sufficient amplitude, the normalized mixing product, measured as the amount of mixed fluid per unit width of the wake, can be increased to levels larger than those seen in high Reynolds number mixing layers. However, no studies examining the velocity and vorticity fields of this flow have been conducted. The present study examines the velocity and vorticity field of a low Reynolds number plane wake within a confining channel in order to better understand the vortex-vortex and vortex-wall interactions in order to shed light on the mechanisms which lead to increases in the amount of mixed fluid within the wake. Molecular Tagging Velocimetry (MTV) is used to measure the velocity field in both the streamwise (u, v velocities in x, y plane) and cross-stream (v, w velocities in y, z plane) measurement planes. The spanwise and streamwise vorticity components are then computed from their respective velocity fields. Measurements in the streamwise plane have found that a distinct spatial periodicity exists in the urms field that is not found in either the unforced case or in unconfined forced flows. A model was developed which relates this spatial periodicity to the phase difference between the forcing input and the rolling up of the vorticity shed from the splitter plate. From these data, it was also determined that the phase at which vorticity is shed is dependent upon the forcing amplitude. The forced wake flow is dominated by the shedding of concentrated, spanwise vortex core rollers. As these cores develop downstream, the levels of peak vorticity within the core decrease. A very small amount of -6w/6z is sufficient to generate a very large decrease in peak vorticity levels. This same quantity has also been found to be a good predictor of the spatial location where mixing enhancement will occur in the forced wake. Mixing enhancement is accomplished by the generation of regions of streamwise vorticity from the reorientation of the primary spanwise vortex cores. A model was developed which describes how these cores develop. The multiple regions of streamwise vorticity are the result of the passage and reorientation of multiple spanwise rollers. These reoriented "legs" of streamwise vorticity interact with the regions of streamwise vorticity resulting from the passage of previous spanwise vortex rollers to generate the additional surface area necessary for mixing enhancement. (Abstract shortened by UMI.)

Cohn, Richard Keith

1999-11-01

259

Dynamics and wake patterns of freely rising and falling spheres at Re = 500  

NASA Astrophysics Data System (ADS)

We study the dynamics of spheres rising or falling freely at Re = 500. All falling spheres, whose mass ratio (or relative density) m*, exceeds 1, descended rectilinearly. For rising spheres, there exists a critical value of the mass ratio below which the sphere undergoes large-amplitude oscillations, m*crit = 0.36. This motion occurs in a vertical plane; no helical trajectories are observed. Initial wake visualizations showed that rather than the two alternately signed vortex loops found in the flow past stationary spheres, the wake of a vibrating freely rising sphere comprised four vortex structures per cycle of oscillation. However, due to the small size and high oscillation frequency of the freely rising spheres, the exact nature and formation of these structures remained unclear. Further studies were performed in a towing tank, prescribing the motion of the sphere based on the measured displacement of the rising spheres. We are able to use much larger, slower-moving bodies while matching the Reynolds numbers of the rising spheres. These experiments result in the same vortex pattern, and reveal that the four structures found in the wake of the rising sphere are vortex rings. What previously appeared to be unusually sharp bends in the counter-rotating vortex pairs are very weak loop-shaped structures, delivering a total of six vortical structures per cycle. Immediately preceding these structures, the two vortices in the pair cross over one another, providing a mechanism for the change in sign of the streamwise vortex pair as the body moves from one half cycle to the next.

Horowitz, M.; Williamson, C. H. K.

2007-11-01

260

Global mode of a sphere turbulent wake controlled by a small sphere  

NASA Astrophysics Data System (ADS)

A small control sphere is displaced in a cross flow plane downstream of a main sphere of diameter D in a uniform flow at Re=33 000. The wake is studied by means of four fixed hot wire probes. It is shown that without the control sphere (reference case), vortex loops are shed from only one side of the sphere at a Strouhal number St=0.19. This reference wake structure has a planar symmetry defining an azimuthal phase ?W that is observed to be random in time. The secondary smaller sphere is treated as a local disturbance of the reference wake. It is found that the shedding occurs closer to the symmetry axis for the control sphere placed at the center of the wake than for the natural case. When the control sphere is off-centered, a subharmonic at half the natural frequency appears and the azimuthal phase becomes imposed by the control sphere position. A pure subharmonic mode is observed when the control sphere reaches the separated shear, suggesting alternative vortex loops shedding from both sides of the wake.

Vilaplana, G.; Grandemange, M.; Gohlke, M.; Cadot, O.

2013-08-01

261

Experimental investigation of the influence of inlet conditions to a bluff body wake  

NASA Astrophysics Data System (ADS)

Wind tunnel experiments have been performed in a bluff body wake with varying inlet conditions in order to enhance the physical understanding of the wake flow instability, which may lead to successful flow control and in turn reduced aerodynamic drag. The geometry consists of a rectangular-based forebody with permeable surfaces, an elliptic leading edge and a blunt trailing edge. Length, width and base height of the forebody is 2.3, 0.5 and 0.04 meters, respectively. Applying continuous suction or blowing, of different levels, through the permeable surfaces along the forebody, varies the wall-normal trailing edge velocity profile in a systematic way and hence the inlet condition to the wake. The streamwise velocity component has been measured both throughout the boundary layer and in the wake behind the body using hot-wire anemometry. High-speed stereo PIV has been used in the wake in order to collect statistics of vortical structures in the wake. The influence of boundary layer parameters on the wake flow characteristics, such as vortex shedding frequency and base pressure, will be presented.

Fallenius, Bengt; Fransson, Jens

2010-11-01

262

Solutions of the Taylor-Green Vortex Problem Using High-Resolution Explicit Finite Difference Methods.  

National Technical Information Service (NTIS)

A computational fluid dynamics code that solves the compressible Navier-Stokes equations was applied to the Taylor-Green vortex problem to examine the code s ability to accurately simulate the vortex decay and subsequent turbulence. The code, WRLES (Wave ...

J. R. DeBonis

2013-01-01

263

Measurement of pressure fluctuation in gas-liquid two-phase vortex street  

NASA Astrophysics Data System (ADS)

The pressure fluctuation in the wake is an important parameter to characterize the shedding process of gas-liquid two-phase Karman vortex street. This paper investigated such pressure fluctuations in a horizontal pipe using air and water as the tested fluid media. The dynamic signal representing the pressure fluctuation was acquired by the duct-wall differential pressure method. Results show that in the wake of the gas-liquid two-phase Karman vortex street, the frequency of the pressure fluctuation is linear with the Reynolds number when the volume void fraction is within the range of 18%. Moreover, the mean amplitude of the pressure fluctuation decreases with the volume void fraction, and the mean amplitude is larger at higher water flowrates under the same volume void fraction. These findings contribute to an in-depth understanding of the gas-liquid two-phase Karman vortex street.

Sun, Zhiqiang; Sang, Wenhui; Zhang, Hongjian

2009-02-01

264

Moving magnetic tubes: fragmentation, vortex streets and the limit of the approximation of thin flux tubes  

NASA Astrophysics Data System (ADS)

Aims.We study the buoyant rise of magnetic flux tubes in a stratified layer over a range of Reynolds numbers (25 ? Re ? 2600) by means of numerical simulations. Special emphasis is placed on studying the fragmentation of the rising tube, its trailing wake and the formation of a vortex street in the high-Reynolds number regime. Furthermore, we evaluate the relevance of the thin flux tube approximation with regard to describing the evolution of magnetic flux tubes in the simulations. Methods: .We used the FLASH code, which has an adaptive mesh refinement (AMR) algorithm, thus allowing the simulations to be carried out at high Reynolds numbers. Results: .The evolution of the magnetic flux tube and its wake depends on the Reynolds number. At Re up to a few hundred, the wake consists of two counter-rotating vortex rolls. At higher Re, the vortex rolls break up and the shedding of flux into the wake occurs in a more intermittent fashion. The amount of flux retained by the central portion of the tube increases with the field line twist (in agreement with previous literature) and with Re. The time evolution of the twist is compatible with a homologous expansion of the tube. The motion of the central portion of the tube in the simulations is very well described by the thin flux tube model whenever the effects of flux loss or vortex forces can be neglected. If the flux tube has an initial net vorticity, it undergoes asymmetric vortex shedding. In this case, the lift force accelerates the tube in such a way that an oscillatory horizontal motion is super-imposed on the vertical rise of the tube, which leaves behind a vortex street. This last result is in accordance with previous simulations reported in the literature, which were carried out at lower Reynolds number.

Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M.

2006-05-01

265

Vortex crystals in fluids  

NASA Astrophysics Data System (ADS)

It is common in geophysical flows to observe localized regions of enhanced vorticity. This observation can be used to derive model equations to describe the motion and interaction of these localized regions, or vortices, and which are simpler than the original PDEs. The best known vortex model is derived from the incompressible Euler equations, and treats vortices as points in the plane. A large part of this dissertation utilizes this particular model, but we also survey other point vortex and weakly viscous models. The main focus of this thesis is an object known as the vortex crystal. These remarkable configurations of vortices maintain their basic shapes for long times, while perhaps rotating or translating rigidly in space. We study existence and stability of families of vortex crystals in the special case where N vortices have small and equal circulation and one vortex has large circulation. As the small circulation tends to zero, the weak vortices tend to a circle centered on the strong vortex. A special potential function of this limiting problem can be used to characterize orbits and stability. Whenever a critical point of this function is nondegenerate, we prove that the orbit can be continued via the Implicit Function Theorem, and its linear stability is determined by the eigenvalues of the Hessian matrix of the potential. For general N, we find at least three distinct families of critical points, one of which continues to a linearly stable class of vortex crystals. Because the stable family is most likely to be observed in nature, we study it extensively. Continuation methods allow us to follow these critical points to nonzero weak vortex strength and investigate stability and bifurcations. In the large N limit of this family, we prove that there is a unique one parameter family of distributions which minimize a "generalized" potential. Finally, we use point vortex and weakly viscous vortex models to analyze vortex crystal configurations observed in hurricane eyes and related numerical simulations. We find striking numerical and analytical agreement, thus validating the use of simplified vortex models to describe geophysical phenomena.

Barry, Anna M.

266

Influence of separated vortex on aerodynamic noise of an airfoil blade  

NASA Astrophysics Data System (ADS)

In order to clarify the mechanism by which aerodynamic noise is generated from separated flow around an airfoil blade, the relation between the attack angle and the aerodynamic noise of the blade was analyzed using a wind tunnel experiment and a CFD code. In the case of rear surface separation, the separated vortex which has a large-scale structure in the direction of the blade chord is transformed into a structure that concentrates at the trailing edge with an increase in the attack angle. The aerodynamic noise level then becomes small according to the vortex scale in the blade chord. When the flow is separated at the leading edge, a separated vortex of low pressure is formed at the vicinity of the trailing edge. The pressure fluctuations on the blade surface at the vicinity of the trailing edge become large due to the vortex in the wake. It is considered that the aerodynamic noise level increases when the flow is separated at the leading edge because the separated vortex is causing the fluctuations due to wake vortex shedding.

Sasaki, Soichi; Takamatsu, Hajime; Tsujino, Masao; Tsubota, Haruhiro; Hayashi, Hidechito

2010-02-01

267

Point vortex model for prediction of sound generated by a wing with flap interacting with a passing vortex.  

PubMed

Acoustic signature of a rigid wing, equipped with a movable downstream flap and interacting with a line vortex, is studied in a two-dimensional low-Mach number flow. The flap is attached to the airfoil via a torsion spring, and the coupled fluid-structure interaction problem is analyzed using thin-airfoil methodology and application of the emended Brown and Michael equation. It is found that incident vortex passage above the airfoil excites flap motion at the system natural frequency, amplified above all other frequencies contained in the forcing vortex. Far-field radiation is analyzed using Powell-Howe analogy, yielding the leading order dipole-type signature of the system. It is shown that direct flap motion has a negligible effect on total sound radiation. The characteristic acoustic signature of the system is dominated by vortex sound, consisting of relatively strong leading and trailing edge interactions of the airfoil with the incident vortex, together with late-time wake sound resulting from induced flap motion. In comparison with the counterpart rigid (non-flapped) configuration, it is found that the flap may act as sound amplifier or absorber, depending on the value of flap-fluid natural frequency. The study complements existing analyses examining sound radiation in static- and detached-flap configurations. PMID:23556563

Manela, A; Huang, L

2013-04-01

268

Optical vortex coronagraph.  

PubMed

We describe a method to observe dim exoplanets that eliminates light from the parent star across the entire exit pupil without sacrificing light from the planet by use of a vortex mask of topological charge m = 2. PMID:16389814

Foo, Gregory; Palacios, David M; Swartzlander, Grover A

2005-12-15

269

Wave–Vortex Interactions  

Microsoft Academic Search

\\u000a This chapter presents a theoretical investigation of wave–vortex interactions in fluid systems of interest to atmosphere and\\u000a ocean dynamics. The focus is on strong interactions in the sense that the induced changes in the vortical flow should be significant. In essence, such strong wave–vortex\\u000a interactions require significant changes in the potential vorticity (PV) of the flow either by advection of

O. Bühler

2010-01-01

270

Improved vortex reactor system  

DOEpatents

An improved vortex reactor system is described for affecting fast pyrolysis of biomass and Refuse Derived Fuel (RDF) feed materials comprising: a vortex reactor having its axis vertically disposed in relation to a jet of a horizontally disposed steam ejector that impels feed materials from a feeder and solids from a recycle loop along with a motive gas into a top part of said reactor. 12 figs.

Diebold, J.P.; Scahill, J.W.

1995-05-09

271

Vortex diode jet  

SciTech Connect

A fluid transfer system is described that combines a vortex diode with a jet ejector to transfer liquid from one tank to a second tank by a gas pressurization method having no moving mechanical parts in the fluid system. The vortex diode is a device that has a high resistance to flow in one direction and a low resistance to flow in the other. 10 figures.

Houck, E.D.

1994-05-17

272

On the effects of free-stream turbulence on axisymmetric disc wakes  

NASA Astrophysics Data System (ADS)

Wind tunnel experiments have been used to study the effects of free-stream turbulence on the axisymmetric wake behind a disc. The disc and its wake were introduced to various turbulent streams having various levels of turbulence intensity and length scale. It was found that the presence of free-stream turbulence enhances the body's drag and hence wake momentum deficit, if it is of sufficient strength, changes the far wake's decay rate and prevents the appearance of self-similarity. The external turbulence causes a significant transformation in the wake's turbulence structure. This gradually evolves towards the character of the free-stream turbulence itself and thus is characterised by much weaker turbulence (cross-stream) transport processes and a consequent dominance of shear stress production, which acts to maintain the shear stress and mean velocity profiles.

Rind, Elad; Castro, Ian P.

2012-08-01

273

Velocity perturbations induced by the longitudinal vortices in a cylinder wake  

Microsoft Academic Search

This paper presents data showing the three-dimensional vortical structures in the near wake region of circular cylinders. The in-plane velocity field was measured using a digital Particle Image Velocimetry (PIV) technique. The vortical structures are found to include inclined counter-rotating longitudinal vortices in the braids joining consecutive Karman vortices. A simple vortex-pair model is proposed to estimate velocity perturbation induced

J. Wu; M. C. Welsh; J. Sheridan

1996-01-01

274

Collisionless Damping of Laser Wakes in Plasma Channels  

SciTech Connect

Excitation of accelerating modes in transversely inhomogeneous plasma channels is considered as an initial value problem. Discrete eigenmodes are supported by plasma channels with sharp density gradients. These eigenmodes are collisionlessly damped as the gradients are smoothed. Using collisionless Landau damping as the analogy, the existence and damping of these "quasi-modes" is studied by constructing and analytically continuing the causal Green's function of wake excitation into the lower half of the complex frequency plane. Electromagnetic nature of the plasma wakes in the channel makes their excitation nonlocal. This results in the algebraic decay of the fields with time due to phase-mixing of plasma oscillations with spatially-varying fequencies. Characteristic decay rate is given by the mixing time, which corresponds to the dephasing of two plasma fluid elements separated by the collisionless skin depth. For wide channels the exact expressions for the field evolution are derived. Implications for electron acceleration in plasma channels are discussed.

Li, X.; Shvets, G.

1998-08-01

275

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

276

Wake survey techniques for objects with highly turbulent wakes  

NASA Astrophysics Data System (ADS)

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, a theoretical analysis of turbulence effects on pressure measurements, and an analysis of wake drag equations. The experimental investigation was performed in the Illinois 3- by 4-foot low-speed wind tunnel. In the test, the wake of a 1-inch diameter cylinder and two airfoils, an S809 and NACA 0012, with and without various ice simulations were surveyed in detail using several Pitot-static probes with different nose shapes and an X-hotwire. The cylinder results were used to validate the wake survey techniques. The drag of the airfoils with and without ice accretions was determined using the validated wake survey techniques. A theoretical analysis of the turbulence effect on total and static pressure measurements was presented and compared with experimental data. Methods for correcting the turbulence effect on pressure measurements were provided, and a technique for estimating the turbulence kinetic pressure using the uncorrected pressure measurements was developed. Turbulence was also found to play an important role in drag determination through Reynolds stresses and static pressure deficit in the wake. A new wake drag equation was derived to include the turbulence effects. It was found the turbulence contribution to profile drag was over 17% in the cylinder test, and over 10% in the test of airfoils with ice accretions. This dissertation for the first time analyzed the turbulence effect on the simplified Jones' equation with measurements using a Pitot probe, and found that this method includes a portion of the turbulence effect into account implicitly, depending on the nose shape of the probe. A method for estimating this implicit correction was developed. Finally, wake survey techniques for determining the drag of objects with highly turbulent wakes were recommended.

Lu, Biao

277

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

278

Simulation of spray dispersion in a simplified heavy vehicle wake  

SciTech Connect

Simulations of spray dispersion in a simplified tractor-trailer wake have been completed with the goal of obtaining a better understanding of how to mitigate this safety hazard. The Generic Conventional Model (GCM) for the tractor-trailer was used. The impact of aerodynamic drag reduction devices, specifically trailer-mounted base flaps, on the transport of spray in the vehicle wake was considered using the GCM. This analysis demonstrated that base flaps including a bottom plate may actually worsen motorist visibility because of the interaction of fine spray with large vortex flows in the wake. This work suggests that to use computational fluid dynamics (CFD) to design and evaluate spray mitigation strategies the jet or sheet breakup processes can be modeled using an array of injectors of small (< 0.1 mm) water droplets; however the choice of size distribution, injection locations, directions and velocities is largely unknown and requires further study. Possible containment strategies would include using flow structures to 'focus' particles into regions away from passing cars or surface treatments to capture small drops.

Paschkewitz, J S

2006-01-13

279

The Energy Distribution Mechanisms of the Near Wakes of Planetary Entry Probes  

NASA Astrophysics Data System (ADS)

A CFD aided theoretical analysis is reported of the energy exchange and conversion processes occurring in the near wakes of bluff bodies in hypersonic flight. The study proceeds by first selecting a point on the Mars atmospheric entry trajectory of the Beagle II spacecraft as the datum case. The freestream values of the system ? groups are then varied in a systematic fashion and the flowfield is recalculated in order to discover the underlying dependence on ? groups of the two phenomena of particular interest. The first of these is the presence an aft facing shock in the reverse flow ahead of the aft stagnation point on the body. The second is a newly identified phenomenon of wake flow thermal inversion in which total temperatures in the near wake flow are elevated above those of the freestream by strong viscous coupling of the external flow driving the wake vortex coupled with poor heat transfer out of the wake. Cyclic heating and cooling behavior is examined for closed streamlines in the wake as further evidence of the energy exchange origins of the thermal inversion observed in the computed flows.

Balage, S.; Boyce, R.; Mudford, N.; O'Byrne, S.

2009-01-01

280

Singularity formation and nonlinear evolution of a viscous vortex sheet model  

NASA Astrophysics Data System (ADS)

We study Dhanak's model [J. Fluid Mech. 269, 265 (1994)] of a viscous vortex sheet in the sharp limit, to investigate singularity formations and present nonlinear evolutions of the sheets. The finite-time singularity does not disappear by giving viscosity to the vortex sheet, but is delayed. The singularity in the sharp viscous vortex sheet is found to be different from that of the inviscid sheet in several features. A discontinuity in the curvature is formed in the viscous sheet, similarly as the inviscid sheet, but a cusp in the vortex sheet strength is less sharpened by viscosity. Exponential decay of the Fourier amplitudes is lost by the formation of the singularity, and the amplitudes of high wavenumbers exhibit an algebraic decay, while in the inviscid vortex sheet, the algebraic decay of the Fourier amplitudes is valid from fairly small wavenumbers. The algebraic decay rate of the viscous vortex sheet is approximately -2.5, independent of viscosity, which is the same rate as the asymptotic analysis of the inviscid sheet. Results for evolutions of the regularized vortex sheets show that the roll-up is weakened by viscosity, and the regularization parameter has more significant effects on the fine-structure of the core than does viscosity.

Sohn, Sung-Ik

2013-01-01

281

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

282

Improving propulsive efficiency through passive mechanisms using a Starling vortex generator  

NASA Astrophysics Data System (ADS)

Ruiz et al. (2011) demonstrated that pulsed propulsion with vortex rings, much like those seen in the wake of jellyfish and squid, can greatly enhance the overall efficiency of submersible vehicles. The objective of the present research is to achieve pulsed propulsion passively using a Starling vortex generator which consists of a collapsible tube within an airtight box. Recent work has shown that a Starling vortex generator is able to generate vortex rings, which indicates enhanced propulsion, while requiring less energy to generate pulsatility than the system by Ruiz et al. (2011). Current work is focused on conducting an experimental parameter study to determine an empirical scaling law suitable for design purposes, with the aim to integrate the device into a full-scale unmanned undersea vehicle.

Whittlesey, Robert; Dabiri, John

2011-11-01

283

Enhancement of vortex induced forces and motion through surface roughness control  

DOEpatents

Roughness is added to the surface of a bluff body in a relative motion with respect to a fluid. The amount, size, and distribution of roughness on the body surface is controlled passively or actively to modify the flow around the body and subsequently the Vortex Induced Forces and Motion (VIFM). The added roughness, when designed and implemented appropriately, affects in a predetermined way the boundary layer, the separation of the boundary layer, the level of turbulence, the wake, the drag and lift forces, and consequently the Vortex Induced Motion (VIM), and the fluid-structure interaction. The goal of surface roughness control is to increase Vortex Induced Forces and Motion. Enhancement is needed in such applications as harnessing of clean and renewable energy from ocean/river currents using the ocean energy converter VIVACE (Vortex Induced Vibration for Aquatic Clean Energy).

Bernitsas, Michael M. (Saline, MI); Raghavan, Kamaldev (Houston, TX)

2011-11-01

284

Irregular sleep-wake syndrome  

MedlinePLUS

... have a different condition, such as shift work sleep disorder or jet lag syndrome. ... Zee PC, Vitello MV. Circadian rhythm sleep disorder: irregular sleep wake rhythm. Sleep Med Clin 4;2009:213-218.

285

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

NASA Astrophysics Data System (ADS)

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 frequency were calculated to evaluate aeroelastic stability of hingeless rotor system. Numerical results of aeroelastic analysis for hingeless rotor blades were presented and compared with results based on experimental data and two-dimensional quasi-steady strip theory in which uniform inflow model was used. It was shown that wakes significantly affect the steady-state deflections and aeroelastic stability.

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

286

Thrust production and wake structure of a batoid-inspired oscillating fin  

PubMed Central

Experiments are reported on the hydrodynamic performance of a flexible fin. The fin replicates some features of the pectoral fin of a batoid fish (such as a ray or skate) in that it is actuated in a travelling wave motion, with the amplitude of the motion increasing linearly along the span from root to tip. Thrust is found to increase with non-dimensional frequency, and an optimal oscillatory gait is identified. Power consumption measurements lead to the computation of propulsive efficiency, and an optimal efficiency condition is evaluated. Wake visualizations are presented, and a vortex model of the wake near zero net thrust is suggested. Strouhal number effects on the wake topology are also illustrated.

CLARK, R. P.; SMITS, A. J.

2009-01-01

287

The Sleep–Wakefulness Cycle  

Microsoft Academic Search

\\u000a Wakefulness (W) is necessary for a thoughtful and precise knowledge of things, allowing us to recognize our essential attributes\\u000a and the changes that we experience in ourselves. We spend about two-thirds of our life in W. This state is circadian and homeostatically\\u000a regulated and precisely meshed with sleep into the sleep–wakefulness cycle (SWC). Sleep is also a necessary, active, periodic,

Fernando Reinoso-Suárez; Isabel Andrés; Miguel Garzón

288

The waking brain: an update  

Microsoft Academic Search

Wakefulness and consciousness depend on perturbation of the cortical soliloquy. Ascending activation of the cerebral cortex\\u000a is characteristic for both waking and paradoxical (REM) sleep. These evolutionary conserved activating systems build a network\\u000a in the brainstem, midbrain, and diencephalon that contains the neurotransmitters and neuromodulators glutamate, histamine,\\u000a acetylcholine, the catecholamines, serotonin, and some neuropeptides orchestrating the different behavioral states. Inhibition

Jian-Sheng Lin; Christelle Anaclet; Olga A. Sergeeva; Helmut L. Haas

2011-01-01

289

Numerical studies of plume-vortex interactions  

NASA Astrophysics Data System (ADS)

A new mixing model was developed to study the initial engine exhaust plume evolution with reduced reaction mechanism for the lower stratosphere. The results show that the effects of the local lack of mixing have an inhibiting effect on ozone depletion in the near-field plume. Inclusion of heterogeneous kinetics involving formation of condensed nitric and sulfuric acid on soot particles showed that 15% of the available NOx, is converted into its inactive form. A comprehensive and efficient aerosol model coupled with chemical kinetics and the jet flow model was developed. The predictions of aerosol number density, surface area density agree with previous simulations qualitatively. The comparison of predictions between with and without the micro-mixing effect also suggests that lack of micro-mixing tends to underpredict the aerosol number density. To study the plume-vortex interaction, a parallel LES code is first validated against experimental free jet data and then applied to the study of the near-field plume-vortex interaction dynamics with gas-phase and heterogeneous chemistry. The simulation results show reasonable agreement with in-situ observations. Results indicate that a significant difference between spatial and temporal simulation exists, which affects the accuracy of the prediction of sulfuric acid aerosols in the wake. Analysis of the results also shows that spatial simulation is more suitable for the near field interaction process.

Wu, Junxiao

1999-11-01

290

Vortex Characterization for Engineering Applications  

SciTech Connect

Realistic engineering simulation data often have features that are not optimally resolved due to practical limitations on mesh resolution. To be useful to application engineers, vortex characterization techniques must be sufficiently robust to handle realistic data with complex vortex topologies. In this paper, we present enhancements to the vortex topology identification component of an existing vortex characterization algorithm. The modified techniques are demonstrated by application to three realistic data sets that illustrate the strengths and weaknesses of our approach.

Jankun-Kelly, M; Thompson, D S; Jiang, M; Shannahan, B; Machiraju, R

2008-01-30

291

Nonlinear spacing and frequency effects of an oscillating cylinder in the wake of a stationary cylinder  

NASA Astrophysics Data System (ADS)

Nonlinear responses to a transversely oscillating cylinder in the wake of a stationary upstream cylinder are studied theoretically by using an immersed-boundary method at Re=100. Response states are investigated in the three flow regimes for a tandem-cylinder system: the ``vortex suppression'' regime, the critical spacing regime, and the ``vortex formation'' regime. When the downstream cylinder is forced to oscillate at a fixed frequency and amplitude, the response state of flow around the two cylinders varies with different spacing between the two cylinders, while in the same flow regime, the response state can change with the oscillating frequency and amplitude of the downstream cylinder. Based on velocity phase portraits, each of the nonlinear response states can be categorized into one of the three states in the order of increasing chaotic levels: lock-in, transitional, or quasiperiodic. These states can also be correlated with velocity spectral behaviors. The discussions are conducted using near-wake velocity phase portraits, spectral analyses, and related vorticity fields. A general trend in the bifurcation diagrams of frequency spacing shows the smaller the spacing, frequency, or amplitude, the less chaotic the response state of the system and more likely the downstream and upstream wakes are in the same response state. The system is not locked-in in any case when the spacing between the cylinders is larger than the critical spacing. The near-wake velocity spectral behaviors correspond to the nonlinear response states, with narrow-banded peaks shown at the oscillation frequency and its harmonics in the lock-in cases. High frequency harmonic peaks, caused by interactions between the upstream wake and the downstream oscillating cylinder, are reduced in the near-wake velocity spectra of the upstream cylinder when the spacing increases.

Yang, Xiaofan; Zheng, Zhongquan Charlie

2010-04-01

292

An examination of the effect of boundary layer thickness on vortex shedding from a square cylinder near a wall  

Microsoft Academic Search

A computational study has been undertaken to examine the effect of boundary layer thickness ?\\/D on vortex shedding from a square cylinder in proximity to a solid wall. The computations were carried out in a second-moment turbulence modeling framework using a finite-volume technique. The computed results show that, in general, thickening of the wall boundary layer causes wake periodicity to

Anthony G. Straatman; Robert J. Martinuzzi

2003-01-01

293

Distribution of spanwise enstrophy in the near wake of three symmetric elongated bluff bodies at high Reynolds number  

NASA Astrophysics Data System (ADS)

Three elongated bluff bodies with a chord-to-thickness ratio of seven have been studied experimentally at a Reynolds number based on body thickness of 3 × 104. The defining feature of elongated bluff bodies is the interaction between trailing edge Kármán vortex shedding and leading edge separation-reattachment. We have used particle image velocimetry with different body geometries to investigate this interaction for three distinct cases: (i) small leading edge separation-reattachment length; (ii) large leading edge separation-reattachment length; and (iii) one case in between these bounds. The leading edge separation-reattachment is a significant source of spanwise enstrophy. Thus, changes in the wake enstrophy distribution are of particular interest. We have examined the time-averaged distribution and production of both the turbulent kinetic energy and the spanwise enstrophy in the near wake region utilizing proper orthogonal decomposition on the vorticity field to distinguish between turbulence and the periodic contribution of the trailing edge vortex shedding. A significant increase in the lateral distribution of spanwise enstrophy is observed - exceeding the typical bounds of the near wake - which is due to the leading edge separation-reattachment and the resulting scale of the flow at the trailing edge. As a result, strengthening the leading edge flow, which tends to weaken the trailing edge vortex shedding, may lead to enhanced mixing in the wake.

Taylor, Zachary J.; Kopp, Gregory A.; Gurka, Roi

2013-05-01

294

Quantum vortex reconnections  

NASA Astrophysics Data System (ADS)

We study reconnections of quantum vortices by numerically solving the governing Gross-Pitaevskii equation. We find that the minimum distance between vortices scales differently with time before and after the vortex reconnection. We also compute vortex reconnections using the Biot-Savart law for vortex filaments of infinitesimal thickness, and find that, in this model, reconnections are time symmetric. We argue that the likely cause of the difference between the Gross-Pitaevskii model and the Biot-Savart model is the intense rarefaction wave which is radiated away from a Gross-Pitaeveskii reconnection. Finally we compare our results to experimental observations in superfluid helium and discuss the different length scales probed by the two models and by experiments.

Zuccher, S.; Caliari, M.; Baggaley, A. W.; Barenghi, C. F.

2012-12-01

295

The structure of vortex breakdown  

Microsoft Academic Search

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

S. Leibovich

1978-01-01

296

Effects of microbubbles on the Taylor-Green vortex flow  

NASA Astrophysics Data System (ADS)

Numerical simulations of the Taylor-Green vortex (TGV) flow laden with microbubbles were performed to study the effects of microbubbles on a simple vortical flow using the two-fluid approach. The study was motivated by our DNS results of a spatially developing turbulent boundary layer laden with microbubbles [J. Fluid Mech. 503 (2004)] which showed that the presence of bubbles results in a local positive divergence of the fluid velocity, ?.U. This velocity divergence displaces the near-wall quasi-streamwise vortical structures away from the wall, thus reducing the skin friction. In the present study, the continuity and momentum equations of both phases (fluid and bubbles) were numerically solved in a cubical domain. The results for Stokes number equal to 0.25 and bubbles volume fraction of 1% show that the magnitude of the vorticity at the center of the vortex decays faster than that of the single-phase flow. After 20 turnover times of the initial vortex, the magnitude of the vorticity at the center of the vortex becomes 30% smaller than that of the single-phase flow. Analysis of the vorticity equation shows that the local positive velocity divergence of the fluid velocity, ?.U, created in the vortex core by the clustering of the bubbles, is responsible for the vorticity decay. Results for different Stokes numbers and bubbles volume fractions will be presented.

Ferrante, A.

2005-11-01

297

Hairpin vortex formation, a case study for unsteady visualization.  

SciTech Connect

To better understand the vortex dynamics of coherent structures in turbulent and transitional boundary layers, we consider direct numerical simulation of the interaction between a flat-plateboundary-layer flow and an isolated hemispherical roughness element. Of principal interest is the evolution of hairpin vortices that form an interlacing pattern in the wake of the hemisphere, lift away from the wall, and are stretched by the shearing action of the boundary layer. Using animations of unsteady three-dimensional representations of this flow, produced by the vtk toolkit and enhanced to operate in a CAVE virtual environment, we identify and study several key features in the evolution of this complex vortex topology not previously observed in other visualization formats.

Fischer, P. F.; Papka, M. E.; Szymanski, M.; Tufo, H. M.

1999-08-10

298

Vortex pairs on surfaces  

SciTech Connect

A pair of infinitesimally close opposite vortices moving on a curved surface moves along a geodesic, according to a conjecture by Kimura. We outline a proof. Numerical simulations are presented for a pair of opposite vortices at a close but nonzero distance on a surface of revolution, the catenoid. We conjecture that the vortex pair system on a triaxial ellipsoid is a KAM perturbation of Jacobi's geodesic problem. We outline some preliminary calculations required for this study. Finding the surfaces for which the vortex pair system is integrable is in order.

Koiller, Jair [Centro de Matematica Aplicada, FGV/RJ, Praia de Botafogo 190 Rio de Janeiro, RJ, 22250-40 (Brazil); Boatto, Stefanella [Instituto de Matematica da UFRJ, C.P. 68530, Cidade Universitaria Rio de Janeiro, RJ 21945-970 (Brazil)

2009-05-06

299

First order vortex dynamics  

SciTech Connect

A non-dissipative model for vortex motion in thin superconductors is considered. The Lagrangian is a Galilean invariant version of the Ginzburg{endash}Landau model for time-dependent fields, with kinetic terms linear in the first time derivatives of the fields. It is shown how, for certain values of the coupling constants, the field dynamics can be reduced to first order differential equations for the vortex positions. Two vortices circle around one another at constant speed and separation in this model. {copyright} 1997 Academic Press, Inc.

Manton, N.S. [Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Silver Street, Cambridge CB3 9EW (England)

1997-05-01

300

Experimental investigation on the aerodynamic loads and wake flow features of a low aspect-ratio circular cylinder  

NASA Astrophysics Data System (ADS)

An experimental investigation on the flow features of the wake generated from a circular cylinder with finite height and placed vertically on a plane is presented. Through force measurements the mean drag coefficient is found to be roughly invariant by varying Reynolds number in a range between 6×104 and 11×104. As for the fluctuating forces, a dominant spectral component is clearly detected for the signals of the cross-flow force. A spectral contribution with roughly the same Strouhal number is detected from velocity signals acquired, through hot-wire anemometry, in proximity to the lateral wake boundary; its energy is found to decrease by moving the probe away from the wake and upwards. Simultaneous velocity measurements showed that these fluctuations can confidently be ascribed to an alternate vortex shedding. Subsequently, dynamic measurements of the pressure field over the lateral surface and the free-end of the model were carried out, which highlight that the spectral component connected to vortex shedding is found over the lateral surface, with maximum energy at an azimuthal position just before the separation of the shear layers. The fluctuating energy connected to vortex shedding decreases by moving towards regions immersed in the separated wake, and with increasing vertical coordinate; as a matter of fact, above about half model height an evident energy peak cannot be detected anymore. This feature highlights that a regular alternate vortex shedding occurs only for the lower half-span of the model and that the remaining part is dominated by the upwash generated by the flow passing over the free-end. From the spectral analysis of the pressure measurements carried out over the model free-end no evidence of the presence of the spectral component connected to the alternate vortex shedding is found, as expected. However, a significant fluctuating energy is observed at lower dominating frequencies.

Iungo, G. V.; Pii, L. M.; Buresti, G.

2012-01-01

301

A vortex panel analysis of circular-arc bluff-bodies in unsteady flow  

SciTech Connect

A method which is capable of calculating the unsteady flow field around circular-arc bluff bodies of zero thickness is presented. This method utilizes linear vortex panels to model the body surface and a portion of the wake surfaces. Discrete vortices are used to model the remainder of the wake surfaces. Separation is assumed to occur at the sharp edges of the bodies. Numerical results for circular-arc bodies with included angles of less than 180/degree/ are compared with experimental data and found to be in good agreement. 31 refs., 15 figs.

Strickland, J.H.

1989-01-01

302

On the far wake and induced drag of aircraft  

NASA Astrophysics Data System (ADS)

A set of matched asymptotic expansions is proposed for the flow far behind an aircraft, with the primary purpose of identifying lift, thrust and drag, particularly induced drag, in a unified manner in integral statements of the momentum equation. The fluid in the far wake is inviscid and incompressible, and variations of total pressure are allowed, as are vortex sheets. A notable feature is that the Trefftz-plane approximation is not invoked; instead the wake is taken as fully rolled-up, and the analysis proceeds without the assumption of light loading. Attention is paid to the absolute convergence of integrals over infinite domains and handling of discontinuities. The expansion includes a sink term, which appears new, so that the mass flux through a transverse plane is non-zero, as is the flux of mechanical energy. The lift can be formally attributed to the velocity induced by the bound vortex of the wing, which is at odds with some treatments, although consistent with Prandtl's analysis over a ground plane. The drag contains the integral of u2)/2, as in many treatments of the subject, u being the perturbation velocity along the wake. The negative sign for u2 appears paradoxical on two counts, one of which is resolved here. First, its very presence instead of the + sign, which would lead to the perturbation kinetic energy and therefore a compelling explanation of induced drag, is explained by the longitudinal energy flux. This energy, the integral of ?u2, is continuously provided by the unsteady starting-vortex system and was deposited earlier by the aircraft. Second, it appears that negative drag could be predicted by this equation. This is shown to be impossible, because of inequalities between the integrals of (v2 + w2) and of u2, but the proof is valid only if the vorticity is of only one sign on each side. A general proof of positivity has not been derived, because of nonlinearities, but neither has a counter-example.

Spalart, Philippe R.

303

Experimental investigation on the wing-wake interaction at the mid stroke in hovering flight of dragonfly  

NASA Astrophysics Data System (ADS)

This paper focuses on flow structures of the wing-wake interaction between the hind wing and the wake of the forewing in hovering flight of a dragonfly since there are arguments whether the wing-wake interaction is useful or not. A mechanical flapping model with two tandem wings is used to study the interaction. In the device, two identical simplified model wings are mounted to the flapping model and they are both scaled up to keep the Reynolds number similar to those of dragonfly in hovering flight since our experiment is conducted in a water tank. The kinetic pattern of dragonfly ( Aeschna juncea) is chosen because of its special interesting asymmetry. A multi-slice phase-locked stereo particle image velocimetry (PIV) system is used to record flow structures around the hind wing at the mid downstroke ( t/ T=0.25) and the mid upstroke ( t/ T=0.75). To make comparison of the flow field between with and without the influence of the wake, flow structures around a single flapping wing (hind wing without the existence of the forewing) at these two stroke phases are also recorded. A local vortex identification scheme called swirling strength is applied to determine the vortices around the wing and they are visualized with the iso-surface of swirling strength. This paper also presents contour lines of ? z at each spanwise position of the hind wing, the vortex core position of the leading edge vortex (LEV) of hind wing with respect to the upper surface of hind wing, the circulation of the hind wing LEV at each spanwise position and so on. Experimental results show that dimension and strength of the hind wing LEV are impaired at the mid stroke in comparison with the single wing LEV because of the downwash from the forewing. Our results also reveal that a wake vortex from the forewing traverses the upper surface of the hind wing at the mid downstroke and its distance to the upper surface is about 40% of the wing chord length. At the instant, the distance of the hind wing LEV to the upper surface is about 20% of the wing chord length. Thus, there must be a wing-wake interaction mechanism that makes the wake vortex become an additional LEV of the hind wing and it can partly compensate the hind wing for its lift loss caused by the downwash from the forewing.

Lai, GuoJun; Shen, GongXin

2012-11-01

304

Vortex patterns in a fast rotating Bose-Einstein condensate  

SciTech Connect

For a fast rotating condensate in a harmonic trap, we investigate the structure of the vortex lattice using wave functions minimizing the Gross-Pitaevskii energy in the lowest Landau level. We find that the minimizer of the energy in the rotating frame has a distorted vortex lattice for which we plot the typical distribution. We compute analytically the energy of an infinite regular lattice and of a class of distorted lattices. We find the optimal distortion and relate it to the decay of the wave function. Finally, we generalize our method to other trapping potentials.

Aftalion, Amandine; Blanc, Xavier; Dalibard, Jean [Laboratoire Jacques-Louis Lions, Universite Paris 6, 175 rue du Chevaleret, 75013 Paris (France); Laboratoire Kastler Brossel, 24 rue Lhomond, 75005 Paris (France)

2005-02-01

305

Non-uniqueness in wakes and boundary layers  

NASA Astrophysics Data System (ADS)

In streamlined flow past a flat plate aligned with a uniform stream, it is shown that (a) the Goldstein near-wake and (b) the Blasius boundary layer are nonunique solutions locally for the classical boundary layer equations, whereas (c) the Rott-Hakkinen (1965) very-near-wake appears to be unique. In each of (a) and (b) an alternative solution exists, which has reversed flow and which apparently cannot be discounted on immediate grounds. So, depending mainly on how the alternatives for (a), (b) develop downstream, the symmetric flow at high Reynolds numbers could have two, four or more steady forms. Concerning nonstreamlined flow, for example past a bluff obstacle, new similarity forms are described for the pressure-free viscous symmetric closure of a predominantly slender long wake beyond a large-scale separation. Features arising include nonuniqueness, singularities and algebraic behavior, consistent with nonentraining shear layers with algebraic decay. Nonuniqueness also seems possible in reattachment onto a solid surface and for nonsymmetric or pressure-controlled flows including the wake of a symmetric cascade.

Smith, F. T.

1984-01-01

306

Current carrying vortex crystals  

NASA Astrophysics Data System (ADS)

Abrikosov vortices in a type-II superconducting film subjected to strong magnetic field B with periodic array of nanoholes of the density npin form sometimes a vortex crystal, even when they are driven by a transport current. It is shown numerically that the crystal melting and transition to the resistive state occurs as a coherent depinning of the single vortex dislocations. For a system with interstitial vortices, f = B/?0npin > 1, the mechanism of depinning depends on the current direction with respect to the pinning array. It was found that slightly above the critical current trajectories of moving vortices are not straight, but rather acquire a snake - like shape enveloping the system of pins. In contrast to the matching field case, f = 1, the transition to a resistive state is not coherent and is developing through formation of the “snake - like” vortex trajectories. It is pointed out that the depinning is closely associated with the appearance of a strongly varying electric field. We calculated the electric fields accompanying vortex crystal melting and found the voltage-current characteristics. When the pinning array is made random, the critical current is reduced.

Rosenstein, B.; Shapiro, I.; Berco, D.; Shapiro, B. Ya

2012-12-01

307

Laminar vortex boundary layers  

NASA Astrophysics Data System (ADS)

The interaction of a Burgers--Rott vortex with a rigid no-slip normal wall is investigated via direct numerical simulations of the unsteady axisymmetric Navier--Stokes equations. The flows in the boundary layer and away from the vortex core have a self-similar structure, i.e. the solutions at time t, radius r, height z, and Reynolds number Re can be reduced to single profiles for the angular momentum and the azimuthal vorticity dependent on a single similarity variable. The similarity variable is the direction normal to the wall scaled by Re^1/2 and a function of r and t. The boundary layer flow near the axis for low-Re consists of a matching between a Bödewadt-like flow near r=0, where the vortex flow is near solid-body rotation, and a potential vortex boundary layer flow. For medium Re, waves form within the core radius resulting from the inflection points in the Bödewadt-like profiles. At large Re, there are also waves that travel vertically along the interface between the rotational core and the irrotational flow outside the core at r? 1.

Arrese, Juan C.; Lopez, John M.

1996-11-01

308

Three-dimensional flow visualization in the wake of a miniature axial-flow hydrokinetic turbine  

NASA Astrophysics Data System (ADS)

Three-dimensional 3-component velocity measurements were made in the near wake region of a miniature 3-blade axial-flow turbine within a turbulent boundary layer. The model turbine was placed in an open channel flow and operated under subcritical conditions (Fr = 0.13). The spatial distribution of the basic flow statistics was obtained at various locations to render insights into the spatial features of the wake. Instantaneous and phase-averaged vortical structures were analyzed to get insights about their dynamics. The results showed a wake expansion proportional to the one-third power of the streamwise distance, within the first rotor diameter. Wake rotation was clearly identified up to a distance of roughly three rotor diameters. In particular, relatively high tangential velocity was observed near the wake core, but it was found to be nearly negligible at the turbine tip radius. In contrast, the radial velocity showed the opposite distribution, with higher radial velocity near the turbine tip and, due to symmetry, negligible at the rotor axis. Larger turbulence intensity was found above the hub height and near the turbine tip. Strong coherent tip vortices, visualized in terms of the instantaneous vorticity and the ? 2 criterion, were observed within the first rotor diameter downstream of the turbine. These structures, influenced by the velocity gradient in the boundary layer, appeared to loose their stability at distances greater than two rotor diameters. Hub vortices were also identified. Measurements did not exhibit significant tip-hub vortex interaction within the first rotor diameter.

Chamorro, Leonardo P.; Troolin, Daniel R.; Lee, Seung-Jae; Arndt, R. E. A.; Sotiropoulos, Fotis

2013-02-01

309

Content Analysis of Dreams and Waking Narratives  

Microsoft Academic Search

This study investigated the specificity of dream content and its continuity with waking life. For each subject (125 men and 125 women, between the ages of 19 and 29 years), a dream and a waking episode were collected according to \\

Alfio Maggiolini; Chiara Cagnin; Franca Crippa; Anna Persico; Pietro Rizzi

2010-01-01

310

Wake structure measurements at the Mod-2 cluster test facility at Goodnoe Hills  

SciTech Connect

A field measurement progam was carried out at the cluster of three MOD-2 wind turbines located at Goodnoe Hills, Washington, to determine the rate of decay of wake velocity deficit with downwind distance in various meteorological conditions. Measurements were taken at hub height (200 ft) between July 12 and August 1, 1982. Wake wind speeds were measured using a radiosonde suspended from a tethered balloon, its position being determined from a grid of ground stakes. Measurments were also made downwind with the turbine off to determine the magnitude of terrain-induced variations in wind speed. The balloon system used to measure downstream wind data proved to be reliable and convenient. Downstream distances of 900, 1500, 2100, and 2700 ft from the turbine were investigated. Differences between the instrumentation systems required that corrections be made to the data. After correction, averaged terrain-induced wind speed variations were regarded as insignificant. Turbine-on velocity ratios showed scatter, suggesting that only some measurements were, in fact, representative of wake centerline velocities, and that others were made off centerline due to wake meander or wind shift. Isolation of the high wind speed (30 to 45 mph) velocity ratios, however, revealed velocity deficits downstream. Measurements at greater downstream distances showed no wake deficit within the limits of resolution of the experiment, indicating that the wake had recovered to free stream conditions. Comparison with the AeroVironment wake model using common values for rotor drag coefficient and turbulence showed similar trends.

Lissaman, P.B.S.; Zambrano, T.G.; Gyatt, G.W.

1983-03-01

311

Single vortex fluctuations in a superconducting chip as generating dephasing and spin flips in cold atom traps  

NASA Astrophysics Data System (ADS)

We study trapping of a cold atom by a single vortex line in an extreme type-II superconducting chip, allowing for pinning and friction. We evaluate the atom's spin flip rate and its dephasing due to the vortex fluctuations in equilibrium and find that they decay rapidly when the distance to the vortex exceeds the magnetic penetration length. We find that there are special spin orientations, depending on the spin location relative to the vortex, at which spin dephasing is considerably reduced while perpendicular directions have a reduced spin flip rate.

Fruchtman, Amir; Horovitz, Baruch

2012-09-01

312

MODEL-BASED CONTROL OF VORTEX SHEDDING USING LOW-DIMENSIONAL GALERKIN MODELS  

Microsoft Academic Search

A model-based flow control strategy is proposed for the suppression of vortex shedding behind a circu- lar cylinder. The control design is based on a hi- erarchy of low-dimensional Galerkin models of the cylinder wake. These models are constructed from a Karhunen-Loeve decomposition of a simulation without actuation. The key enablers are an addi- tional physical mode in the Karhunen-Loeve

Johannes Gerhard; Mark Pastoor; Rudibert King; Bernd R. Noack; Andreas Dillmann; Marek Morzynskik; Gilead Tadmor

313

Island wakes in the Southern California Bight  

Microsoft Academic Search

Wind- and current-induced island wakes were investigated using a multiplatform approach of in situ, remote sensing, and numerical model simulations for the Southern California Bight (SCB). Island wind wakes are a result of sheltering from the wind, with weak wind mixing, strong heat storage, and consequent high sea surface temperature (SST). Wind wakes around Santa Catalina Island are most persistent

R. M. A. Caldeira; P. Marchesiello; N. P. Nezlin; P. M. DiGiacomo; J. C. McWilliams

2005-01-01

314

Simulation of vortex shedding in a turbine stage  

SciTech Connect

Vortex shedding in a turbomachine blade row is affected by passing of blades in the adjacent downstream blade row, but these effects have not been examined in the literature. A series of flow simulations has been performed to study vortex shedding in a turbine stage, and to quantify the blade interaction effects on the unsteady pressure response. The numerical issues of spatial order of accuracy and the use of Newton subiterations were investigated first. Second-order spatial accuracy was shown to be inadequate to model the shedding frequency response and time-averaged base pressure accurately. For the small time step employed for temporal accuracy, Newton iterations were shown to be unnecessary. The effects of the adjacent blade row were examined by comparing the shedding frequency response for the stage simulations to the response for isolated cascades. The vane shedding was shown to occur exactly an a series of harmonics of the blade passing frequency for the stage case, compared to a single predominant frequency for the isolated cascade. Losses were also examined in the wake region. It was shown that close to the trailing edge, losses were mainly due to wake mixing. Farther downstream of the trailing edge, losses were predominantly due to the trailing edge shock wave.

Sondak, D.L. [Boston Univ., MA (United States); Dorney, D.J. [GMI Engineering and Management Inst., Flint, MI (United States)

1999-07-01

315

Heat Transfer Enhancement in Separated and Vortex Flows  

SciTech Connect

This document summarizes the research performance done at the Heat Transfer Laboratory of the University of Minnesota on heat transfer and energy separation in separated and vortex flow supported by DOE in the period September 1, 1998--August 31, 2003. Unsteady and complicated flow structures in separated or vortex flows are the main reason for a poor understanding of heat transfer under such conditions. The research from the University of Minnesota focused on the following important aspects of understanding such flows: (1) Heat/mass transfer from a circular cylinder; (2) study of energy separation and heat transfer in free jet flows and shear layers; and (3) study of energy separation on the surface and in the wake of a cylinder in crossflow. The current study used three different experimental setups to accomplish these goals. A wind tunnel and a liquid tunnel using water and mixtures of ethylene glycol and water, is used for the study of prandtl number effect with uniform heat flux from the circular cylinder. A high velocity air jet is used to study energy separation in free jets. A high speed wind tunnel, same as used for the first part, is utilized for energy separation effects on the surface and in the wake of the circular cylinder. The final outcome of this study is a substantial advancement in this research area.

Richard J. Goldstein

2004-05-27

316

Cooling Signs in Wake Debate  

ERIC Educational Resources Information Center

|More than a year after dismantling a student-assignment policy based on socioeconomic diversity and setting off a wave of reaction that drew national attention, the Wake County, North Carolina, school board took a step that may turn down the temperature of the intense debate. The board, which has been deeply split on an assignment plan for the…

Samuels, Christina A.

2011-01-01

317

Nighttime Wakefulness Associated with Infant  

Microsoft Academic Search

Parent-infant cosleeping occurs in human and nonhuman primates, yet stud- ies on the impact of cosleeping on parental sleep patterns have been limited to human mothers. We examined the effects of cosleeping on the nighttime wake- fulness of a biparental New World primate, Wied's black tufted-ear marmoset (Callithrix kuhlii). We compared the sleep patterns of marmoset parents caring for young

Jeffrey E. Fite; Jeffrey A. French; Kimberly J. Patera; Elizabeth C. Hopkins; Michael Rukstalis; Heather A. Jensen; Corinna N. Ross

318

Control of sleep and wakefulness.  

PubMed

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making. PMID:22811426

Brown, Ritchie E; Basheer, Radhika; McKenna, James T; Strecker, Robert E; McCarley, Robert W

2012-07-01

319

Two new vortex liquids  

NASA Astrophysics Data System (ADS)

In 1967, Reatto and Chester proposed that solid helium-4 might exhibit superfluidity, and in 1970, Leggett suggested what was thought to be a definitive experimental test: to find non-classical rotational inertia in a toroidal sample. More than three decades later, the observation by Kim and Chan of exactly that effect generated great interest and has been repeated and confirmed by a number of groups. However, many attempts to find actual superflow in truly solid samples have failed. Here, I draw an analogy with a second example of anomalous response to vorticity in a dissipative fluid, the vortex liquid phase in the pseudogap region of high-temperature superconductors, and propose that the solid helium experiments have been mischaracterized: what is observed is not supersolidity but an incompressible vortex liquid. This state is distinct from a conventional liquid in that its properties are dominated by conserved supercurrents flowing around a thermally fluctuating tangle of vortices.

Anderson, Philip W.

2007-03-01

320

Control of vortex shedding on a circular cylinder using self-adaptive hairy-flaps  

NASA Astrophysics Data System (ADS)

Experiments on separation control using flexible self-adaptive hairy-flaps are presented herein. The wake-flow behind a circular cylinder is investigated without and with flexible hairy-flaps at the aft-part of the cylinder. Flow dynamics and hair motion were measured by particle image velocimetry and image processing in a range of Reynolds number 5000vortex separation cycle in such a way that the vortices do not shed in a zig-zag like arrangement as in the classical von Kármán vortex street but in line in a row with the cylinder wake axis. Thus, the wake-deficit is largely reduced. Furthermore, flow fluctuations are considerably reduced about 42% in streamwise and 35% in transversal direction compared to the reference case without hairy-flaps, too. The condition for this mode change is the lock-in of the vortex shedding with a traveling wave running through the flexible hair bundles in transversal direction at the aft-part of the cylinder. As a consequence, the vortex shedding frequency is increased, the length of the separation bubble is decreased and drag force is decreased, too. The lock-in appears as a jump-like change of the shedding frequency and a jump in the Strouhal-Reynolds number diagram. However, when the characteristic length for the normalized frequency is chosen as the length of the separation bubble instead of the cylinder diameter, the Str-Re dependence is regular again. This hints on the relevance of the resonator model as proposed by Sigurdson and Roshko (1988) [16] on vortex shedding mechanism when boundary conditions are changed such as in our case, where the hairy-flap bundle imposes a flexible wall with visco-elastic coupling in transversal direction.

Kunze, Sebastian; Brücker, Christoph

2012-01-01

321

Aeroacoustics of viscous vortex reconnection  

NASA Astrophysics Data System (ADS)

Reconnection of two anti-parallel vortex tubes is studied by direct numerical simulations and large-eddy simulations of the incompressible Navier-Stokes equations over a wide range (2000-50,000) of the vortex Reynolds number (Re). A detailed investigation of the flow dynamics is performed and at high Re, multiple reconnections are observed as the newly formed ``bridges'' interact by self and mutual induction. To investigate acoustics produced by the recoil action of the vortex threads, M"ohring's theory of vortex sound is applied to the flow field and evaluated at varying far-field locations. The acoustic solver is verified against calculations of laminar vortex ring collision. For anti-parallel vortex reconnection, the resulting far-field spectra are shown to be grid converged at low-to-mid frequencies. To assess the relevance to fully turbulent jet noise, the dependence of reconnection upon Reynolds number is investigated.

Paredes, Pedro; Nichols, Joseph W.; Duraisamy, Karthik; Hussain, Fazle

2011-11-01

322

High sensitivity vortex shedding flowmeter  

SciTech Connect

This patent describes an apparatus for measuring fluid flows. It comprises: a flowmeter body including a flow passage; a vortex generator of an elongated cylindrical shape disposed across a cross section of the flow passage, wherein at lest one extremity of the vortex generator is secured to the flowmeter body; a transducer contained in a container vessel secured to the flowmeter body, wherein the transducer is pressed onto a thin wall of the container vessel; and a flexible coupling connecting the thin wall of the container vessel to a deflective portion of the vortex generating, wherein the flexible coupling enhances relative deflection between the vortex generator and the container vessel. Wherein fluctuating fluid dynamic forces resulting from vortices shed from the vortex generator and experienced by vortex generator generate fluctuating electrical signals from the transducer as a measure of fluid flow through the flow passage.

Lew, H.S.

1989-12-05

323

Confined Vortex Scrubber  

SciTech Connect

The program objective is to demonstrate efficient removal of fine particulates to sufficiently low levels to meet proposed small scale coal combustor emission standards. This is to be accomplished using a novel particulate removal device, the Confined Vortex Scrubber. This is the first quarterly technical progress report under this contract. Accordingly, a summary of the cleanup concept and the structure of the program is given here.

Not Available

1990-02-01

324

The vortex flap  

NASA Astrophysics Data System (ADS)

The Vortex Flap is a new type of mechanically driven high-lift device consisting of a rotating cylinder placed underneath and near the trailing edge of an airfoil. Wind tunnel tests were designed and conducted in the Washington University Low-Speed Wind Tunnel. Wind tunnel tests indicate that the Vortex Flap produces notable lift coefficient increments and increases maximum lift coefficients, particularly for the low Reynolds number range tested. The best configurations of the configurations investigated (not necessarily optimal) produce lift increments of 300-900% at low-to-moderate angles of attack, and increase the maximum lift coefficient on the order of 200%. The large lift increments found, particularly at low angles of attack, underscore the ability to drive the airfoil to high lift coefficients even at low angles of attack, a potentially useful characteristic for certain flight maneuvers. Regions of fairly high L/D (on the order of 10) as well as low L/D performance were identified. The nondimensional cylinder rotation speed was found to be the most important experimental parameter. Methods for correcting wind tunnel data were developed and outlined, and a Response Surface Method was applied to the corrected data for ease of interpretation. Performance comparisons between the Vortex Flap and other trailing-edge high-lift devices are included. To demonstrate the potential of the device, a Navy mission specification for a VTOL ship-borne UAV, currently filled by a rotary-wing aircraft, is analyzed using a hypothetical fixed wing aircraft and the Vortex Flap. It is demonstrated that, under certain reasonable wind-over-deck conditions, such an aircraft could hypothetically fill a VTOL mission.

Buerge, Brandon T.

325

Vortex generating mass flowmeter  

SciTech Connect

This patent describes a vortex generating flowmeter. It comprises: a first port; a second port; a flow measurement passage between the ports; a restriction formed in the flow passage between the first and second ports, the restriction comprising a converging diverging nozzle; a rod producing drag in the passage between the restriction and the second port; and means for sensing fluid pressure in the vicinity if the rod.

Hughes, N.

1990-01-30

326

Vortex Design Problem  

Microsoft Academic Search

In this investigation we are concerned with a family of solutions of the 2D steady--state Euler equations, known as the Prandtl--Batchelor flows, which are characterized by the presence of finite--area vortex patches embedded in an irrotational flow. We are interested in flows in the exterior of a circular cylinder and with a uniform stream at infinity, since such flows are

Bartosz Protas

2007-01-01

327

Confined vortex scrubber  

SciTech Connect

The program objective is to demonstrate efficient removal of fine particulates to sufficiently low levels to meet proposed small scale coal combustor emission standards. This is to be accomplished using a novel particulate removal device, the Confined Vortex Scrubber (CVS). The CVS consists of a cylindrical vortex chamber with tangential flue gas inlets. The clean gas exit is via tangent slots in a central tube. Liquid is introduced into the chamber and is confined with the vortex chamber by the centrifugal force generated by the gas flow itself. This confined liquid forms a layer through which the flue gas is then forced to bubble, producing a strong gas/liquid interaction, high inertial separation forces and efficient particulate cleanup. In effect, each of the sub-millimeter diameter gas bubbles in the liquid layer acts as a micro-cyclone, inertially separating particles into the surrounding liquid. The CVS thus obtains efficient particle removal by forcing intimate and vigorous interaction between the particle laden flue gas and the liquid scrubbing medium.

Not Available

1990-05-01

328

Vortex structures in dipolar condensates  

SciTech Connect

We investigate the properties of single vortices and of the vortex lattice in a rotating dipolar condensate. We show that vortices in this system possess several features induced by the long-range anisotropic dipolar interaction between particles. For example, when the dipoles are polarized along the rotation axis, vortices may display a craterlike structure; when dipoles are polarized orthogonal to the rotation axis, the vortex cores take an elliptical shape and the vortex lattice no longer possesses hexagonal symmetry.

Yi, S. [Department of Physics and Astronomy and Rice Quantum Institute, Rice University, Houston, Texas 77251-1892 (United States); Institute of Theoretical Physics, Chinese Academy of Science, Beijing, 100080 (China); Pu, H. [Department of Physics and Astronomy and Rice Quantum Institute, Rice University, Houston, Texas 77251-1892 (United States)

2006-06-15

329

Application of Wake Characteristics to Prediction of Broadband Noise of a Multiblade Fan  

NASA Astrophysics Data System (ADS)

In this study, the wake characteristics of an arc blade were measured by the wind tunnel experiment; the characteristics were defined as the width of the wake, diameter of the vortex, ratio of the vortex scale, and the local lift. The influence of the angle of attack on the aerodynamic noise of the blade was quantitatively predicted by using these characteristics. It was clarified experimentally that the sound pressure of the aerodynamic noise becomes small since the gradient of the differential of the lift fluctuation was reduced according to the increase of the angle of attack. The wake characteristics were applied to the prediction of the broadband noise generated from a multiblade fan; the fan noise level distribution was estimated with high accuracy to be in the range from 1000-3000 Hz and was used to analyze the broadband noise of the fan. From the analysis of the fan noise level, it was found that the difference in the relative velocity caused by the biased internal flow was related to the noise levels.

Sasaki, Soichi; Hayashi, Hidechito

330

Prediction of aerodynamic noise in a ring fan based on wake characteristics  

NASA Astrophysics Data System (ADS)

A ring fan is a propeller fan that applies an axial-flow impeller with a ring-shaped shroud on the blade tip side. In this study, the entire flow field of the ring fan is simulated using computational fluid dynamics (CFD); the accuracy of the CFD is verified through a comparison with the aerodynamic characteristics of a propeller fan of current model. Moreover, the aerodynamic noise generated by the fan is predicted on the basis of the wake characteristics. The aerodynamic characteristic of the ring fan based on CFD can represent qualitatively the variation in the measured value. The main flow domain of the ring fan is formed at the tip side of the blade because blade tip vortex is not formed at that location. Therefore, the relative velocity of the ring fan is increased by the circumferential velocity. The sound pressure levels of the ring fan within the frequency band of less than 200 Hz are larger than that of the propeller fan. In the analysis of the wake characteristics, it revealed that Karman vortex shedding occurred in the main flow domain in the frequency domain lower than 200 Hz; the aerodynamic noise of the ring fan in the vortex shedding frequency enlarges due to increase in the relative velocity and the velocity fluctuation.

Sasaki, Soichi; Fukuda, Masaharu; Tsujino, Masao; Tsubota, Haruhiro

2011-06-01

331

Investigation of the near-field tip vortex behind an oscillating wing  

NASA Astrophysics Data System (ADS)

The near-field tip-vortex flow structure behind an oscillating NACA 0015 wing was investigated at {Re = 1.86 × 10^{5}. For attached-flow and light-stall oscillations, a small hysteretic property existed between the pitch-up and pitch-down motion, and many of the vortex flow features were found to be qualitatively similar to those of a static wing. For deep-stall oscillations, the wing oscillations imposed a strong discrepancy in contour shapes and magnitudes between the pitch-up and pitch-down phases of the oscillation cycle. The vortex was less organized during pitch-down (as a result of leading-edge-vortex-induced massive flow separation) than during pitch-up. The tangential velocity, circulation and lift-induced drag increased progressively with the airfoil incidence, and had higher magnitudes during pitch-up than during pitch-down, while varying slightly with the downstream distance. The vortex size, however, was larger during pitch-down than during pitch-up. The axial flow was always wake-like during the deep-stall oscillation cycle. The normalized circulation within the inner region of the tip vortex also exhibited a self-similar structure, similar to that of a static wing, and was insensitive to the reduced frequency.

Birch, D.; Lee, T.

2005-12-01

332

The evolution of microphysical and optical properties of an A380 contrail in the vortex phase  

NASA Astrophysics Data System (ADS)

A contrail from a large-body A380 aircraft at cruise in the humid upper troposphere has been probed with in-situ instruments onboard the DLR research aircraft Falcon. The contrail was sampled during 700 s measurement time at contrail ages of about 1-4 min. The contrail was in the vortex regime during which the primary wake vortices were sinking 270 m below the A380 flight level while the secondary wake remained above. Contrail properties were sampled separately in the primary wake at 90 and 115 s contrail age and nearly continously in the secondary wake at contrail ages from 70 s to 220 s. The scattering phase functions of the contrail particles were measured with a polar nephelometer. The asymmetry parameter derived from these data is used to distinguish between quasi-spherical and aspherical ice particles. In the primary wake, quasi-spherical ice particles were found with concentrations up to 160 cm-3, mean effective diameter Deff of 3.7 ?m, maximum extinction of 7.0 km-1, and ice water content (IWC) of 3 mg m-3 at slightly ice-subsaturated conditions. The secondary and primary wakes were separated by an almost particle-free wake vortex gap. The secondary wake contained clearly aspherical contrail ice particles with mean Deff of 4.8 ?m, mean (maximum) concentration, extinction, and IWC of 80 (350) cm-3, 1.6 (5.0) km-1, and 2.5 (10) mg m-3, respectively, at conditions apparently above ice-saturation. The asymmetry parameter in the secondary wake decreased with contrail age from 0.87 to 0.80 on average indicating a preferential aspherical ice crystal growth. A retrieval of ice particle habit and size with an inversion code shows that the number fraction of aspherical ice crystals increased from 2% initially to 56% at 4 min contrail age. The observed crystal size and habit differences in the primary and secondary wakes of an up to 4 min old contrail are of interest for understanding ice crystal growth in contrails and their climate impact. Aspherical contrail ice particles cause less radiative forcing than spherical ones.

Gayet, J.-F.; Shcherbakov, V.; Voigt, C.; Schumann, U.; Schäuble, D.; Jessberger, P.; Petzold, A.; Minikin, A.; Schlager, H.; Dubovik, O.; Lapyonok, T.

2012-07-01

333

Study on the near-wake flow behind EBMC flameholder  

NASA Astrophysics Data System (ADS)

The detailed near-wake flow behind the Edge Blowing Mixture Curtain flameholder and the V-gutter flameholder were investigated by using 2D cross-correlation on-line PIV in a low speed wind tunnel. Experiments were conducted at the main flow velocity of 10m/s and 20m/s, the jet velocity of 10m/s and 20m/s respectively. Results show that near-wake flow structure of both flameholders is governed mainly by large-scale vortices in the form of vortex street, and the jet increase the size of the vortices. But symmetrical large-scale vortices appear when the momentum ratio of the jet to the main flow is high, which is disadvantage to combustion. Based on the research on the instantaneous flow, the instantaneous viewpoint of the flame stability mechanism was brought forwad. Important effect of alternate formation and shedding of vortices on the flame stability was discussed. All of these offer important theoretic basis for further studying combustion performance of the novel flameholder.

Yue, Lianjie; Liu, Bao-jie; Yang, Maolin

2003-04-01

334

Investigation of the variability of the structure of a stratified wake behind a horizontal cylinder using optical and acoustic methods  

Microsoft Academic Search

The shadow flow pattern behind a horizontal cylinder uniformly towed in a stratified fluid with constant buoyancy frequency\\u000a (in the imbedded vortex and turbulent wake regime) is recorded synchronously with acoustic echo sounding (basic frequency\\u000a equal to 1 MHz) in a laboratory tank. Using computer processing, the illumination profiles in the schlieren pattern are constructed\\u000a on scales comparable with the

V. V. Mitkin; V. E. Prokhorov; Y. D. Chashechkin

1998-01-01

335

Development of a new free wake model considering a blade vane interaction for a low noise axial fan planform optimization  

NASA Astrophysics Data System (ADS)

Multidisciplinary Design Optimization (MDO) is an essential part for low noise axial fan design since various parameters, such as flow rate, efficiency, noise etc., should be considered. For this reason, Response Surface Method (RSM) design technique is adopted as an axial fan design method. RSM has an advantage of choosing objective functions and constraint conditions unrestrictedly on a design space. However, RSM needs a lot of independent variables to construct a proper response surface. Thus an efficient and accurate flow analysis tool is indispensable for optimization. In an axial fan, the discrete (commonly called Blade-Passage-Frequency) components are usually dominant in the noise spectrum. Especially the blade guide vane interaction is one of most important noise sources. In order to predict this noise component efficiently at the design stage, a new free wake model named Finite Vortex Element (FVE) is devised to simulate this blade guide vane interaction, which is very difficult to analyze numerically in a conventional free wake model. In this new free wake model, the blade wake guide vane interaction is described by cutting a vortex filament when the filament collides with a guide vane. This FVE model is compared with a conventional curved vortex methodology and verified by a comparison with measured data to show its effectiveness and validity. Then FVE model is coupled with RSM to implement a low noise axial fan blade optimization. Using this method, a reduction of 8 dB(A) at 2 m from fan hub in the overall noise level is achieved while the flow rate and the efficiency are maintained as the values of the baseline blade, which implies that FVE wake model coupled with RSM is very effective methodology for MDO problems such as a low noise axial fan design.

Shin, Hyungki; Sun, Hyosung; Lee, Soogab

2006-03-01

336

Laser velocimetry and blade pressure measurements of a blade-vortex interaction  

NASA Astrophysics Data System (ADS)

An investigation of the flowfield chracteristics around a rotor blade during a blade-vortex interaction (BVI) was conducted at the NASA Langley Research Center by the Army's Aeroperformance Division and the Boeing Defense and Space Group, Helicopter Division, during a wind-tunnel test in the 14 by 22-foot Subsonic Tunnel. A two-component laser velocimeter was used to measure the blade pressure during a BVI. This paper presents velocity measurements that indicate the presence of a vortex in the streamlines and vectors of the induced velocity, when studied in conjunction with the blade surface pressures, indicate how the flowfield is behaving during a BVI. The following conclusions can be made from this investigation: (1) The streamlines and vectors of the induced velocity, when studied in conjunction with the blade surface pressures, indiacte how the flowfield is behaving during a BVI. The blade approaches and intersects a vortex, and the vortex slides beneath the blade. (2) The data provide detailed flowfield information for validating computational predictions of BVI and also for evaluating and improving current wake models. Among the options investigated, only the free-wake calculation by TECH-01 indicated any BVI activity in the first quadrant.

Gorton, Susan Althoff; Poling, David R.; Dadone, Leo

1995-04-01

337

The dynamics of vortex shedding of flow past a vertical flat plate; LES studies  

NASA Astrophysics Data System (ADS)

Although, apparently flow-induced vibrations represent a canonical problem, a complete understanding of the fluid-structure mechanism of interaction has not yet been achieved. One of the issues associated with flow induced vibrations stems from the vortex shedding phenomenon. It is well known that vortex streets are formed in the wake of bluff bodies over a wide range of Reynolds numbers. The periodic shedding of vortices may result in significant fluctuating loading on the body. When the shedding frequency is close to one of the characteristic frequencies of the body, the resonant oscillations of the body can be excited, causing damaging instabilities. Various studies regarding the vortex shedding resonance or "lock-on" phenomenon have been conducted. However, the effect of Reynolds number on the "lock-on" regime is yet to be fully understood. The "lock-on" phenomenon is of critical importance for the analysis of flow-induced vibration, when the aeroelastic response of the structure is considered. In the present research the influence of sweeping angle on the plate tip vortex formation is studied numerically using large eddy simulation (LES). The results show an increase in magnitude and size of vertical structures developed in the wake of the plate.

Velez, Carlos; Ilie, Marcel

2010-11-01

338

Physics behind vortex-induced vibration reduction using an oblique trailing edge hydrofoil  

NASA Astrophysics Data System (ADS)

The issue of vortex-induced vibration based on the phenomenon of vortex shedding behind a bluff body is a major problem in hydraulic machinery. Resulting fluctuating forces may lead to excessive vibrations and premature cracks. It is well known that a hydrofoil with an oblique trailing edge reduces vibration as compared to that with a blunt trailing edge. However physics behind this is not fully understood. The purpose of the present work is to conduct an experimental investigation of vortex shedding dynamics in the wake of an oblique trailing edge hydrofoil to understand the phenomena and the reasons for vibration reduction. This could help optimize the trailing edge shape and diminish the induced vibration. A velocity survey in the hydrofoil wake is performed via Laser-Doppler and Particle Image velocimetry using the Proper-Orthogonal-Decomposition technique for post-processing. In addition, flow induced vibration measurements and high speed visualization are performed. The high-speed videos clearly demonstrate alternate shedding of the vortices transforming into nearly simultaneous shedding at the hydrofoil trailing edge. As a result, partial cancellation is observed for upper and lower vortices, accompanied by the thickening of the lower vortex core that is believed to be the primary reason of the vibration reduction.

Zobeiri, Amirreza; Avellan, Francois; Farhat, Mohamed

2010-11-01

339

Wingtip vortex control via the use of a reverse half-delta wing  

NASA Astrophysics Data System (ADS)

The effect of a 65° sweep reverse half-delta wing (RHDW), mounted at the squared tip of a rectangular NACA 0012 wing, on the tip vortex was investigated experimentally at Re = 2.45 × 105. The RHDW was found to produce a weaker tip vortex with a lower vorticity level and, more importantly, a reduced lift-induced drag compared to the baseline wing. In addition to the lift increment, the RHDW also produced a large separated wake flow and subsequently an increased profile drag. The reduction in lift-induced drag, however, outperformed the increase in profile drag and resulted in a virtually unchanged total drag in comparison with the baseline wing. Physical mechanisms responsible for the RHDW-induced appealing aerodynamics and vortex flow modifications were discussed.

Lee, T.; Su, Y. Y.

2012-06-01

340

Finite-temperature vortex dynamics in Bose-Einstein condensates  

SciTech Connect

We study the dynamics of a vortex in an atomic Bose-condensed gas at finite temperature within the Zaremba-Nikuni-Griffin formalism. In a harmonically trapped pancake-shaped condensate, an off-centered vortex is known to decay by spiraling out toward the edge of the condensate. We quantify the dependence of this decay on temperature, atomic collisions, and thermal cloud rotation. Near the trap center where the density varies slowly, we show that our numerical results agree with the predictions of the Hall-Vinen phenomenological friction force model used to describe quantized vorticity in superfluid systems. Our result thus clarifies the microscopic origin of the friction and provides an ab initio determination of its value.

Jackson, B.; Proukakis, N. P.; Barenghi, C. F. [School of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne NE1 7RU (United Kingdom); Zaremba, E. [Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, Ontario, K7L 3N6 (Canada)

2009-05-15

341

Vortex Characteristic and Flow Discharge In Vortex Settling Chamber  

NASA Astrophysics Data System (ADS)

In water treatment field, separation of sediment from raw water is one of the most important problems we must face today, especially separation of fine sediment particle from muddy water - water with very high sediment concentration in natural resources such as river, reservoir, etc. There are so many different methods to solve this problem including tunnel type, vortex tubes, rectangular settling basins and the vortex type settling chamber. Among them the vortex settling chamber has recently studied so much because of its advantage. The vortex settling chamber is a device which is used to extract sediment from the diverted water by the vortex flow and centrifugal force in chamber. It can be said that vortex settling basin is an economical, efficient, and water-conserving choice compared with the other available devices especially for excluding fine suspended sediment particles. This research presents the new model of vortex settling chamber which will be focused on the separation of fine sediment from muddy water. This paper firstly presents the model in detail and some experimental cases which are carried out in this study. The relationship between flow discharge and water level will be considered and then some respective results will be presented and discussed. Finally, some conclusions are made about vortex characteristic in chamber as well as its effect on flow discharge.

Nguyen, Q.; Jan, C.

2008-12-01

342

Disrupted states of vortex flow and vortex breakdown  

Microsoft Academic Search

Flow visualization studies and laser Doppler anemometer measurements on swirling water flows reveal six distinct types of very large amplitude disturbance modes of the vortex core. Three, 'axisymmetric' and spiral vortex breakdowns, and the 'double helix,' have been described by others. A definite order of evolution in parameter space (Reynolds number and circulations) occurs, and is described. Puzzling responses of

J. H. Faler; S. Leibovich

1977-01-01

343

Geometric Methods for Vortex Extraction  

Microsoft Academic Search

This paper presents two vortex detection methods which are based on the geometric properties of streamlines. Unlike traditional vortex detection meth- ods, which are based on point-samples of physical quantities, one of our methods is also effective in detecting weak vortices. In addition, it allows for quantitative feature extraction by calculating numerical attributes of vortices. Results are pre- sented of

I. Ari Sadarjoen; Frits H. Post

344

Instability of a junction vortex  

Microsoft Academic Search

The flow field in the region where a moving wall, started from rest, slides under a stationary one, produces an interesting flow phenomena with relatively simple generation geometry. Experiments show that if the wall speed is high enough a vortex forms close to the junction of the moving wall with the stationary one. Vortex formation was observed for the range

Takashi Naitoh; James Allen

2005-01-01

345

Numerical simulation of the effects of rotor-stator spacing and wake/blade count ratio on turbomachinery unsteady flows  

SciTech Connect

A two-dimensional time-accurate Navier-Stokes solver for incompressible flows is used to simulate the effects of the axial spacing between an upstream rotor and a stator, and the wake/blade count ratio on turbomachinery unsteady flows. The code uses a pressure-based method. A low-Reynolds number two-equation turbulence model is incorporated to account for the turbulence effect. By computing cases with different spacing between an upstream rotor wake and a stator, the effect of the spacing is simulated. Wake/blade count ratio effect is simulated by varying the number of rotor wakes in one stator passage at the computational inlet plane. Results on surface pressure, unsteady velocity vectors, blade boundary layer profiles, rotor wake decay and loss coefficient for all the cases are interpreted. It is found that the unsteadiness in the stator blade passage increases with a decrease in the blade row spacing and a decrease in the wake/blade count ratio. The reduced frequency effect is dominant in the wake/blade count ratio simulation. The time averaged loss coefficient increases with a decrease in the axial blade row spacing and an increase in the wake/blade count ratio.

Yu, W.S.; Lakshminarayana, B. [Pennsylvania State Univ., University Park, PA (United States). Center for Gas Turbines and Power

1995-12-01

346

Optical Influence of Ship Wakes  

NASA Astrophysics Data System (ADS)

The optical variations observed within ship wakes are largely due to the generation of copious amounts of air bubbles in the upper ocean, a fraction of which accumulate as foam at the surface, where they release scavenged surfactants. Field experiments were conducted to test previous theoretical predictions of the variations in optical properties that result from bubble injection in the surface ocean. Variations in remote-sensing reflectance and size distribution of bubbles within the ship-wake zone were determined in three different optical water types: the clear equatorial Pacific Ocean, moderately turbid coastal waters, and very turbid coastal waters, the latter two of which were offshore of New Jersey. Bubbles introduced by moving vessels increased the backscattering in all cases, which in turn enhanced the reflectance over the entire visible and infrared wave bands. The elevated reflectance had different spectral characteristics in the three locations. The color of ship wakes appears greener in the open ocean, whereas little change in color was observed in near-coastal turbid waters, consistent with predictions. Colorless themselves, bubbles increase the reflected radiance and change the color of the ocean in a way that depends on the spectral backscattering and absorption of the undisturbed background waters. For remote observation from aircraft or satellite, the foam and added surfactants further enhance the reflectance to a degree dependent on the illumination and the viewing geometry.

Zhang, Xiaodong; Lewis, Marlon; Bissett, W. Paul; Johnson, Bruce; Kohler, Dave

2004-05-01

347

Kuroshio-induced wake in the lee of Green Island off Taiwan  

NASA Astrophysics Data System (ADS)

The wake of Green Island was investigated in the alongshore flowing Kuroshio east of Taiwan, where current speeds range from 1 to 1.5 ms-1. Vertical profiling with shipboard lowered acoustic Doppler current profiler and conductivity-temperature-depth profiler revealed sizable anomalies in flow and water characteristics in the lee of Green Island. Two different stages of wake evolution were observed from two shipboard surveys. In the first stage, a recirculation developed leeward of the island, followed by a wavy (wave-like) tail that resembled a weak vortex street. In the second stage, a cold eddy probably originating from the leeward side of the island showed up 14 km downstream of the island. The wake water was colder, saltier, higher in chlorophyll-a concentration, and produced isopycnal doming up to 60 m. In the recirculation area, the relative vorticity, either positive or negative, was 10 times of planetary vorticity, and the horizontal divergence or convergence was O (10-4 s-1) on average. Flow divergence and convergence in the wake were expected to form upwelling and downwelling zones, producing a vertical circulation that vertically displaces isotherms. High inverse Richardson number, produced by strong vertical shear of horizontal currents, was associated with intense overturning events in the wake. High vertical shear of horizontal currents drove the mixing. The dissipation rate of turbulent kinetic energy in the overturn regions is O (10-7-10-5) W Kg-1; the corresponding eddy diffusivity is O (10-3-10-1) m2 s-1. The wake water properties are vertically diffused via upwelling and turbulence and can be delivered downstream through eddy shedding or advection. The extent of downstream influence remains to be investigated.

Chang, Ming-Huei; Tang, Tswen Yung; Ho, Chung-Ru; Chao, Shenn-Yu

2013-03-01

348

Prediction of BVI noise patterns and correlation with wake interaction locations  

NASA Astrophysics Data System (ADS)

High resolution fluctuating airloads data were acquired during a test of a contemporary design United Technologies model rotor in the Duits-Nederlandse Windtunnel (DNW). The airloads are used as input to the noise prediction program WOPWOP, in order to predict the blade-vortex interaction (BVI) noise field on a large plane below the rotor. Trends of predicted advancing and retreating side BVI noise levels and directionality as functions of flight condition are presented. The measured airloads have been analyzed to determine the BVI locations on the blade surface, and are used to interpret the predicted BVI noise radiation patterns. Predicted BVI locations are obtained using the free wake model in CAMRAD/JA, the UTRC Generalized Forward Flight Distorted Wake Model, and the UTRC FREEWAKE analysis. These predicted BVI locations are compared with those obtained from the measured pressure data.

Marcolini, Michael A.; Martin, Ruth M.; Lorber, Peter F.; Egolf, T. A.

349

Drag and wakes of freely falling 60° cones at intermediate Reynolds numbers  

NASA Astrophysics Data System (ADS)

The drag coefficient for freely falling cones with a vertex angle of 60° was determined experimentally in the Reynolds number range from 90 to 8×103 and described by empirical equations. The drag was determined by measurement of the terminal velocity of the cones falling through water. Flow-visualization experiments showed the different regimes of the wake structure for a wide range of the Reynolds numbers covering the successive destabilizations of the wake on the way to turbulence. Especially, a very regular staggered array of two rows of ring-shaped hairpin vortices appeared behind the cones in the Reynolds number range from 170 to 235. The Strouhal number was determined in the Reynolds number range from 170 to 1.2×103. The arrangement of the double row of vortex rings and the oscillatory motion of the cones were given in some detail.

Yaginuma, Takayuki; Ito, Hidesato

2008-11-01

350

Study of mechanisms and factors that influence the formation of vortical wake of a heaving airfoil  

NASA Astrophysics Data System (ADS)

A two-dimensional numerical study is performed to investigate the relation between the direction of a deflected wake and the vortex pairing mechanisms. The deflection angle can be correlated with two effective phase velocities defined to represent the trends of symmetry breaking and symmetry holding, respectively. The deflection angle increases with the strength of the vortex pairs, which is associated with the heaving amplitude, frequency, and the free stream Reynolds number. Furthermore, not only the influence of Strouhal number but also those of the two heaving motion components - amplitude and frequency - are studied individually under different Reynolds numbers. The study shows that the deflection angle consistently increases with the difference between the symmetry-breaking phase velocity and symmetry-holding phase velocity.

Zheng, Z. C.; Wei, Z.

2012-10-01

351

System identification of the kinematics of an oscillating cylinder using wake velocities  

NASA Astrophysics Data System (ADS)

A classical problem in vortex-induced vibration is to know the flow field past an oscillating cylinder. In this paper we use system theory to identify the oscillatory behaviour of a circular cylinder from flow variables in the wake. We use numerical simulations (CFD) of the flow past a cylinder oscillating in the cross-flow direction at different oscillation frequencies and amplitudes to construct a transfer function that relates the displacement of the cylinder and the resulting flow field. This transfer function can then be inverted to 'predict' the displacement of the cylinder given the flow field (as determined by simulations or measurements). We investigate this technique in the so-called lock-in region, where the vortex shedding frequency is synchronised with the oscillation frequency of the cylinder.

Runacres, M. C.; De Troyer, T.; Shirzadeh, R.; Guillaume, P.

2013-08-01

352

Large-scale structures in the wake of a circular cylinder  

NASA Astrophysics Data System (ADS)

The authors have reported (1984) that the two-dimensional Karman vortices behind a circular cylinder with diameter d are broken into lengths of about 8(d) and they form chains of spoon-shaped vortex couples. In the present experiment, disks were attached to the cylinder so that the Karman vortices were artificially cut to fixed lengths of 4(d), 6(d), 8(d), and 10(d). The structures with 8(d) were most stable forming a vortex chain with least irregularities, and the wake was much wider than without the disks, while the neighboring vortices with a length 4(d) merged to a scale of 8(d). These results show that the length 8(d) is a unique scale in the deformation of the Karman vortices to a three-dimensional large scale structure.

Yamane, Ryuichiro; Mochimaru, Yoshihiro; Yagita, Miki; Tanaka, Yutaka; Shirakashi, Masataka

1986-11-01

353

Neurophysiology of sleep and wakefulness.  

PubMed

Wakefulness, NREM sleep, and REM sleep are three distinct states of existence. Each state has characteristic behavioral and physiologic patterns,and each has specific neurophysiologic mechanisms associated with its generation and control. Structures in the brainstem use various neurotransmitters to influence higher brain structures in the midbrain and cortex. The ARAS provides cholinergic, noradrenergic, and glutaminergic stimulation to the thalamus, hypothalamus, and basal forebrain resulting in cholinergic and glutaminergic excitation of the cortex. An active cortex that exhibits a characteristic pattern of desynchronized EEG manifests wakefulness. Various factors affect the need and timing of sleep onset. These factors influence the nucleus tractus solitarius, causing its noradrenergic projections to midbrain and forebrain structures to inhibit activity in the ARAS, resulting inactivation of inhibitory GABAergic thalamocortical projections to the cor-tex. During a state of decreased activation, the cortex exhibits a pattern of synchronized EEG. Transition between NREM sleep and REM sleep is controlled by noradrenergic neurons in the loci coeruleus and serotoninergic neurons in the raphe called REM-off cells and cholinergic neurons in the nucleus reticularis pontis oralis called REM-on cells. Other brain structures are involved in generation and control of REM sleep-related phenomena, such as eye movement and muscle atonia. During wakefulness, there is increased sympathetic tone and decreased parasympathetic tone that maintains most organ systems in a state of action or readiness. During NREM sleep, there is decreased sympathetic tone and increased parasympathetic activity that creates a state of reduced activity. REM sleep is characterized by increased parasympathetic activity and variable sympathetic activity associated with increased activation of certain brain functions. The states of wakefulness and sleep are characterized as stages that are defined by stereotypical EEG, EMG, and EOG patterns. Wakefulness stage has an EEG pattern predominated by the alpha rhythm. With onset of stage 1 sleep, the alpha rhythm attenuates, and an EEG pattern of relatively low voltage and mixed frequency is seen. Progression to stage 2 sleep is defined by the appearance of sleep spindles or K-complexes. Further progression into the deepest sleep stages 3 and 4 is defined by the occurrence of high-amplitude, low-frequency EEG activity. The progression of sleep stages occurs in cycles of 60 to 120 minutes throughout the sleep period. Various circadian environmental and ontologic factors affect the pattern of sleep stage occurrence. PMID:16303589

Harris, Cameron D

2005-12-01

354

The Effect of Shape on the Wake of Low-Aspect-Ratio Wall-Mounted Obstacles  

NASA Astrophysics Data System (ADS)

Wall-mounted bodies in boundary layer flows are ubiquitous in nature and engineering applications. We evaluate the role of shape on the wakes around three different low-aspect-ratio wall-mounted obstacles in shallow boundary-layer flow: semi-ellipsoids with the major axis of the base ellipse aligned in the transverse and streamwise directions, and a sphere. Despite their geometric simplicity, the obstacles create extremely complex, highly three-dimensional and unsteady flow fields for which the transport mechanisms of momentum and scalars are still not well-understood. All three obstacles have the same height and the aspect ratios considered are 0.67, 0.89 and 1, respectively. DPIV was used to interrogate the flow. Streamwise structures observed in the mean wake include tip, base, and horseshoe vortex pairs, which vary significantly in strength with changes in obstacle geometry. Significant variation in the strength of these structures with streamwise location suggests a complex connectivity with the mean spanwise arch structure in the near wake. The three-dimensional topology of the mean wake will be discussed.

Hajimirzaie, Seyed; Buchholz, James

2011-11-01

355

All-optical discrete vortex switch  

SciTech Connect

We introduce discrete vortex solitons and vortex breathers in circular arrays of nonlinear waveguides. The simplest vortex breather in a four-waveguide coupler is a nonlinear dynamic state changing its topological charge between +1 and -1 periodically during propagation. We find the stability domain for this solution and suggest an all-optical vortex switching scheme.

Desyatnikov, Anton S. [Nonlinear Physics Center, Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200 (Australia); Dennis, Mark R. [H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL (United Kingdom); Ferrando, Albert [Interdisciplinary Modeling Group, InterTech and Departament d'Optica, Universitat de Valencia, E-46100 Burjassot (Spain)

2011-06-15

356

Evaluation of state-of-the-art parametric building wake models using computational fluid dynamics (CFD) computer codes, and development of building wake, plume rise, and dispersion models  

NASA Astrophysics Data System (ADS)

The close location of most DOE non-reactor nuclear facilities to site boundaries and the potential for having receptors in the proximity of such facilities makes it extremely important to accurately address the impact of plume rise and building wake effects on the consequences to such individuals. Unfortunately, there is no current single computer code or model that adequately address the consequences to receptors postulated to be located within the building wake of such facilities. Existing state-of-the-art models have relied on over- simplistic plume rise and parametric wake models that were developed based on very limited amount of data or assumptions, thus potentially leading to large errors in calculations. Building wake and plume rise models implemented in existing consequence computer codes have been identified and evaluated. These models come from an extensive literature review of dispersion, transport, and consequence modeling of airborne radioactive material releases that extends over 25 years. This dissertation focuses on the evaluation of existing state-of-the-art parametric building wake dispersion models by the use of computational fluid dynamic (CFD) codes, developing potential improvements to such models, and comparing the results of such improvements to those generated by CFD models and models implemented in state- of-the-art computer codes. This dissertation also presents new dispersion models and a new analytical parametric model to deal with transient releases that decay or transform during transport.

Restrepo, Louis Fernando

357

Ion-wake-mediated particle interaction in a magnetized-plasma flow.  

PubMed

The interaction forces between dust grains in a flowing plasma are strongly modified by the formation of ion wakes. Here, we study the interparticle forces mediated by ion wakes in the presence of a strong magnetic field parallel to the ion flow. For increasing magnetic flux densities a continuous decay of the interaction force is observed. This transition occurs at parameters, where the ion cyclotron frequency starts to exceed the ion plasma frequency, which is in agreement with theoretical predictions. The modification of the interparticle forces is important for the understanding of the structure and dynamics of magnetized dusty plasmas. PMID:23030094

Carstensen, Jan; Greiner, Franko; Piel, Alexander

2012-09-27

358

Threshold for Trapping Positrons in the Wake Driven by a Ultra-relativistic Electron Bunch  

SciTech Connect

We have recently proposed a new concept for generating, injecting and accelerating positrons in a plasma using a double-pulse electron bunch. Monte Carlo simulations show that the number of the positrons produced in a foil target has an exponentially decay energy spectrum. The energy threshold for the trapping of these positrons in a ultra-relativistic electron wake is investigated numerically. For a typical 28.5 GeV electron drive bunch, the trapping threshold for the positrons is a few MeV, and therefore a majority of positrons generated in the foil target are focused and accelerated by the plasma wake.

Wang, X.; Muggli, P.; Katsouleas, T. [University of Southern California, Los Angeles, CA 90089 (United States); Ischebeck, R.; Hogan, M. J. [Stanford Linear Accelerator Center, Stanford, CA 94025 (United States); Joshi, C.; Mori, W. B. [University of California at Los Angeles, Los Angeles, CA 90095 (United States)

2009-01-22

359

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

NASA Astrophysics Data System (ADS)

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

Rule, John Allen

1997-08-01

360

Self-preservation in stratified momentum wakes  

Microsoft Academic Search

A general model is described for drag wakes in a linearly stratified fluid, based on the self-preservation of the flow. It is assumed that the buoyancy-controlled self-similar wake expands in the horizontal direction due to turbulent diffusion and in the vertical direction due to viscous diffusion. The mean characteristics of the wake (height, width and velocity defect) are analytically derived

Patrice Meunier; Peter J. Diamessis; Geoffrey R. Spedding

2006-01-01

361

Unsteady turbulent round jets and vortex motion  

NASA Astrophysics Data System (ADS)

A new model to predict the velocity distribution in round jets with time-varying injection profiles has been formulated as an extension of steady jet theory. The approach introduces an effective injection velocity within the jet based on a representative response time. It is assumed that the instantaneous injection velocity affects the velocity within the jet with an exponential response function and that the response time is related to the fluid particle's residence time within the jet, consistent with the theory of translation of jet vortex rings from Helmholtz's vortex motion analysis [P. G. Tait, London Edinburgh Dublin Philos. Mag. J. Sci. 33, 485 (1867)]. The Helmholtz theory is also shown to reduce to the well-known velocity decay rate in the case of steady turbulent gas jets. A Duhamel superposition integral is used to determine the effective injection velocity for time-varying injection rates. The model is tested with different injection profiles and different ambient densities. The results are also compared with numerical results from a computational fluid dynamics code. The comparisons agree very well and the new model is shown to offer an efficient method to predict jet tip penetrations for unsteady jets.

Abani, Neerav; Reitz, Rolf D.

2007-12-01

362

Laser-induced wakes in ion crystals  

NASA Astrophysics Data System (ADS)

Wakes in a Coulomb crystal are produced by ``pushing'' with radiation pressure on a rotating spheroidal cloud of laser-cooled 9Be+ ions. The wakes are stationary in the lab frame and are caused by the interference of ``drum-head'' type oscillations. Velocity images of these wakes are obtained directly through the dependence of the ion fluorescence on Doppler shifts, and new analytical calculations accurately reproduce these experimental wake images. The technique demonstrates a way to excite and study modes, that were not accessible with previous techniques. .

Kriesel, J. M.; Bollinger, J. J.; Mitchell, T. B.; King, L. B.; Dubin, D. H. E.

2002-01-01

363

Tunneling decay of false vortices  

NASA Astrophysics Data System (ADS)

We consider the decay of vortices trapped in the false vacuum of a theory of scalar electrodynamics in 2+1 dimensions. The potential is inspired by models with intermediate symmetry breaking to a metastable vacuum that completely breaks a U(1) symmetry, while in the true vacuum, the symmetry is unbroken. The false vacuum is unstable through the formation of true vacuum bubbles; however, the rate of decay can be extremely long. On the other hand, the false vacuum can contain metastable vortex solutions. These vortices contain the true vacuum inside in addition to a unit of magnetic flux and the appropriate topologically nontrivial false vacuum outside. We numerically establish the existence of vortex solutions which are classically stable; however, they can decay via tunneling. In general terms, they tunnel to a configuration which is a large, thin-walled vortex configuration that is now classically unstable to the expansion of its radius. We compute an estimate for the tunneling amplitude in the semiclassical approximation. We believe our analysis would be relevant to superconducting thin films or superfluids.

Lee, Bum-Hoon; Lee, Wonwoo; MacKenzie, Richard; Paranjape, M. B.; Yajnik, U. A.; Yeom, Dong-han

2013-10-01

364

On the lock-on of vortex shedding to oscillatory actuation around a circular cylinder  

NASA Astrophysics Data System (ADS)

We numerically investigate the influence of sinusoidal flow control on the von Kármán vortex shedding behind a circular cylinder in two-dimensional flow. Actuator location, direction, frequency, and amplitude are varied to examine their effects on the wake and the corresponding change in drag on the cylinder. We place focus on the conditions for which the cylinder wake locks onto the actuation frequency. The lock-on region is found to be consistent with stability horns observed in oscillator dynamics. Under certain conditions, the actuation reduces drag by elongating the wake structure to appear more streamlined than the wake without flow control. In other cases, the use of actuation led to less streamlined wakes, resulting in no significant drag reduction or for some instances in a drag increase. Purely steady and oscillatory actuation components are examined to highlight their individual influence on the lock-on and drag characteristics. We also note that low frequency oscillations are observed for cases in the neighborhood of the lock-on boundaries due to the competition between low and high-drag states.

Munday, Phillip M.; Taira, Kunihiko

2013-01-01

365

Electromagnetic scattering model of the Kelvin wake and turbulent wake by a moving ship  

Microsoft Academic Search

Taking the attenuation character of the Kelvin wake and the limitation of the traditional two-scale method into account, the practical electromagnetic (EM) scattering model of the Kelvin wake is obtained by using a facet-based model; and for a turbulent ship wake, it is produced by dealing with the wave energy loss rate due to turbulence with the width of turbulent

Rong-Qing Sun; Gen Luo; Min Zhang; Chao Wang

2011-01-01

366

Pediatric sleep-wake disorders.  

PubMed

Sleep-wake problems are common during childhood and adolescence. They are of diverse cause, and can contribute significantly to alterations in behavior, cognition, and learning. Obstructive sleep apnea, central hypoventilation syndrome, narcolepsy, periodic hypersomnia, delayed sleep phase syndrome, restless legs syndrome, parasomnias, and sleep disruption consequent to psychiatric disorders are some of the commonly encountered conditions. Some aspects of sleep architecture and its organization change with age and maturation. Diagnostic criteria and sleep laboratory techniques and findings for some childhood sleep disorders differ from those of adults. Most pharmacologic agents used to treat pediatric sleep disorders are off-label. PMID:23099134

Kotagal, Suresh; Chopra, Amit

2012-11-01

367

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

NASA Astrophysics Data System (ADS)

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

Kecskemety, Krista Marie

368

Low-frequency unsteadiness in the vortex formation region of a circular cylinder  

NASA Astrophysics Data System (ADS)

The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = UrefD/?) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of fm = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.

Lehmkuhl, O.; Rodríguez, I.; Borrell, R.; Oliva, A.

2013-08-01

369

Flow past a circular cylinder at low Reynolds number: Oblique vortex shedding  

NASA Astrophysics Data System (ADS)

Oblique shedding in the laminar regime for the flow past a nominally two-dimensional circular cylinder has been investigated numerically via a stabilized finite element method. No-slip condition on one of the sidewalls leads to the formation of a boundary layer which promotes oblique vortex shedding. Computations are carried out for three values of Reynolds number (Re): 60, 100, and 150. Cellular shedding is observed in all cases. Three cells are observed along the span for the Re=60 flow while only two cells are formed at Re=100 and 150. Spotlike vortex dislocations form at the junction of the cells. The frequency of the appearance of the dislocations increases with Re. Cellular shedding leads to low frequency modulation in the time histories of aerodynamic coefficients. Lowest value of drag is achieved at a time instant corresponding to the appearance of a new dislocation in the near wake. The vortex shedding frequency as well as the oblique angle of the primary vortices is found to vary with time for the Re=60 flow. Their variation is also related to the appearance of dislocations in the near wake. It is found that the vortex shedding frequency (St?) is related to the frequency observed for parallel shedding (St0) and the angle of the oblique vortices (?) by the relation: St?=St0 cos ?. This relationship was proposed earlier for the case when the vortex shedding frequency and the oblique angle do not change with time. The velocity fluctuations are found to decrease with increase in ?. For the Re=100 and 150 flow, the oblique angle of the vortices and the shedding frequency outside the end cell do not change with time. However, ? and St? depend on the aspect ratio of the cylinder. The oblique shedding angle, for various lengths of endplate and Re, is found to vary linearly with the thickness of the boundary layer on the side wall.

Behara, Suresh; Mittal, Sanjay

2010-05-01

370

Vortex dynamics in anisotropic traps  

SciTech Connect

We investigate the dynamics of linear vortex lattices in anisotropic traps in two dimensions and show that the interplay between the rotation and the anisotropy leads to a rich but highly regular dynamics.

McEndoo, S.; Busch, Th. [Department of Physics, University College Cork, Cork (Ireland)

2010-07-15

371

The Lunar Wake Current Systems  

NASA Astrophysics Data System (ADS)

Direct impact of the solar wind plasma with the non-conductive body of the Moon, with no atmosphere and no global magnetic field, neutralizes the plasma in the lunar day-side, leaves a plasma void and forms an expanding rarefaction region, confined into a plasma Mach cone downstream. We show that in the transition regions between the plasma void, the rarefaction region, and the interplanetary plasma there are three main currents flowing around these regions in the lunar wake. The generated currents induce magnetic fields within these regions, perturb the field lines there and confine the field perturbations within the lunar Mach cone. We use a three-dimensional, self-consistent hybrid model of plasma (particle ions and fluid electrons) to show the flow of these three currents. First we identify the different plasma regions, separated by the currents, then we show how the currents depend on the interplanetary magnetic field direction. Finally we discuss the current closures in the lunar wake.

Fatemi, Shahab; Holmström, Mats; Futaana, Yoshifumi; Barabash, Stas; Lue, Charles

2013-04-01

372

Numerical analysis of vortex cell efficiency in laminar and turbulent flows past a circular cylinder with embedded rotating bodies  

Microsoft Academic Search

The effect of flow intensification in small-sized vortex cells on the flow pattern in the near wake downstream of a cylinder\\u000a and the cylinder drag in laminar and turbulent flows is analyzed on the basis of a numerical simulation of the two-dimensional\\u000a steady-state flow past a circular cylinder with rotating cylindrical bodies built into the cylinder contour.

S. A. Isaev; Yu. S. Prigorodov; A. G. Sudakov

2000-01-01

373

Analysis of helicopter blade-vortex interaction noise with application to adaptive-passive and active alleviation methods  

Microsoft Academic Search

This study focuses on detection and analysis methods of helicopter blade-vortex interactions (BVI) and applies these methods to two different BVI noise alleviation schemes---an adaptive-passive and an active scheme. A standard free-wake analysis based on relaxation methods is extended in this study to compute high-resolution blade loading, to account for blade-to-blade dissimilarities, and dual vortices when there is negative loading

Lionel Christian Tauszig

2002-01-01

374

Discrete vortex representation of magnetohydrodynamics  

SciTech Connect

We present an alternative approach to statistical analysis of an intermittent ideal MHD fluid in two dimensions, based on the hydrodynamical discrete vortex model applied to the Elsasser variables. The model contains negative temperature states which predict the formation of magnetic islands, but also includes a natural limit under which the equilibrium states revert to the familiar twin-vortex states predicted by hydrodynamical turbulence theories. Numerical dynamical calculations yield equilibrium spectra in agreement with the theoretical predictions.

Kinney, R.; Tajima, T.; Petviashvili, N. [Texas Univ., Austin, TX (United States). Inst. for Fusion Studies; McWilliams, J.C. [National Center for Atmospheric Research, Boulder, CO (United States)

1993-02-01

375

Ion cyclotron waves in Io's wake region  

NASA Astrophysics Data System (ADS)

When Galileo flew almost directly through the center of Io's wake in December 1995 it encountered a region of depressed field strength, devoid of the strong ion cyclotron waves that were present inbound and outbound on either side of the wake. In August 2001 Galileo made an almost orthogonal pass, parallel to the wake axis, passing within 200 km of the Io surface, and staying close to 1 RIo above the center of the wake. Downstream from Io (but not upstream) strong ion cyclotron waves were seen. The very low frequency of these waves, well below the local SO 2+ gyro frequency, suggests that they were created in the low field of Io's wake. Thus in contrast to implications of the December 1995, near-Io, wake passage, ion cyclotron waves are produced in the Io wake. Furthermore, these waves persisted beyond 6.7 RIo downstream, with little sign of recovery in frequency, indicating that Io's depressed field wake extends well downstream of Io.

Russell, C. T.; Blanco-Cano, X.; Kivelson, M. G.

2003-03-01

376

Turbulence characteristics in wind-turbine wakes  

Microsoft Academic Search

An analysis of the evolution of turbulence characteristics in wind-turbine wakes has been carried out. Based on experimental results and on numerical results obtained with a CFD code, complemented with some theoretical considerations, simple analytical expressions are proposed for the estimation of the turbulent kinetic energy, k, and its dissipation rate, ?. To obtain the turbulence spectra in the wake

A. Crespo; J. Herna´ndez

1996-01-01

377

Modelling Wind Turbine Wakes in Complex Terrain  

Microsoft Academic Search

Existing engineering-type models for wakes have been developed and calibrated for flat terrain applications. However to consider the effects of the atmospheric boundary layer in a complex terrain environment, including flow separation and wind rose narrowing, requires the application of advanced methods. A method of modeling the wind turbine wakes using a Navier-Stokes solver along with the k-? turbulence model

J. M. Prospathopoulos; E. S Politis; P. K. Chaviaropoulos

378

Dynamics and control of hydrofoil wakes  

Microsoft Academic Search

The problem of rotor-stator interaction has been an issue in the turbomachinery field for more than forty years. Manipulation of the stator wakes is one method to minimize the problem. In order to explore this concept, a comprehensive experimental program was carried out in a water tunnel utilizing a series of NACA 0015 hydrofoils. Baseline wake data were collected with

Roger Arndt; Morten Kjeldsen; Martin Wosnik

2006-01-01

379

Wake Filling by Active Tail Articulation  

Microsoft Academic Search

In a viscous fluid, the no slip boundary condition results in a surface drag force on a moving streamlined body, such as a hydrofoil, which causes a downstream wake velocity defect. In this paper, experimental results are presented which show that articulation of a trailing edge tail flap on a hydrofoil is sufficient to diminish the mean wake velocity defect.

Daniel Macumber; David Beal; Anuradha Annaswamy; Charles Henoch; Stephen Huyer

2004-01-01

380

Transition Wake-Fields in Resistive Tube  

Microsoft Academic Search

The transition wake fields excited by relativistic point charge in cylindrical resistive round pipe are studied. The problem has been solved for the infinite pipe with abrupt change of the walls conductivity from perfect boundary condition to resistive. The analytical presentation of the longitudinal monopole wake field is given.

M. Ivanyan; V. Tsakanov; A. Tsakanian

2005-01-01

381

Unpinning triggers for superfluid vortex avalanches  

NASA Astrophysics Data System (ADS)

The pinning and collective unpinning of superfluid vortices in a decelerating container is a key element of the canonical model of neutron star glitches and laboratory spin-down experiments with helium II. Here the dynamics of vortex (un-)pinning is explored using numerical Gross-Pitaevskii calculations, with a view to understanding the triggers for catastrophic unpinning events (vortex avalanches) that lead to rotational glitches. We explicitly identify three triggers: rotational shear between the bulk condensate and the pinned vortices, a vortex proximity effect driven by the repulsive vortex-vortex interaction, and sound waves emitted by moving and repinning vortices. So long as dissipation is low, sound waves emitted by a repinning vortex are found to be sufficiently strong to unpin a nearby vortex. For both ballistic and forced vortex motion, the maximum inter-vortex separation required to unpin scales inversely with pinning strength.

Warszawski, L.; Melatos, A.; Berloff, N. G.

2012-03-01

382

Vortex Dislocations and Body Forces in Flow-Structure Interactions  

NASA Astrophysics Data System (ADS)

DNS is used in order to simulate 3D flow past stationary, rigid or flexible cylinders subject to forced vibrations (FV) or vortex induced vibrations (VIV). Different flow and structural parameters such as shear rate, Reynolds number, cylinder aspect ratio and end-conditions are investigated in order to examine structural responses and hydrodynamic forces. The relationship of these quantities with the dynamics of the near wake flow is studied. In the case of VIV due to uniform inflow, at turbulent Reynolds number of 1000, past cylinders with a small or large aspect ratio we propose an appropriate modification to the drag coefficient empirical formula of Skop, Griffin & Ramberg (1977). For the stationary and the FV cases subject to shear inflow we obtained cells of constant frequency. These cells are correlated with significant variations in the drag coefficient along the span of the cylinder.

Lucor, Didier; Evangelinos, Constantinos; Karniadakis, George Em

1999-11-01

383

Propeller tip and hub vortex dynamics in the interaction with a rudder  

NASA Astrophysics Data System (ADS)

In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1-11, 2006a, Exp Fluids 46(1):147-1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments, the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment of the vortex filaments in the pressure and suction sides of the appendage.

Felli, Mario; Falchi, Massimo

2011-11-01

384

Propulsive force calculations in swimming frogs. II. Application of a vortex ring model to DPIV data.  

PubMed

Frogs propel themselves by kicking water backwards using a synchronised extension of their hind limbs and webbed feet. To understand this propulsion process, we quantified the water movements and displacements resulting from swimming in the green frog Rana esculenta, applying digital particle image velocimetry (DPIV) to the frog's wake. The wake showed two vortex rings left behind by the two feet. The rings appeared to be elliptic in planform, urging for correction of the observed ring radii. The rings' long and short axes (average ratio 1.75:1) were about the same size as the length and width of the propelling frog foot and the ellipsoid mass of water accelerated with it. Average thrust forces were derived from the vortex rings, assuming all propulsive energy to be compiled in the rings. The calculated average forces (F(av)=0.10+/-0.04 N) were in close agreement with our parallel study applying a momentum-impulse approach to water displacements during the leg extension phase. We did not find any support for previously assumed propulsion enhancement mechanisms. The feet do not clap together at the end of the power stroke and no "wedge-action" jetting is observed. Each foot accelerates its own water mantle, ending up in a separate vortex ring without interference by the other leg. PMID:15802668

Stamhuis, Eize J; Nauwelaerts, Sandra

2005-04-01

385

Wake dynamics and fluid forces of turning maneuvers in sunfish.  

PubMed

While experimental analyses of steady rectilinear locomotion in fishes are common, unsteady movement involving time-dependent variation in heading, speed and acceleration probably accounts for the greatest portion of the locomotor time budget. Turning maneuvers, in particular, are key elements of the unsteady locomotor repertoire of fishes and, by many species, are accomplished by generating asymmetrical forces with the pectoral fins. The development of such left-right asymmetries in force production is a critical and as yet unstudied aspect of aquatic locomotor dynamics. In this paper, we measure the fluid forces exerted by the left and right pectoral fins of bluegill sunfish (Lepomis macrochirus) during turning using digital particle image velocimetry (DPIV). DPIV allowed quantification of water velocity fields, and hence momentum, in the wake of the pectoral fins as sunfish executed turns; forces exerted during turning were compared with those generated by the immediately preceding fin beats during steady swimming. Sunfish generate the forces required for turning by modulating two variables: wake momentum and pectoral fin stroke timing. Fins on opposite sides of the fish play functionally distinct roles during turning maneuvers. The fin nearer the stimulus inducing the turn (i.e. the strong side fin) generates a laterally oriented vortex ring with a strong central jet whose associated lateral force is four times greater than that produced during steady swimming. Little posterior (thrust) force is generated by the strong-side fin, and this fin therefore acts to rotate the body away from the source of the stimulus. The contralateral (weak-side) fin generates a posteriorly oriented vortex ring with a thrust force nine times that produced by the fin during steady swimming. Minimal lateral force is exerted by the weak-side fin, and this fin therefore acts primarily to translate the body linearly away from the stimulus. Turning with the paired fins is not simply steady swimming performed unilaterally. Instead, turning involves asymmetrical fin movements and fluid forces that are distinct in both direction and magnitude from those used to swim forward at constant speed. These data reflect the plasticity of the teleost pectoral fin in performing a wide range of steady and unsteady locomotor tasks. PMID:11171296

Drucker, E G; Lauder, G V

2001-02-01

386

An examination of wake effects and power production for a group of large wind turbines  

NASA Astrophysics Data System (ADS)

Data from a group of three MOD-2 wind turbines and two meteorological towers at Goodnoe Hills were analyzed to evaluate turbine power output and wake effects (losses in power production due to operation of upwind turbines), and atmospheric factors influencing them. The influences of variations in the ambient wind speed, wind direction, and turbulence intensity were the primary factors evaluated. Meteorological and turbine data collected at the Goodnoe Hills site from April 1 to October 17, 1985, were examined. Wind data from the two meteorological towers were evaluated to estimate the effect of a wake from an upwind turbine on the wind flow measured at the downwind tower. Maximum velocity deficits were about 25 percent and 12 percent at downwind distances of 5.8 and 8.3 rotor diameters (D), respectively. However, the maximum deficits at 5.8 D were about 14 degrees off the centerline orientation between the turbine and the tower, indicating significant wake curvature. Velocity deficits were found to depend on the ambient wind speed, ranging from 27 percent at lower speeds (15 to 25 mph) to 20 percent at higher speeds (30 to 35 mph). Turbulence intensity increases dramatically in the wake by factors of about 2.3 and 1.5 over ambient and conditions at 5.8 D and 8.3 D, respectively. An analysis of the ambient (non-wake) power production for all three turbines showed that the MOD-2 power output depends, not only on wind speed, but also on the turbulence intensity. At wind speeds below rated, there was a dramatic difference in turbine power output between low and high turbulence intensities for the same wind speed. One of the turbines had vortex generators on the blades. This turbine produced from 10 percent to 13 percent more power than the other two turbines when speeds were from 24 to 31 mph.

Elliott, D. L.; Buck, J. W.; Barnard, J. C.

1988-04-01

387

Rolling with the flow: bumblebees flying in unsteady wakes.  

PubMed

Our understanding of how variable wind in natural environments affects flying insects is limited because most studies of insect flight are conducted in either smooth flow or still air conditions. Here, we investigate the effects of structured, unsteady flow (the von Karman vortex street behind a cylinder) on the flight performance of bumblebees (Bombus impatiens). Bumblebees are 'all-weather' foragers and thus frequently experience variable aerial conditions, ranging from fully mixed, turbulent flow to unsteady, structured vortices near objects such as branches and stems. We examined how bumblebee flight performance differs in unsteady versus smooth flow, as well as how the orientation of unsteady flow structures affects their flight performance, by filming bumblebees flying in a wind tunnel under various flow conditions. The three-dimensional flight trajectories and orientations of bumblebees were quantified in each of three flow conditions: (1) smooth flow, (2) the unsteady wake of a vertical cylinder (inducing strong lateral disturbances) and (3) the unsteady wake of a horizontal cylinder (inducing strong vertical disturbances). In both unsteady conditions, bumblebees attenuated the disturbances induced by the wind quite effectively, but still experienced significant translational and rotational fluctuations as compared with flight in smooth flow. Bees appeared to be most sensitive to disturbance along the lateral axis, displaying large lateral accelerations, translations and rolling motions in response to both unsteady flow conditions, regardless of orientation. Bees also displayed the greatest agility around the roll axis, initiating voluntary casting maneuvers and correcting for lateral disturbances mainly through roll in all flow conditions. Both unsteady flow conditions reduced the upstream flight speed of bees, suggesting an increased cost of flight in unsteady flow, with potential implications for foraging patterns and colony energetics in natural, variable wind environments. PMID:24031057

Ravi, Sridhar; Crall, James D; Fisher, Alex; Combes, Stacey A

2013-09-12

388

Numerical and experimental analysis of the near wake flow over a square cylinder  

NASA Astrophysics Data System (ADS)

The flow that develops behind a cylinder is very complex because it is fully three-dimensional, unsteady, including transition regions to turbulence as well as flow separations along the sidewall. The formation of a vortex street is generally considered to be the result of a coupling between KelvinHelmholtz instabilities within the separated shear layers and the Karman instability in the near wake. In the present paper we propose a joint experimental / numerical study in order to investigate the flow features in the near wall region of a square cylinder at Re = 21400 (ERCOFTAC benchmark). The interaction between KH vortical structures in the separating shear layer and Karman vortex shedding in the near wake will be discussed based on both visualisations and frequency analysis. In particular, the dependency with Reynolds number of the ratio from the shear layer frequency to the fundamental Karman frequency by Bloor (1964) will be investigated for the square cylinder. The controversial resulting square root law discussed by Rajagopalan and Antonia (2005) will be focused for the square cylinder case as well.

Serre, Eric; Minguez, Matthieu; Brun, Christophe; Pasquetti, Richard

2010-11-01

389

Wake dynamics of external flow past a curved circular cylinder with the free stream aligned with the plane of curvature  

NASA Astrophysics Data System (ADS)

Three-dimensional spectral/hp computations have been performed to study the fundamental mechanisms of vortex shedding in the wake of curved circular cylinders at Reynolds numbers of 100 and 500. The basic shape of the body is a circular cylinder whose centreline sweeps through a quarter section of a ring and the inflow direction lies on the plane of curvature of the quarter ring: the free stream is then parallel to the geometry considered and the part of the ring that is exposed to it will be referred to as the . Different configurations were investigated with respect to the leading-edge orientation. In the case of a convex-shaped geometry, the stagnation face is the outer surface of the ring: this case exhibited fully three-dimensional wake dynamics, with the vortex shedding in the upper part of the body driving the lower end at one dominant shedding frequency for the whole cylinder span. The vortex-shedding mechanism was therefore not governed by the variation of local normal Reynolds numbers dictated by the curved shape of the leading edge. A second set of simulations were conducted with the free stream directed towards the inside of the ring, in the so-called concave-shaped geometry. No vortex shedding was detected in this configuration: it is suggested that the strong axial flow due to the body's curvature and the subsequent production of streamwise vorticity plays a key role in suppressing the wake dynamics expected in the case of flow past a straight cylinder. The stabilizing mechanism stemming from the concave curved geometry was still found to govern the wake behaviour even when a vertical extension was added to the top of the concave ring, thereby displacing the numerical symmetry boundary condition at this point away from the top of the deformed cylinder. In this case, however, the axial flow from the deformed cylinder was drawn into the wake of vertical extension, weakening the shedding process expected from a straight cylinder at these Reynolds numbers. These considerations highlight the importance of investigating flow past curved cylinders using a full three-dimensional approach, which can properly take into account the role of axial velocity components without the limiting assumptions of a sectional analysis, as is commonly used in industrial practice. Finally, towing-tank flow visualizations were also conducted and found to be in qualitative agreement with the computational findings.

Miliou, A.; de Vecchi, A.; Sherwin, S. J.; Graham, J. M. R.

390

Dynamic vortex dust structures in a nuclear-track plasma  

NASA Astrophysics Data System (ADS)

Results are presented from Monte Carlo calculations of the electric charge on dust grains in a plasma produced during the slowing down of radioactive decay products of californium nuclei in neon. The dust grain charging is explained as being due to the drift of electrons and ions in an external electric field. It is shown that the charges of the grains depend on their coordinates and strongly fluctuate with time. The time-averaged grain charges agree with the experimental data obtained on ordered liquid-like dust structures in a nuclear-track plasma. The time-averaged dust grain charges are used to carry out computer modelling of the formation of dynamic vortex structures observed in experiments. Evidence is obtained for the fact that the electrostatic forces experienced by the dust grains are potential in character. The paper is supplemented by a video clip showing the typical dynamics of the simulated vortex dust structure.

Rykov, V. A.; Khudyakov, A. V.; Filinov, V. S.; Vladimirov, V. I.; Deputatova, L. V.; Krutov, D. V.; Fortov, V. E.

2003-10-01

391

Numerical simulation of vortex ring formation in the presence of background flow with implications for squid propulsion  

NASA Astrophysics Data System (ADS)

Numerical simulations are used to study laminar vortex ring formation under the influence of background flow. The numerical setup includes a round-headed axisymmetric body with an opening at the posterior end from which a column of fluid is pushed out by a piston. The piston motion is explicitly included into the simulations by using a deforming mesh. A well-developed wake flow behind the body together with a finite-thickness boundary layer outside the opening is taken as the initial flow condition. As the jet is initiated, different vortex evolution behavior is observed depending on the combination of background flow velocity to mean piston velocity (U/Up) ratio and piston stroke to opening diameter (Lm/D) ratio. For low background flow (U/Up =0.2) with a short jet (Lm/D =6), a leading vortex ring pinches off from the generating jet, with an increased formation number. For intermediate background flow (U/Up =0.5) with a short jet (Lm/D =6), a leading vortex ring also pinches off but with a reduced formation number. For intermediate background flow (U/Up =0.5) with a long jet (Lm/D =15), no vortex ring pinch-off is observed. For high background flow (U/Up =0.75) with both a short (Lm/D =6) and a long (Lm/D =15) jet, the leading vortex structure is highly deformed with no single central axis of fluid rotation (when viewed in cross-section) as would be expected for a roll-up vortex ring. For Lm/D =6, the vortex structure becomes isolated as the trailing jet is destroyed by the opposite-signed vorticity of the background flow. For Lm/D =15, the vortex structure never pinches off from the trailing jet. The underlying mechanism is the interaction between the vorticity layer of the jet and the opposite-signed vorticity layer from the initial wake. This interaction depends on both U/Up and Lm/D. A comparison is also made between the thrust generated by long, continuous jets and jet events constructed from a periodic series of short pulses having the same total mass flux. Force calculations suggest that long, continuous jets maximize thrust generation for a given amount of energy expended in creating the jet flow. The implications of the numerical results are discussed as they pertain to adult squid propulsion, which have been observed to generate long jets without a prominent leading vortex ring.

Jiang, Houshuo; Grosenbaugh, Mark A.

2006-04-01

392

Ozone profile measurements within, at the edge of, and outside the Antarctic polar vortex in the spring of 1988  

SciTech Connect

Ozone and temperature were measured during 38 balloon soundings at McMurdo Station, Antarctica (78{degree}S), in the spring of 1988. Because of the motion of the Antarctic polar vortex, measurements were obtained within, at the edge of, and outside the vortex. Although the polar vortex did not remain over McMurdo as it did in 1986 and 1987, it was overhead long enough to establish that ozone depletion was less extensive and ended earlier than in either 1986 or 1987. In the vortex the ozone mixing ratio at 18 km decayed with an exponential half-life of 29 days compared to 25 and 12 days in 1986 and 1987. While ozone partial pressure in the 16-18 km layer decayed to values as low as 10 nbar in 1986 and 3 nbar in 1987, ozone partial pressure dropped to only 60-70 nbar in 1988 in the depleted region, a reduction of 30 to 50%. Even with these differences in degree of ozone depletion there were similarities to previous measurements. Ozone depletion was caused by a sink between 12 and 20 km, and primary depletion was episodic, occurring in periods of <10 days. Measurements at the edge of the vortex displayed the ozone layering observed in 1986 and 1987 and suggest the exchange of ozone rich and poor air across the vortex wall in the 12-20 km layer. Outside the vortex, vertical profiles displayed a region of high ozone and constant temperature above 20 km.

Deshler, T.; Hofmann, D.J.; Hereford, J.V. (Univ. of Wyoming, Laramie (USA))

1990-06-20

393

The hairpin vortex illusion  

NASA Astrophysics Data System (ADS)

It has long been customary to assume that vortical structures in turbulent flows are synonymous with regions of rotational motion. Mathematical implementations of this idea using numerical and experimental velocity data from turbulent boundary layers reveal the presence of hairpin vortices, both singly and in groups called packets. However, vorticity may be present that does not directly cause rotation, and by failing to take this into account it is possible to be misled as to the true nature of the vortical structures. In this work a vortex filament scheme is applied to boundary layer flow that allows for a view of structures unrestrained by the requirement that they be regions of rotational motion. It is found that furrow-like streamwise aligned eruptions of the nominally spanwise near-wall vorticity overlying low-speed streaks are the primary structural feature of the transitioning boundary layer. These progress from an arch-like form at their upstream end to either one or two-lobed mushroom-shapes at their downstream end. The rotational motion associated with the furrows has the appearance of hairpins. Mushroom-shaped structures rapidly breakdown into complex forms in the post-transitional region that may have rotational signatures similar to that of packets.

Bernard, Peter S.

2011-12-01

394

Aerodynamic, aeroacoustic, and aeroelastic investigations of airfoil-vortex interaction using large-eddy simulation  

NASA Astrophysics Data System (ADS)

In helicopters, vortices (generated at the tip of the rotor blades) interact with the next advancing blades during certain flight and manoeuvring conditions, generating undesirable levels of acoustic noise and vibration. These Blade-Vortex Interactions (BVIs), which may cause the most disturbing acoustic noise, normally occur in descent or high-speed forward flight. Acoustic noise characterization (and potential reduction) is one the areas generating intensive research interest to the rotorcraft industry. Since experimental investigations of BVI are extremely costly, some insights into the BVI or AVI (2-D Airfoil-Vortex Interaction) can be gained using Computational Fluid Dynamics (CFD) numerical simulations. Numerical simulation of BVI or AVI has been of interest to CFD for many years. There are still difficulties concerning an accurate numerical prediction of BVI. One of the main issues is the inherent dissipation of CFD turbulence models, which severely affects the preservation of the vortex characteristics. Moreover this is not an issue only for aerodynamic and aeroacoustic analysis but also for aeroelastic investigations as well, especially when the strong (two-way) aeroelastic coupling is of interest. The present investigation concentrates mainly on AVI simulations. The simulations are performed for Mach number, Ma = 0.3, resulting in a Reynolds number, Re = 1.3 x 106, which is based on the chord, c, of the airfoil (NACA0012). Extensive literature search has indicated that the present work represents the first comprehensive investigation of AVI using the LES numerical approach, in the rotorcraft research community. The major factor affecting the aerodynamic coefficients and aeroacoustic field as a result of airfoil-vortex interaction is observed to be the unsteady pressure generated at the location of the interaction. The present numerical results show that the aerodynamic coefficients (lift, moment, and drag) and aeroacoustic field are strongly dependent on the airfoil-vortex vertical miss-distance, airfoil angle of attack, vortex characteristics, and aeroelastic response of airfoil to airfoil-vortex interaction. A decay of airfoil-vortex interactions with the increase of vertical miss-distance and angle of attack was observed. Also, a decay of airfoil-vortex interactions is observed for the case of a flexible structure when compared with the case of a rigid structure. The decay of vortex core size produces a decrease in the aerodynamic coefficients.

Ilie, Marcel

395

Vorticity and potential vorticity in mountain wakes  

SciTech Connect

A wake is traditionally defined as the region of nearly stagnant flow downstream of a body in a uniform stream. In a stratified fluid, the motions and density surfaces downstream of an obstacle become primarily horizontal; the vertical component of the vorticity associated with the wake, coexisting with the stable vertical density stratification, implies that there is potential vorticity (PV) in the lake. Recent work has demonstrated that dissipation aloft, associated with a breaking mountain wave over an isolated peak, produces a dipole in PV downstream; the dipolar vertical vorticity of the wake is associated with the PV dipole. Although one may infer the existence of vorticity downstream, the PV argument is silent on the question. Where does the wake vorticity come from? To answer this question, a weakly nonlinear model for PV production and wake formation in the case of a small-amplitude mountain has been analyzed, and numerical simulations pertaining to the strongly nonlinear large-amplitude case have been carried out. The simple model indicates that even with dissipation in the system, the vertical vorticity of the wake arises through the tilting of baroclinically generated horizontal vorticity by the dissipating mountain wave. This analysis shows that there need not be any direct effect of friction in the vorticity equation on the tilting term. Analysis of numerical simulations of the large-amplitude case shows that the conclusions from the weakly nonlinear model regarding the source of wake vorticity continue to hold in the strongly nonlinear type.

Rotunno, R.; Grubisic, V.; Smolarkiewicz, P.K. [National Center for Atmospheric Research, Boulder, CO (United States)

1999-08-15

396

Temperatures of wakes in Saturn's A ring  

NASA Astrophysics Data System (ADS)

The physical temperatures of the Saturn's A ring measured by the Cassini Composite Infrared Spectrometer (CIRS) show quadraople azimuthal modulations besides temperature drops in Saturn's shadow. These azimuthal modulations are likely to be caused by self-gravity wakes. In this paper, we develop a new thermal model in which wakes are modeled as elliptical cylinders ignoring inter-wake particles. All the heat fluxes are calculated explicitly taking into account inter-wake shadowing and heating. We apply our model to azimuthal scans of the A ring obtained by the CIRS. The thermal inertia estimated from the eclipse data (data only inside and near Saturn's shadow) of the low phase scans is found to be about 10 in MKS units. With this value of the thermal inertia, the amplitude of the azimuthal temperature modulation is overestimated in our model as compared with those observed. This is likely to be because our model ignores inter-wake particles. The bolometric reflectance of wakes is estimated to be 0.35-0.4 although lower values are required to reproduce temperatures at low solar phase angles. This apparent phase dependence of the reflectance indicates that roughness on the wake surfaces is necessary.

Morishima, Ryuji; Spilker, L.; Turner, N.; Cassini CIRS ring Team

2013-10-01

397

Status of wake and array loss research  

SciTech Connect

In recent years, many projects have evaluated wind turbine wake effects and resultant array losses in both Europe and the United States. This paper examines the status of current knowledge about wake effects and array losses and suggests future research. Single-turbine wake characteristics have been studied extensively and are generally described well by existing theoretical models. Field measurements of wake effects in wind turbine arrays are largely limited to small arrays, with 2 to 4 rows of turbines. Few data have been published on wake effects within large arrays. Measurements of wake deficits downwind of large arrays that deficits are substantially larger and extend farther downwind than expected. Although array design models have been developed, these models have been tested and verified using only limited data from a few rows of wind turbines in complex terrain, whereas some of the largest arrays have more than 40 rows of wind turbines. Planned cooperative efforts with the wind industry will obtain existing data relevant to analyzing energy deficits within large arrays and identifying data sets for potential use in array model verification efforts. Future research being considered include a cooperative research experiment to obtain more definitive data on wake deficits and turbulence within and downwind of large arrays. 16 refs., 9 figs., 1 tab.

Elliott, D.L.

1991-09-01

398

Large HAWT wake measurement and analysis  

NASA Astrophysics Data System (ADS)

From the theoretical fluid dynamics point of view, the wake region of a large horizontal-axis wind turbine has been defined and described, and numerical models of wake behavior have been developed. Wind tunnel studies of single turbine wakes and turbine array wakes have been used to verify the theory and further refine the numerical models. However, the effects of scaling, rotor solidity, and topography on wake behavior are questions that remain unanswered. In the wind tunnel studies, turbines were represented by anything from scaled models to tea strainers or wire mesh disks whose solidity was equivalent to that of a typical wind turbine. The scale factor compensation for the difference in Reynolds number between the scale model and an actual turbine is complex, and not typically accounted for. Though it is wise to study the simpler case of wakes in flat topography, which can be easily duplicated in the wind tunnel, current indications are that wind turbine farm development is actually occurring in somewhat more complex terrain. Empirical wake studies using large horizontal-axis wind turbines have not been thoroughly composited, and, therefore, the results have not been applied to the well-developed theory of wake structure. The measurement programs have made use of both in situ sensor systems, such as instrumented towers, and remote sensors, such as kites and tethered, balloonborne anemometers. We present a concise overview of the work that has been performed, including our own, which is based on the philosophy that the MOD-2 turbines are probably their own best detector of both the momentum deficit and the induced turbulence effect downwind. Only the momentum deficit aspects of the wake/machine interactions have been addressed. Both turbine power output deficits and wind energy deficits as measured by the onsite meteorological towers have been analyzed from a composite data set. The analysis has also evidenced certain topographic influences on the operation of spatially diverse wind turbines.

Miller, A. H.; Wegley, H. L.; Buck, J. W.

1995-05-01

399

Flow visualization study of role of coherent structures in a tab wake  

NASA Astrophysics Data System (ADS)

A simple surface-mounted tapered tab has recently attracted fluids research both for its ability to enhance mixing and heat transfer (for which it is known as high-efficiency vortab mixer) and for its generation of coherent structures that are topologically similar to those found in natural turbulent boundary layers. Two types of structures, namely pressure-driven counter-rotating vortex pair (CVP) and hairpin vortices were previously identified in the tab wake, but the contribution of individual structures to the mixing enhancement process and how they interact are not known. In the present study, flow visualization using a planar laser-induced fluorescence (PLIF) technique is carried out to probe into the flow dynamics in the wake of the mixing tab. By injecting dye at an appropriate location and illuminating the flow in various planes, the structures are visualized clearly. The results show, in contrary to earlier observations, that the two types of structures dominate different regions. At the Reynolds number of 700 based on tab height ( h), the CVP has more influence in the region 0< x/ h<1.5. The counter-rotating action of the vortex pair induces a pumping action along the symmetry by which the low-speed fluid from the boundary layer is transported to the high-speed outer shear layer. The displaced fluid is entrained by the recirculating counter-rotating vortices and is mixed well while convecting downstream. Beyond this region, fully developed hairpin structures contribute more to mixing in a similar way as in a turbulent boundary layer. It is observed that the shedding frequency of hairpin vortices is slightly higher than the pumping frequency of the counter-rotating vortex pair. It is also observed that the hairpin structures loses their identity beyond x/ h>15, and there is no large-scale cross-stream mixing visible in this region.

Elavarasan, R.; Meng, Hui

2000-09-01

400

Complex wakes behind objects in multispecies plasmas  

NASA Astrophysics Data System (ADS)

Many space, astrophysical, technological and laboratory plasmas are multispecies. Using particle-in-cell (PIC) numerical simulations, the novel features of the wake structures behind finite-sized objects in multispecies collisionless plasma flows are studied. In electropositive plasmas, different ion species can form separate wakes with distinct ion focus regions, and give rise to polarization of the plasma. In electronegative plasmas, with a large fraction of heavy negative ions, the polarization can be stronger due to substantial ion density reduction in the wake.

Miloch, W. J.; Vladimirov, S. V.; Yaroshenko, V. V.

2013-01-01

401

Point vortex dynamics in a magnetized plasma  

Microsoft Academic Search

A self-consistent theory describing vortex-wave dynamics in a magnetized plasma has been formulated based on the Hasegawa-Mima equation by extending the modulated point vortex model so as to include the vortex-wave interactions. The energy and entrophy are shown to be conserved in contrast with the previous modulated point vortex description. Dynamical behaviors of vortices under the interaction with waves are

Mitsuo Kono; Hideaki Shibahara; Kentaro Yabuki

1994-01-01

402

Wake-induced unsteady flows: Their impact on rotor performance and wake rectification  

SciTech Connect

The impact of wake-induced unsteady flows on blade row performance and the wake rectification process is examined by means of numerical simulation. The passage of a stator wake through a downstream rotor is first simulated using a three-dimensional unsteady viscous flow code. The results from this simulation are used to define two steady-state inlet conditions for a three-dimensional viscous flow simulation of a rotor operating in isolation. The results obtained from these numerical simulations are then compared to those obtained form the unsteady simulation both to quantify the impact of the wake-induced unsteady flow field on rotor performance and to identify the flow processes which impact wake rectification. Finally, the results from this comparison study are related to an existing model, which attempts to account for the impact of wake-induced unsteady flows on the performance of multistage turbomachinery.

Adamczyk, J.J. [National Aeronautics and Space Administration, Brook Park, OH (United States). Lewis Research Center; Celestina, M.L. [Sverdrup Technology, Inc., Brook Park, OH (United States). Dept. of Aeromechanics; Chen, J.P. [Mississippi State Univ., MS (United States). NSF Engineering Research Center

1996-01-01

403

Numerical Simulation of Ultrafine Particle-Laden Cylinder Wake Flow with Coherent Structures  

NASA Astrophysics Data System (ADS)

The phenomena of the wake flows containing ultrafine particles such as dust and pollutant in the atmosphere are usually observed downwind of the mountains and buildings. Different from the usually heavy particles, the ultrafine particles suspended in fluid undergo the processes of nucleation, growth and coagulation. Coherent structure in typical occurrence of the stretching of the Karman vortex street can be clearly seen in above mentioned gas-particle wakes. The aim of the present study is to explore the effects of coherent structures on the coagulation and growth of ultrafine particles suspended in wake flow. The particle field is represented by employing a moment method to approximate the ultrafine particle general dynamic equation. The fluid flow is obtained by solving the continuity and momentum equations with the large eddy simulation method and the subgrid-scale stresses were parametrized using the dynamic eddy viscosity model. The calculated contours of vorticity were compared with the relevant experimental results. The instantaneous spatial-temporal distribution of the particles are given and analyzed. The effects of the coherent structure on the diffusion and distribution of particle number concentration, polydispersity are discussed in detail.

Wang, Changbin; Zhi, Shujie; Wan, Zhanhong; Sun, Zhilin; Ding, Hai

404

On the wake of a circular cylinder with nodal and saddle attachment  

NASA Astrophysics Data System (ADS)

Flow field of a cylinder with a mid-span curvature was experimentally investigated in a wind tunnel and a water tunnel. The azimuthal orientation of the cylinder was changed to obtain a nodal, saddle and a mixed nodal-saddle type of flow attachment. Surface flow topology suggested that the nature of the attachment strongly influenced the spanwise distributions of foci structures that play a significant role in introducing three-dimensionality in the immediate wake. Flow visualization in the water tunnel revealed that the length of a vortex formation region also followed the changes in the nature of the attachment. A symmetric shedding of vortices was observed with a saddle type of attachment. Wake mean velocity profiles showed that the velocity defect and therefore the drag of a curved cylinder was minimum for nodal, and maximum for saddle type of attachment. Nomenclature of the wake was compared with asymptotic profiles and equilibrium parameters. Approach to self-preservation, similarity and other features are discussed.

Ahmed, Anwar

2010-01-01

405

Turnable CO2 laser measurements of composition and concentrations in multicomponent wake behind supersonic civil aircraft  

NASA Astrophysics Data System (ADS)

The paper describes experimental and theoretical investigations on tunable CO2-laser-assisted measurements of the composition and concentrations in multicomponent wake behind supersonic civil aircraft. An experimental facility based on a tunable CO2 laser and multipass cell is presented and results of experiments on determination of substances typical for civil aircraft wake (carbon and nitrogen oxides, ozone, water vapor, etc.) are given. The conditions characteristic of the aircraft wake are simulated. A possibility to perform real-time measurements of substances in question at a level of 80 ppb (80 billion-1) is demonstrated. Calculative models were created and results of generation and evolution of substances in question in a turbulent twisted flow are reported. A mathematical model and program code were validated for numerical simulations of two-phase turbulent flow in a single trailing vortex in initial stage of contrail generation behind supersonic aircraft. The work was carried out under the project #1477 supported financially by the International Scientific-Research Center, Moscow.

Ilyin, Sergey P.; Adamenkov, Anatoly A.; Asmolov, Evgeniy S.; Bulkin, Yuri N.; Vyskubenko, Boris A.; Deryugin, Maxim Y.; Kolobyanin, Yuriy V.; Kudryashov, Evgeniy A.; Rusyanov, Dmitriy A.; Shustov, Andrey V.

2005-03-01

406

Review of CFD for wind-turbine wake aerodynamics  

Microsoft Academic Search

This article reviews the state of the art of the numerical calculation of wind-turbine wake aerodynamics. Different CFD techniques for modeling the rotor and the wake are discussed. Regarding rotor modeling, recent advances in the generalized actuator approach and the direct model are discussed, as far as it attributes to the wake description. For the wake, the focus is on

B. Sanderse; Pijl van der S. P; B. Koren

2010-01-01

407

Interaction of wake turbulence with a free surface  

Microsoft Academic Search

The turbulent wake of a flat plate aligned with a uniform water flow and extending through the free surface was investigated experimentally. Laser-Doppler velocimetry (LDV) measurements show good agreement with published data for a two-dimensional wake, except in a shallow layer near the free surface. In this surface layer, the wake width is observed to double while the wake centerline

Larry M. Logory; Amir Hirsa; Douglas G. Anthony

1996-01-01

408

Exploring a flight deck based wake turbulence situational awareness tool  

Microsoft Academic Search

As NextGen concepts move toward increasing en route and terminal throughput, wake turbulence separation may become a limiting factor in the pursuit of capacity improvements. Better knowledge of the probable location of wakes (for air traffic controllers as well as pilots) could help provide safe separation from wake turbulence while avoiding unnecessary restrictions to operations. The Wake Turbulence Avoidance Automation

Clark Lunsford; Marshall Koch; H. Peter Stassen; Steven Estes; Brendan Hogan

2012-01-01

409

Optical vortex coronagraphy with an elliptical aperture.  

PubMed

An optical vortex coronagraph that makes efficient use of a larger fraction of the clear aperture of a Cassegrain-type telescope is described. This design incorporates an elliptical subaperture rather than the conventional circular subaperture. We derive a new vortex phase mask that maintains the same theoretical contrast of a circularly symmetric vortex coronagraph. PMID:23314632

Ruane, Garreth J; Swartzlander, Grover A

2013-01-10

410

Photographic studies of quantized vortex lines  

Microsoft Academic Search

A study of the behavior of systems of quantized vortex lines in rotating superfluid4He is described. Using a photographic technique, the positions of the vortex cores at the free surface of the liquid are recorded in the form of time-lapse motion pictures. The observation of stationary arrays of vortices are discussed and a comparison with the predictions of rectilinear vortex

E. J. Yarmchukt; R. E. Packard

1982-01-01

411

Instability of vortex pair leapfrogging  

NASA Astrophysics Data System (ADS)

Leapfrogging is a periodic solution of the four-vortex problem with two positive and two negative point vortices all of the same absolute circulation arranged as co-axial vortex pairs. The set of co-axial motions can be parameterized by the ratio 0 < ? < 1 of vortex pair sizes at the time when one pair passes through the other. Leapfrogging occurs for ? > ?2, where ?=2-1 is the silver ratio. The motion is known in full analytical detail since the 1877 thesis of Gröbli and a well known 1894 paper by Love. Acheson [``Instability of vortex leapfrogging,'' Eur. J. Phys. 21, 269-273 (2000)] determined by numerical experiments that leapfrogging is linearly unstable for ?2 < ? < 0.382, but apparently stable for larger ?. Here we derive a linear system of equations governing small perturbations of the leapfrogging motion. We show that symmetry-breaking perturbations are essentially governed by a 2D linear system with time-periodic coefficients and perform a Floquet analysis. We find transition from linearly unstable to stable leapfrogging at ? = ?2 ~ 0.381966, where ?=12(5-1) is the golden ratio. Acheson also suggested that there was a sharp transition between a ``disintegration'' instability mode, where two pairs fly off to infinity, and a ``walkabout'' mode, where the vortices depart from leapfrogging but still remain within a finite distance of one another. We show numerically that this transition is more gradual, a result that we relate to earlier investigations of chaotic scattering of vortex pairs [L. Tophøj and H. Aref, ``Chaotic scattering of two identical point vortex pairs revisited,'' Phys. Fluids 20, 093605 (2008)]. Both leapfrogging and ``walkabout'' motions can appear as intermediate states in chaotic scattering at the same values of linear impulse and energy.

Tophøj, Laust; Aref, Hassan

2013-01-01

412

The Role of Wakes in Modelling Tidal Current Turbines  

NASA Astrophysics Data System (ADS)

The eventual proper development of arrays of Tidal Current Turbines (TCT) will require a balance which maximizes power extraction while minimizing environmental impacts. Idealized analytical analogues and simple 2-D models are useful tools for investigating questions of a general nature but do not represent a practical tool for application to realistic cases. Some form of 3-D numerical simulations will be required for such applications and the current project is designed to develop a numerical decision-making tool for use in planning large scale TCT projects. The project is predicated on the use of an existing regional ocean modelling framework (the Regional Ocean Modelling System - ROMS) which is modified to enable the user to account for the effects of TCTs. In such a framework where mixing processes are highly parametrized, the fidelity of the quantitative results is critically dependent on the parameter values utilized. In light of the early stage of TCT development and the lack of field scale measurements, the calibration of such a model is problematic. In the absence of explicit calibration data sets, the device wake structure has been identified as an efficient feature for model calibration. This presentation will discuss efforts to design an appropriate calibration scheme which focuses on wake decay and the motivation for this approach, techniques applied, validation results from simple test cases and limitations shall be presented.

Conley, Daniel; Roc, Thomas; Greaves, Deborah

2010-05-01

413

[Central mechanisms of sleep-wakefulness cycle].  

PubMed

Brief anatomical, physiological and neurochemical basics of the regulation of wakefulness, slow wave (NREM) sleep and paradoxical (REM) sleep are regarded as representing by the end of the first decade of the second millennium. PMID:21950094

Koval'zon, V M

414

Wake collapse in a stratified fluid.  

PubMed

A two-dimensional model is used to obtain quantitative data on characteristics of turbulently mixed wakes of bodies submerged in stratified fluids (more dense below than above). The time between turbulent mixing and maximum expansion of the wake before vertical collapse starts is 0.44 T, where T is the local Väisälä-Brunt period. Time after mixing for maximum rate of horizontal spreading is about 2.0 T. The average Väisälä-Brunt period for the oceans and atmosphere is discussed. It is predicted that the wake collapse phenomenon is not unusual in these environments. The characteristic time for the most active phase of vertical wake collapse should be between a few minutes to several tens of minutes. Qualitative observations of aircraft vapor trails tend to confirm that the phenomenon does occur at full scale. PMID:17798697

Schooley, A H

1967-07-28

415

[Physiology of sleep-wakefulness rhythms].  

PubMed

As the most important site for sleep-wakefulness change, the role of the hypothalamus is particularly emphasized based on our long term experimental data. After rostral midbrain transection, ECoG of the isolated forebrain recovered sleep-wakefulness change, with circadian rhythm, one week after transection. In this preparation, after additional bilateral preoptic or posterior hypothalamic lesions, ECoG "insomnia" or "coma" pattern appeared, respectively. Therefore, the mechanism of sleep-wakefulness change of the forebrain primarily does not depend on the lower brain stem structures such as the raphe nuclei and midbrain reticular formation. Hypothalamic sleep-wakefulness mechanism usually receives strong influence from the suprachiasmatic nucleus, but it can exert its own ultradian "rhythms" (very irregular though) in intact rats or in rats with suprachiasmatic lesions. PMID:9503822

Kawamura, H

1998-02-01

416

Vortex ice in nanostructured superconductors  

SciTech Connect

We demonstrate using numerical simulations of nanostructured superconductors that it is possible to realize vortex ice states that are analogous to square and kagome ice. The system can be brought into a state that obeys either global or local ice rules by applying an external current according to an annealing protocol. We explore the breakdown of the ice rules due to disorder in the nanostructure array and show that in square ice, topological defects appear along grain boundaries, while in kagome ice, individual defects appear. We argue that the vortex system offers significant advantages over other artificial ice systems.

Reichhardt, Charles [Los Alamos National Laboratory; Reichhardt, Cynthia J [Los Alamos National Laboratory; Libal, Andras J [Los Alamos National Laboratory

2008-01-01

417

Vortex ring formation at the open end of a shock tube: A particle image velocimetry study  

NASA Astrophysics Data System (ADS)

The vortex ring generated subsequent to the diffraction of a shock wave from the open end of a shock tube is studied using particle image velocimetry. We examine the early evolution of the compressible vortex ring for three-exit shock Mach numbers, 1.1, 1.2, and 1.3. For the three cases studied, the ring formation is complete at about tUb/D=2, where t is time, Ub is fluid velocity behind shock as it exits the tube and D is tube diameter. Unlike in the case of piston generated incompressible vortex rings where the piston velocity variation with time is usually trapezoidal, in the shock-generated vortex ring case the exit fluid velocity doubles from its initial value Ub before it slowly decays to zero. At the end of the ring formation, its translation speed is observed to be about 0.7 Ub. During initial formation and propagation, a jet-like flow exists behind the vortex ring. The vortex ring detachment from the tailing jet, commonly referred to as pinch-off, is briefly discussed.

Arakeri, J. H.; Das, D.; Krothapalli, A.; Lourenco, L.

2004-04-01

418

HISTAMINE IN THE REGULATION OF WAKEFULNESS  

PubMed Central

The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H1 and/or H3 receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H1 receptor agonists induced wakefulness, whereas administration of H1 receptor antagonists promoted sleep. The H3 receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H3 receptor decreased histamine release and promoted sleep. Conversely, blockade of H3 receptor promoted wakefulness. Histamine release in the hypothalamus and other target regions was highest during wakefulness. The histaminergic neurons displayed maximal activity during the state of vigilance, and cease their activity during NREM and REM sleep. The cerebrospinal levels of histamine were reduced in diseased states where hypersomnolence was a major symptom. The histamine deficient HDC KO mice displayed sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results were obtained when histamine neurons were lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness.

Thakkar, Mahesh M.

2010-01-01

419

Statistics of waves within a ship wake  

NASA Astrophysics Data System (ADS)

High-amplitude water waves induced by high-speed ferries are regularly observed in Tallinn Bay, the Baltic Sea causing intense beach erosion and disturbing marine habitants in the coastal zone. Such a strong impact on coast can be a result of a group structure of the wake and it is studied experimentally at Pikakari beach, Tallinn Bay. The most energetic vessel waves at this location have amplitudes of about 1 m and periods of 8-10 sec with maximum run-up heights up to 1.4 m. These wakes represent a certain structure, where the largest and longest waves come first and waves of smaller amplitude and period after. Sometimes the groups of different heights and periods can be separated even within one wake. The wave heights within a wake are well-described by the Weibull distribution, which has different parameters for wakes from different ships. Wave runup heights can also be described by Weibull distribution and its parameters can be connected to the parameters of the distribution of wave heights. Finally, the runup of individual waves within a wake is studied. It is shown that the largest amplification occurs for waves of weak amplitude and is in a good agreement with an estimate for the nonbreaking runup of a sinusoidal wave. The largest waves are strongly affected by the wave breaking and their runup is modeled numerically in the framework of the nonlinear shallow-water theory.

Didenkulova, I.; Rodin, A.

2012-04-01

420

Histamine in the regulation of wakefulness.  

PubMed

The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H? and/or H? receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H? receptor agonists induces wakefulness, whereas administration of H? receptor antagonists promotes sleep. The H? receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H? receptor reduces histamine release and promotes sleep. Conversely, blockade of H? receptor promotes wakefulness. Histamine release in the hypothalamus and other target regions is highest during wakefulness. The histaminergic neurons display maximal activity during the state of high vigilance, and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The cerebrospinal levels of histamine are reduced in diseased states where hypersomnolence is a major symptom. The histamine deficient L-histidine decarboxylase knockout (HDC KO) mice display sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results have been obtained when histamine neurons are lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness. PMID:20851648

Thakkar, Mahesh M

2010-09-20

421

Aspects of the influence of an oscillating mini-flap upon the near wake of an airfoil NACA 4412  

NASA Astrophysics Data System (ADS)

A NACA 4412 airfoil was tested, in a boundary layer wind tunnel, with the aim to study the effect of a Gurney mini-flap, as an active and passive flow control device submitted to a turbulent flow field. The main objective was the experimental determination of flow pattern characteristics downstream the airfoil in the near wake. The untwisted wing model used for the experiments had 80cm wingspan and 50cm chord, with airfoil NACA 4412. The mini-flap was located on the lower surface at a distance, from the trailing edge, of 8%c (c airfoil chord). The Reynolds number, based upon the wing chord and the mean free stream velocity was 326,000 and 489,000. The turbulence intensity was 1.8%. The model was located into the wind tunnel between two panels, in order to assure a close approximation to two-dimensional flow over the model. As an active control device a rotating mini-flaps, geared by an electromechanical system (which rotate to a 30°) was constructed. The wake pattern and pressure values near the trailing edge were measured. The results obtained, for this mechanism, show us that the oscillating mini-flap change the wake flow pattern, alleviating the near wake turbulence and enhancing the vortex pair near the trailing edge at the mini-flap level and below that level, magnifying the effect described first by Liebeck [1]. That effect grows with the oscillating frequency. Additionally, the wake alleviation probably affects also the far wake. All of these facts suggest us to continue with the experiments, trying to measure the pressure distribution around the airfoil in all the cases, obtaining the lift and drag characteristics.

Delnero, J. S.; Marañón Di Leo, J.; Colman, J.; García Sainz, M.; Muñoz, F.; Hérouard, N.; Camocardi, M. E.

2011-05-01

422

A parabolized Navier-Stokes analysis of wake/boundary-layer flow along a cable in tow  

SciTech Connect

A parabolized Navier-Stokes analysis of a turbulent, compressible, wake/boundary-layer flow field for a cable in tow is discussed. It is assumed that the cable is being towed by a missile-like configuration whose total drag coefficient is known. The cable is assumed to be perfectly aligned with the missile axis and is subjected to its wake. Modeled in the analysis is the far wake behind the missile, coupled with the turbulent boundary layer growth along the cable. An analytical starting solution for a parabolized Navier-Stokes code is presented. The starting solution is applicable downstream of the towing body's near wake and, therefore, circumvents the complex task of computing the towing body's flow field. An algebraic wake/boundary-layer turbulence model is used to simulate turbulent flow in both the decaying wake and growing boundary layer along the cable. Results are presented for a towing-body freestream Mach number of 5 and a Reynolds number of 36.0 {times} 10{sup 6} per ft at select distances along a thin cable. 18 refs., 7 figs., 2 tabs.

Barnette, D.W.

1991-01-01

423

A flow physics study of flap-mounted vortex generators on a multi-element airfoil  

NASA Astrophysics Data System (ADS)

Vortex generators are a commonly used aerodynamic "fix" for flow separation problems. They are typically used to remedy flow separation due to design shortcomings or changes in operating conditions that exceed the original design point. Flow separation is often encountered with high lift systems. Flaps and slats can be difficult to design due to complicated flow phenomena and large Reynolds number effects. Previous research has indicated the effectiveness of vortex generators in correcting flow separation over a flap. In fact, significant aerodynamic performance improvements were predicted for high-lift systems that incorporate vortex generators in the original design. Before this may be attempted, a better understanding of vortex generator flow physics must be obtained for the development of appropriate design tools and analysis methods. The research contained herein is focused on a detailed flow physics study of vortex generators mounted to the flap of a three-element high-lift airfoil. Detailed velocity measurements taken using a three-component laser Doppler velocimeter were used to vortex/boundary layer interactions and global flowfield effects. The full Reynolds stress tensor and mean velocity field was measured in addition to surface pressures. Three basic vortex generator arrangements were studied: upflow, downflow, and corotating. Although not optimized, all three types of vortex generators were effective at eliminating boundary layer separation. The vortices demonstrated a tendency to rise from the flap surface regardless of orientation and decayed rapidly, with cross-stream vorticity dropping below measurable levels by 75% flap chord. However, the embedded vortices produced significant perturbations in the turbulence field and mean flow of the flap boundary layer that persisted to the flap trailing edge.

Klausmeyer, Steven Michael

424

Kelvin Waves of Quantized Vortex Lines in Trapped Bose-Einstein Condensates  

SciTech Connect

We have theoretically investigated Kelvin waves of quantized vortex lines in trapped Bose-Einstein condensates. Counterrotating perturbation induces an elliptical instability to the initially straight vortex line, driven by a parametric resonance between a quadrupole mode and a pair of Kelvin modes of opposite momenta. Subsequently, Kelvin waves rapidly decay to longer wavelengths emitting sound waves in the process. We present a modified Kelvin wave dispersion relation for trapped superfluids and propose a simple method to excite Kelvin waves of specific wave number.

Simula, T. P.; Mizushima, T.; Machida, K. [Department of Physics, Okayama University, Okayama 700-8530 (Japan)

2008-07-11

425

Vortex Dynamics in Magnetized Plasmas  

Microsoft Academic Search

Low frequency dynamics of electrostatic fluctuations in strongly magnetized plasmas have been studied. It was found that perturbations in density and potential can be very localized, indicating the applicability of an approximate description based on a finite number of vortices. A model based on a few isolated vortical structures is discussed, with particular attention to vortex collapse, where three vortices

M. Kono; B. Krane; H. L. Pécseli; J. Trulsen

1998-01-01

426

Vortex imaging in unconventional superconductors  

NASA Astrophysics Data System (ADS)

The real space imaging of vortices in unconventional superconductors not only provides important information about the effectiveness of flux pinning that can inform high current applications, but also yields crucial insights into the form of the superconducting order parameter. For example, the structure of the vortex lattice reflects effective mass and order parameter anisotropies within the material, and profiles of isolated vortices provide a local measure of the magnetic penetration depth that can be used to infer the superfluid density. We describe here the analysis of recent studies whereby state-of-the-art scanning Hall probe microscopy (SHPM) has been used to perform vortex-resolved magnetic imaging on two distinct families of unconventional superconductors. Two sets of results will be analysed in detail; (i) vortex lattice structural transitions in the p-wave superconductor Sr2RuO4 that reflect underlying anisotropies in the system and (ii) a quantitative analysis of vortex profiles in Co-doped 122 pnictide superconductors (SrFe2?xCoxAs2 & BaFe2?xCoxAs2) that allows one to infer the temperature-dependent superfluid density. The latter has then been compared with predictions for different order parameter models for a multiband superconductor.

Bending, S. J.; Curran, P. J.; Desoky, W. M. A.; Khotkevych, V. V.; Gibbs, A.; Mackenzie, A. P.; Tamegai, T.; Sebastian, S. E.

2012-09-01

427

Vortex waves and vertical motion in oceanic eddies  

NASA Astrophysics Data System (ADS)

Oceanic eddies are present anywhere in the World Ocean. They modulate the surface and internal dynamics and contain most of the ocean kinetic energy. Eddies eventually transport phytoplankton and modify the flux of nutrients from the deep ocean into the sunlit layers, affecting the primary production in the open ocean surface waters. The conceptual models generally used to explain the impact of eddies on the marine ecosystem link vortex growth and decay to the shoaling or deepening of isopycnal surfaces, and relate observed primary production variations to corresponding nutricline displacements. Here I analyse the evolution of a cyclonic eddy through combined satellite-in situ observations and a higher order dynamical approximation. I show that vortex azimuthal oscillations dominate the semi-geostrophic vertical velocity field. These waves are compatible with the propagation of potential vorticity disturbances on the radial gradient of the potential vorticity associated with the basic-state eddy, known in literature as vortex Rossby waves (VRW). VRW have been widely analysed in theoretical studies, laboratory experiments, and numerical models, but difficult to measure directly in the oceans. Synthetic Lagrangian trajectories within the eddy indicate that VRW drive vertical oscillations at the base of the euphotic layer in a range of frequencies for which intense biogeochemical responses are expected. These findings open a new perspective on the vertical fluxes of nutrients at mesoscale and point to a revision of conventional conceptual models of biophysical interactions in oceanic eddies.

Buongiorno Nardelli, Bruno

2013-04-01

428

The effect of an axial flow component on a circular cylinder wake  

NASA Astrophysics Data System (ADS)

A series of experiments were conducted to determine what effect a variation in the level of streamwise vorticity would have on the wake of a bluff body. These experiments investigate changes in wake dynamics caused by the introduction of a steady spanwise component of flow, which creates a steady influx of streamwise vorticity. To accomplish this, a cylinder in a uniform flow field is suddenly moved at constant velocity in the spanwise direction. The steady state flow condition is studied. The control cylinder technique is used to control the end conditions, to make the unperturbed flow parallel. Two primary modes of shedding are observed in the laminar cylinder wake, depending on the ratio of the axial cylinder velocity to the uniform flow velocity (V/U). For ratios of V/U less than four, oblique laminar shedding occurs. The direction and angle of the oblique shedding are controlled by the sign and magnitude of the ratio V/U. For larger ratios of V/U, shedding occurs in the form of trailing vortices. At a critical ratio of V/U, the vortex sheet appears to breakdown, and spectra are completely broadband. The steady-state motion can be compared, via Galilean transformation, to that of the yawed cylinder in a steady flow. Oblique shedding and trailing vortices have also been observed in yawed cylinder wakes. The relationship between shedding angle, frequency, wavelength, mean vorticity, and V/U is discussed. These results are compared to relationships found to occur in naturally occurring oblique shedding and to yawed cylinder results.

Lewis, Christine Gladding

1993-01-01

429

A CFD model of the wake of an offshore wind farm: using a prescribed wake inflow  

Microsoft Academic Search

An CFD model of the wake of an offshore wind farm, expanding existing measurements is proposed. The method is based on solving the Navier Stokes equation in a large domain downstream an offshore wind farm. The inflow of the domain is estimated using existing met mast measurements from both free stream and directly in-wake conditions. A comparison between the simulation

P-E Réthoré; A Bechmann; N N Sørensen; S T Frandsen; J Mann; H E Jørgensen; O Rathmann; S E Larsen

2007-01-01

430

Transverse force on a vortex and vortex mass: Effects of free bulk and vortex-core bound quasiparticles  

NASA Astrophysics Data System (ADS)

The paper reassesses the old but still controversial problem of the transverse force on a vortex and the vortex mass. The transverse force from free bulk quasiparticles on the vortex, both in the Bose and the Fermi liquids, originates from the Aharonov-Bohm effect. However, in the Fermi liquid, one should take into account peculiarities of the Aharonov-Bohm effect for BCS quasiparticles described by two-component spinor wave functions. There is no connection between the transverse force (either from free bulk quasiparticles or from vortex-core bound quasiparticles) and the spectral flow in the vortex core in superfluid Fermi liquid, in contrast to widely known claims. In fact, there is no steady spectral flow in the core of the moving vortex, and the analogy with the Andreev bound states in the superconductor-normal-metal-superconductor junction, where the spectral flow is really possible, is not valid in this respect. The role of the backflow on the vortex mass is clarified. The backflow is