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1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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.

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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.

21

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.

22

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

23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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.

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

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

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92

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

93

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

94

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

95

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

96

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

97

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 ...

98

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

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

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

101

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

102

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

103

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

104

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

105

Delayed Orexin Signaling Consolidates Wakefulness and Sleep: Physiology and Modeling  

PubMed Central

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

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

2011-01-01

106

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

107

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

108

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

109

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

110

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

111

A mathematical model of the sleep\\/wake cycle  

Microsoft Academic Search

We present a biologically-based mathematical model that accounts for several features of the human sleep\\/wake cycle. These\\u000a features include the timing of sleep and wakefulness under normal and sleep-deprived conditions, ultradian rhythms, more frequent\\u000a switching between sleep and wakefulness due to the loss of orexin and the circadian dependence of several sleep measures.\\u000a The model demonstrates how these features depend

Michael J. Rempe; Janet Best; David Terman

2010-01-01

112

Modeling the temporal architecture of rat sleep-wake behavior.  

PubMed

The fine architecture of sleep-wake behavior shows a distinct dynamic structure with distributions of rat sleep and wake bout durations displaying qualitatively different profiles. Wake bout durations follow a power-law relation whereas sleep bout durations are exponentially distributed. We show that a physiologically-based sleep-wake regulatory network model with an underlying deterministic structure governing neuronal interactions can generate realistic rat sleep-wake behavior as assessed by both standard summary statistics and survival analysis of bout distributions. Obtaining appropriate bout duration distributions depended on stochastic elements included in the model, the existence of multiple mechanisms for state transitions, and specific relationships among time constants governing state maintenance. This model provides a novel framework for exploring the disruptions of sleep-wake architecture associated with pharmacological, genetic, and disease states. PMID:22255390

Diniz Behn, Cecilia G; Booth, Victoria

2011-01-01

113

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

114

Numerical Validation of Quasigeostrophic Ellipsoidal Vortex Model  

NASA Astrophysics Data System (ADS)

In geophysical flows, coherent vortex structures persist for long time and their interactions dominate the dynamics of geophysical turbulence. Meacham et al.1,2) obtained a series of exact unsteady solution of the quasigeostrophic equation, which represents a uniform ellipsoidal vortex patch embedded in a uniform 3D shear field. Miyazaki et al.3,4) have derived a Hamiltonian dynamical system of 3N degrees of freedom, describing the interactions of N ellipsoidal vortices, where each coherent vortex was modeled by an ellipsoid of uniform potential vorticity. The center of vorticity and the angular momentum are conserved, besides the total energy and Casimirs of the system, such as the vortex height and the vortex volume. There are three Poisson-commutable invariants, which is less than the degree of freedom for N>=2, and chaotic motions are observed even in a two-body system. In this paper, direct numerical simulations based on a Contour Advective Semi-Lagrangian algorithm (CASL) are performed in order to assess the validity of the Hamiltonian model. First, the instability of a tilted spheroid is investigated. A prolate spheroid becomes unstable against the third Legendre mode when the aspect ratio is less than 0.44 and the inclination angle is larger than 0.48.5) Weakly unstable flatter spheroidal vortices emit thin filaments from their top and bottom, whereas strongly unstable slender spheroidal vortices are broken up into two pieces. Secondly, the interaction of two co-rotating spheroidal vortices on slightly different vertical levels, which plays a key role in the turbulence dynamics, is studied in detail. The Hamiltonian model can predict the critical distance of symmetric mergers very well, except for mergers of vortices on the same horizontal plane. The model gives poorer predictions in asymmetric cases, where vorticity exchange occurs (instead of merger) along the threshold determined by the Hamiltonian model. The slenderer vortex loses half of its original volume, and the flatter vortex expands slightly absorbing some of the filaments ejected from the slenderer vortex. This is a new dynamical process linked with the energy and enstrophy cascades. Considerable amounts of energy and enstrophy are dissipated in these events. The correlation between the energy dissipation and the enstrophy dissipation is good, suggesting the existence of a simple deterministic reset-rule. 1)S. P. Meacham, et al.: Dyn. Atmos. Oceans 21 (1994) 167. 2)S. P. Meacham, et al: Phys. Fluids 9 (1997) 2310. 3)T. Miyazaki, et al.: J. Phys. Soc. Jpn. 69 (2000) 3233. 4)T. Miyazaki, et al.: J. Phys. Soc. Jpn. 70 (2001) 1942. 5)T. Miyazaki, et al.: J. Phys. Soc. Jpn. 68 (1999) 2592.

Miyazaki, T.; Fujishima, S.

2002-05-01

115

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

116

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

117

A mathematical model of the sleep/wake cycle.  

PubMed

We present a biologically-based mathematical model that accounts for several features of the human sleep/wake cycle. These features include the timing of sleep and wakefulness under normal and sleep-deprived conditions, ultradian rhythms, more frequent switching between sleep and wakefulness due to the loss of orexin and the circadian dependence of several sleep measures. The model demonstrates how these features depend on interactions between a circadian pacemaker and a sleep homeostat and provides a biological basis for the two-process model for sleep regulation. The model is based on previous "flip-flop" conceptual models for sleep/wake and REM/NREM and we explore whether the neuronal components in these flip-flop models, with the inclusion of a sleep-homeostatic process and the circadian pacemaker, are sufficient to account for the features of the sleep/wake cycle listed above. The model is minimal in the sense that, besides the sleep homeostat and constant cortical drives, the model includes only those nuclei described in the flip-flop models. Each of the cell groups is modeled by at most two differential equations for the evolution of the total population activity, and the synaptic connections are consistent with those described in the flip-flop models. A detailed analysis of the model leads to an understanding of the mathematical mechanisms, as well as insights into the biological mechanisms, underlying sleep/wake dynamics. PMID:19557415

Rempe, Michael J; Best, Janet; Terman, David

2009-06-26

118

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

119

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

120

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

121

Vortex Sheet Model for a Turbulent Mixing Layer  

NASA Astrophysics Data System (ADS)

The primary aim of this work is to study instability induced roll up of a slightly perturbed vortex sheet in an Euler fluid. A point vortex model tends to evolve into a chaotic cloud of point vortices instead of smooth double branched spirals. The present model uses linear splines to interpolate the vortex sheet. Computer simulation of this vortex sheet is numerically prohibitive. However, the evolution of the vortex sheet can be performed conveniently using a closed form equation of motion which derived from the basic equations of vortex dynamics. The vortex sheet rolls up into a smooth double branched spiral. A vortex core is formed by regular windings of the vortex sheet and irrotational fluid in between the layers. Various statistical quantities like the growth rate and mean velocity profiles are computed along with the evolution of the vortex sheet. The problem of spontaneous appearance of singularity in an evolving vortex sheet is treated in detail. The critical time for the present vortex sheet model is calculated analytically and compared to the numerical value.

Paul, Ujjayan; Narasimha, Roddam; Alam, Meheboob

2011-03-01

122

A New Analytical Model for Wind-Turbine Wakes  

NASA Astrophysics Data System (ADS)

The intention of this study is to propose and validate a simple and efficient analytical model for the prediction of the wake velocity downwind of a stand-alone wind-turbine. Extensive efforts have been carried out to model the wake region analytically. One of the most popular models, proposed by Jensen, assumes a top-hat distribution of the velocity deficit at any plane perpendicular to the wake. That model has been extensively used in the literature and commercial softwares, but it has two important limitations that should be pointed out: (a) Even though this model is supposed to satisfy momentum conservation, in reality mass conservation is only used to derive it; (b) the assumption of a top-hat distribution of the velocity deficit is expected to underestimate that deficit in the center of the wake, and overestimate it near the edge of the wake. In order to overcome the above-mentioned limitations, here we propose an alternative analytical model that satisfies both mass and momentum conservation, and assumes a Gaussian distribution of the velocity deficit. For this purpose, we apply momentum and mass conservation to two different control volumes which have been previously used in the context of analytical modeling of wakes. The velocity profiles obtained with our proposed model are in good agreement with large-eddy simulation data and experimental measurements. By contrast, the top hat models, as expected, clearly underestimate the velocity deficit at the center of the wake region and overestimate it near the edge of the wake.

Bastankhah, Majid; Porté-Agel, Fernando

2013-04-01

123

Numerical Validation of Quasigeostrophic Ellipsoidal Vortex Model  

NASA Astrophysics Data System (ADS)

In geophysical flows, coherent vortex structures persist for long time and their interactions dominate the dynamics of geophysical turbulence. Meacham et al. obtained a series of exact unsteady solution of the quasigeostrophic equation, which represents a uniform ellipsoidal vortex patch embedded in a uniform 3D shear field. Miyazaki et al. derived a Hamiltonian dynamical system describing the interactions of N ellipsoidal vortices, where each coherent vortex was modeled by an ellipsoid of uniform potential vorticity. In this paper, direct numerical simulations based on a Contour Advective Semi-Lagrangian algorithm (CASL) are performed in order to assess the validity of the Hamiltonian model. First, the instability of a tilted spheroid is investigated. A prolate spheroid becomes unstable against the third Legendre mode when the aspect ratio is less than 0.44 and the inclination angle is larger than 0.48. Weakly unstable flatter spheroidal vortices emit thin filaments from their top and bottom, whereas strongly ustable slender spheriodal vortices are broken up into two pieces. Secondly, the interaction of two co-rotaing spheroidal vortices on slightly different vertical levels is studied in detail. It is shown that the Hamiltonian model can predict the critical merger distance fairly well. Considerable amounts of energy and enstrophy are dissipated in these events. The correlation between the energy dissipation and the enstrophy dissipation is good, suggesting the existence of a deterministic reset-rule.

Miyazaki, Takeshi; Asai, Akinori; Yamamoto, Masahiro; Fujishima, Shinsuke

2002-11-01

124

Wake II model for hydrodynamic forces on marine pipelines including waves and currents  

Microsoft Academic Search

The Wake II model for the determination of the hydrodynamic forces on marine pipelines is extended to include currents and waves. There are two main differences between the Wake II and the traditional model. First, in the Wake II model the velocity is modified to include the pipe's encounter with the wake flow when the velocity reverses. Second, the model

Said R Sabag; Billy L Edge; Iwan Soedigdo

2000-01-01

125

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

126

Vortex filament model and multifractal conjecture  

NASA Astrophysics Data System (ADS)

We develop a theory of turbulence based on the inviscid Navier-Stokes equation. We get a simple but exact stochastic solution (vortex filament model) which allows us to obtain a power law for velocity structure functions in the inertial range. Combining the model with the multifractal conjecture, we calculate the scaling exponents without using the extended self-similarity approach. The results obtained are shown to be in very good agreement with numerical simulations and experimental data. The role of more general stochastic solutions of the Navier-Stokes equation is discussed.

Zybin, K. P.; Sirota, V. A.

2012-05-01

127

Vortex filament model and multifractal conjecture.  

PubMed

We develop a theory of turbulence based on the inviscid Navier-Stokes equation. We get a simple but exact stochastic solution (vortex filament model) which allows us to obtain a power law for velocity structure functions in the inertial range. Combining the model with the multifractal conjecture, we calculate the scaling exponents without using the extended self-similarity approach. The results obtained are shown to be in very good agreement with numerical simulations and experimental data. The role of more general stochastic solutions of the Navier-Stokes equation is discussed. PMID:23004872

Zybin, K P; Sirota, V A

2012-05-30

128

Approaches to Validation of CFD Models for Far Ship Wake  

NASA Astrophysics Data System (ADS)

The centerline wake of surface ships can extend to tens of kilometers on synthetic aperture radar (SAR) images. However, the hydrodynamics of far wakes of ships are not well understood. Our assumption is that far from the ship, the pattern of flows comprising the wake is represented by longitudinal coherent vortices, which gross parameters only slowly change in the direction along the wake. In order to model this process, we have run a set of non-hydrostatic simulations. The simulations also included dynamics of freshwater plumes in the upper ocean because such types of formations often produce sharp fronts, which can be confused with ship wakes. We have used 2D and 3D setups with slippery and free upper boundary and with several turbulence closure schemes including k-?, standard LES, and Hybrid LES. The models have been implemented in CFD Fluent and simulated such processes as wind-wake and wind-plume interactions and formation of sharp frontal lines on the sea surface. In order to justify the choice of grid and model parameters, we have performed a series of validation tests. These tests included grid and time convergence, sensitivity to geometric parameters, and comparison to available experimental data including photo and SAR images of ship wakes. The Hybrid LES turbulence model has demonstrated a more realistic performance than the other two tested turbulence closure models. The application of CFD to these problems has resulted in a qualitative level of information. Providing information on the level of absolute quantities requires validation with the data from specialized field and laboratory experiments.

Fujimura, A.; Soloviev, A.

2008-12-01

129

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

130

WIND TUNNEL AND GAUSSIAN PLUME MODELING OF BUILDING WAKE DISPERSION  

EPA Science Inventory

This paper summarizes a study of the relationship between Gaussian plume models and wind-tunnel models. ind-tunnel measurements of the distribution of tracer concentrations downwind of a point source in the near wake of a rectangular model building were evaluated. rofiles of mean...

131

An inviscid model for vortex shedding from a deforming body  

Microsoft Academic Search

An inviscid vortex sheet model is developed in order to study the unsteady separated flow past a two-dimensional deforming\\u000a body which moves with a prescribed motion in an otherwise quiescent fluid. Following Jones (J Fluid Mech 496, 405–441, 2003)\\u000a the flow is assumed to comprise of a bound vortex sheet attached to the body and two separate vortex sheets originating

Ratnesh K. Shukla; Jeff. D. Eldredge

2007-01-01

132

SIMULATION MODELING OF UNSTEADY MANEUVERS USING A TIME ACCURATE FREE WAKE  

Microsoft Academic Search

The transient response of a helicopter to unsteady maneuvers using a time accurate free wake model coupled with a comprehensive flight dynamics simu- lation model was investigated. The simulation model is a coupled rotor-fuselage model with flexible blade modelling in flap, lag and torsion. The free wake model uses a time marching scheme to solve the gov- erning wake equations,

Maria Ribera; Roberto Celi

2004-01-01

133

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

134

Feedback control of the cylinder wake using balanced reduced order models  

NASA Astrophysics Data System (ADS)

Feedback control is most successful when an accurate model of the system-to-be-controlled is available. For fluids, this can be achieved using a reduced order model which is balanced (meaning the input-output behaviour is properly captured). With this in mind, we consider feedback control of the cylinder wake in low Reynolds number simulations. Actuation is via blowing and suction on the cylinder's surface, and a single velocity sensor in the wake is used. Balanced reduced order models are formed using the Eigensystem Realization Algorithm (ERA) at a number of Reynolds numbers. The reduced order models, validated by comparing their impulse responses to the full system, are then used in two ways. First, the "gain window" phenomenon seen in previous feedback control studies is reproduced (and therefore explained) by the models. We see that this gain window shrinks with increasing Reynolds number, the consequence being that feedback control with a simple proportional gain is not possible at higher Reynolds numbers. Second, H? loop-shaping techniques are used to design "dynamic" controllers that are effective at higher Reynolds numbers, achieving complete suppression of vortex shedding at Reynolds numbers in excess of 100.

Illingworth, Simon; Naito, Hiroshi; Fukagata, Koji

2010-11-01

135

A Neuron-Based Model of Sleep-Wake Cycles  

NASA Astrophysics Data System (ADS)

In recent years it was discovered that a neuropeptide orexin/hypocretin plays a main role in sleep processes. This peptide is produced by the neurons in the lateral hypothalamus, which project to almost all brain areas. We present a computational model of sleep-wake cycles, which is based on the Hodgkin-Huxley type neurons and considers reciprocal glutaminergic projections between the lateral hypothalamus and the prefrontal cortex. Orexin is released as a neuromodulator and is required to keep the neurons firing, which corresponds to the wake state. When orexin is depleted the neurons are getting silent as observed in the sleep state. They can be reactivated by the circadian signal from the suprachiasmatic nucleus and/or external stimuli (alarm clock). Orexin projections to the thalamocortical neurons also can account for their transition from tonic firing activity during wakefulness to synchronized burst discharges during sleep.

Postnova, Svetlana; Peters, Achim; Braun, Hans

2008-03-01

136

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

137

Modeling and simulation of high-speed wake flows  

Microsoft Academic Search

High-speed, unsteady flows represent a unique challenge in computational hypersonics research. They are found in nearly all applications of interest, including the wakes of reentry vehicles, RCS jet interactions, and scramjet combustors. In each of these examples, accurate modeling of the flow dynamics plays a critical role in design performance. Nevertheless, literature surveys reveal that very little modern research effort

Michael Daniel Barnhardt

2009-01-01

138

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

139

On the need of nonlinear control for efficient model-based wake stabilization  

NASA Astrophysics Data System (ADS)

The mitigation of oscillatory vortex shedding behind a cylinder is chosen as a well-investigated benchmark problem to compare model-based feedback flow control approaches. The flow is sensed by a single velocity signal in the wake and is manipulated via a single volume force actuator. A low-dimensional proper orthogonal decomposition Galerkin model is adopted as a control-oriented fluid flow representation. An extended Kalman filter is used as an effective means for online dynamic state estimation. Investigated strategies of linear and nonlinear controller design include pole placement, linear parameter-varying, input-output linearization, Lyapunov-based backstepping, and nonlinear model predictive control. These strategies are applicable to a large class of flows with oscillatory dynamics and to experimental conditions, where variants have already been used. Controllers are evaluated and compared based on their application to the full plant, that is, to the direct numerical simulation of the wake, emulating an experiment with a single hot-wire sensor. Overall, nonlinear closed-loop control is shown to be distinctly superior to linear approaches. As is often the case, physics dictates a similarity of successful control commands, irrespective of the design approach, and differentiates these controllers, as a group, from less successful approaches.

Aleksi?-Roeßner, Katarina; King, Rudibert; Lehmann, Oliver; Tadmor, Gilead; Morzy?ski, Marek

2013-03-01

140

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

141

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

142

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

143

Prediction of hydrodynamic forces on submarine pipelines using an improved Wake II Model  

Microsoft Academic Search

The hydrodynamic force model for prediction of forces on submarine pipelines as described includes flow history effect (wake effects) and time dependence in the force coefficients. The wake velocity correction is derived by using a closed-form solution to the linearized Navier–Stokes equations for oscillatory flow. This is achieved by assuming that the eddy viscosity in the wake is only time

Iwan R. Soedigdo; K. F. Lambrakos; Billy L. Edge

1998-01-01

144

CFD modeling issues of wind turbine wakes under stable atmospheric conditions  

Microsoft Academic Search

Summary In the present work, two Navier-Stokes solvers are applied to predict wake velocity deficits and turbulence intensity in the wake of a single wind turbine for different atmospheric stratification conditions. The two solvers use different turbulence closure, the k-? and k-? model respectively. Results from both solvers showed underestimation of the near wake velocity deficit. This can be justified

K. G. Rados; J. M. Prospathopoulos; E. S. Politis; P. K. Chaviaropoulos; A. Zervos

145

Applying canopy flow model for estimation of wind turbine wake  

NASA Astrophysics Data System (ADS)

For the planning of large offshore wind farm the optimal spatial placing of wind turbines as well as wind farms relatively to each other is highly important to reduce the wake losses of energy. Conventional instrumental investigations of airflow characteristics around and inside an offshore wind farm aimed at understanding of far-wake behavior are very difficult and expensive. Computational fluid dynamic (CFD) models can provide the information on spatial patterns of wind and turbulence and thus, help to develop the optimal wind farm design. With limited level of model resolution, however, there is still a problem of how to describe the effect of a wind turbine itself on air flow. Having this problem solved the joint effects of a given number of wind turbines could be easily estimated. In present work, to describe the influence of a wind turbine on the flow a coupled canopy-atmospheric boundary-layer model SCADIS is implemented. It has been shown that this model, based on two-equation closure and modified to account for plant drag, is able to simulate airflow through a wide range of vegetation reasonably. In the numerical experiment with SCADIS the turbine's rotor was replaced by a disk of limited thickness, with diameter (D) and location of real rotor but with properties of vegetation. Aerodynamic drag values for this rotor with some 'plant' surface density can be derived from the trust coefficient Cp of the wind turbine of interest. Model results were compared with measurements from the Danish offshore wind farm Vindeby consisted of 11 Bonus 450 kW turbines (hub height and rotor diameter are 38 m and 35 m, respectively). The comparison show that the approach can describe well the single- and double-wake cases (at distance 9.6D behind the last turbine), and quintuple-wake case (at distance 8.6D). Taking in account relatively low the computing time demands of the approach, it is a promising tool for further studies of wakes of offshore wind turbines of any size and composition.

Sogachev, A.; Joergensen, H. E.; Mann, J.; Frandsen, S.; Ott, S.

2008-12-01

146

Spherical Shell Vortex Model For Compound Drops  

NASA Astrophysics Data System (ADS)

A new generalization of the classical solution for inviscid incompressible flow with vorticity known as Hill's spherical vortex is presented here. The new solution includes a concentric sphere inside the spherical body produced by Hill's vortex. The internal sphere can be filled with a different fluid,with limiting cases of a spherical cavity, or solid sphere. This solution is applicable to coated pill and pellet manfacture.The present solution also includes internal swirling about the axis parallel to the direction of oncoming flow.

Shusser, Michael; Weihs, Daniel

2003-11-01

147

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

148

Reduced-order models for closed-loop wake control.  

PubMed

We review a strategy for low- and least-order Galerkin models suitable for the design of closed-loop stabilization of wakes. These low-order models are based on a fixed set of dominant coherent structures and tend to be incurably fragile owing to two challenges. Firstly, they miss the important stabilizing effects of interactions with the base flow and stochastic fluctuations. Secondly, their range of validity is restricted by ignoring mode deformations during natural and actuated transients. We address the first challenge by including shift mode(s) and nonlinear turbulence models. The resulting robust least-order model lives on an inertial manifold, which links slow variations in the base flow and coherent and stochastic fluctuation amplitudes. The second challenge, the deformation of coherent structures, is addressed by parameter-dependent modes, allowing smooth transitions between operating conditions. Now, the Galerkin model lives on a refined manifold incorporating mode deformations. Control design is a simple corollary of the distilled model structure. We illustrate the modelling path for actuated wake flows. PMID:21382828

Tadmor, Gilead; Lehmann, Oliver; Noack, Bernd R; Cordier, Laurent; Delville, Joël; Bonnet, Jean-Paul; Morzy?ski, Marek

2011-04-13

149

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

150

Vortex modeling of gaseous jets in a compressible crossflow  

NASA Astrophysics Data System (ADS)

This paper discusses an analytical/numerical model developed to describe the behavior of gaseous jets injected transversely into a subsonic (but compressible) crossflow. The cross-section of the jet is modeled as an inviscid compressible vortex pair, consistent with experimental observations of the transversejet cross-section. The numerically computed behavior of the vortex pair is used as an input to mass and momentum balances along the jet, forming a model which describes the trajectory, entrainment, and mixing of jets injected into subsonic compressible crossflows.

Heister, S. D.; Karagozian, A. R.

1988-07-01

151

Center vortex model and the G(2) gauge group  

SciTech Connect

The thick center vortex model is applied to G(2) gauge group to obtain the potentials between static sources of the fundamental and adjoint representations. The group G(2) has only one trivial center element and therefore it does not have any vortices which are defined based on non trivial center elements. To obtain the potential from the thick center vortex model, the idea of the vacuum domain structure is used. The intermediate string tensions from this model are in rough agreement with the G(2) lattice results and the Casimir ratio. We argue that the SU(3) subgroup of G(2) may be responsible for the linear potential at the intermediate distances.

Deldar, Sedigheh; Lookzadeh, Hadi; Nejad, Seyed Mohsen Hosseini [Department of Physics, University of Tehran, P.O. Box 14395/547, Tehran 1439955961 (Iran, Islamic Republic of)

2011-05-23

152

Vortex ring modelling of toroidal bubbles  

Microsoft Academic Search

During the collapse of a bubble near a surface, a high-speed liquid jet often forms and subsequently impacts upon the opposite\\u000a bubble surface. The jet impact transforms the originally singly-connected bubble to a toroidal bubble, and generates circulation\\u000a in the flow around it. A toroidal bubble simulation is presented by introducing a vortex ring seeded inside the bubble torus\\u000a to

Q. X. Wang; K. S. Yeo; B. C. Khoo; K. Y. Lam

2005-01-01

153

Approaches to Validation of CFD Models for Far Ship Wake  

Microsoft Academic Search

The centerline wake of surface ships can extend to tens of kilometers on synthetic aperture radar (SAR) images. However, the hydrodynamics of far wakes of ships are not well understood. Our assumption is that far from the ship, the pattern of flows comprising the wake is represented by longitudinal coherent vortices, which gross parameters only slowly change in the direction

A. Fujimura; A. Soloviev

2008-01-01

154

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

155

Impacts of Wake Effect and Time Delay on the Dynamic Analysis of Wind Farms Models  

ERIC Educational Resources Information Center

|This article investigates the impacts of proper modeling of the wake effects and wind speed delays, between different wind turbines' rows, on the dynamic performance accuracy of the wind farms models. Three different modeling scenarios were compared to highlight the impacts of wake effects and wind speed time-delay models. In the first scenario,…

El-Fouly, Tarek H. M.; El-Saadany, Ehab F.; Salama, Magdy M. A.

2008-01-01

156

Model for particle balance in pumped divertors (pre-VORTEX)  

Microsoft Academic Search

An internally consistent model for particle transport in an open divertor geometry has been developed. Embodied in a new code, pre-VORTEX, the model couples the particle balance in the plasma core, the scrape-off layer, the open divertor channels, and the vacuum'' regions. This mutual coupling is particularly important in determining the conditions required for high recycling in the divertor. The

Hogan

1990-01-01

157

Gradient descent assimilation for the point-vortex model  

NASA Astrophysics Data System (ADS)

Data assimilation is concerned with incorporating (noisy) observations into (imperfect) models that describe the underlying dynamics of the system, in order to infer the properties of the current state, by ensuring that the assimilated trajectories are consistent with both the observations and model dynamics. For many physical systems, particularly in oceanography, observations are usually available in the form of Lagrangian (particle trajectory) data that are augmented into models describing the flow fields. The incorporation of Lagrangian data into models of flow presents several challenges concerning the potential complexity of the Lagrangian trajectories in relatively simple flow fields, for example the appearance of nonlinear effects that are triggered by the exponential rate of separation of tracer trajectories in the region of saddle points [1]. As such, standard linear-based data assimilation methods, such as the Kalman filter, can fail. A nonlinear approach known as gradient descent assimilation [2] is presented, in which analysis trajectories are found by minimising a cost function in an extended state space. The gradient descent approach is demonstrated in the context of assimilating Lagrangian tracer trajectories in two-dimensional flows of point-vortex systems. The point-vortex model plays an important role as a simplified version of many physical systems, including Bose-Einstein condensates, certain plasma configurations and inviscid turbulence, in which the model dynamics are described by a relatively simple system of nonlinear ODEs, which can exhibit regular or chaotic motion for the 2-point vortex or 3-point vortex system respectively. A set of tracer advection equations augment the point vortex model equations, allowing the observed tracer positions to update the state information about the unobserved vortex postions. The gradient descent approach to the two-point vortex system has been successfully demonstrated for the case of both full and partial observations in a wide variety of test cases. [1] K. Ide, L. Kuznetsov and C. K. R. T. Jones. Lagrangian data assimilation for point vortex systems, Journal of Turbulence, 3, 053 (2002). [2] K. Judd, L. A. Smith and A. Weisheimer. Gradient free descent: Shadowing and state estimation using limited derivative information, Physica D, 190, 153-166 (2004).

Suckling, E. B.; Smith, L. A.

2012-04-01

158

Microburst Simulation via Vortex-Ring and Turbulent Jet Models.  

NASA Astrophysics Data System (ADS)

Microbursts, suggested as primary causes of many aircraft fatal crashes, are the subject of this research. A microburst, or low-level intense wind shear, is generated by a thunderstorm or a small rain cloud, and presents hazardous conditions for aircraft during take-off and landing maneuvers. Recently released data show that a microburst resembles a transient vortex ring. Three microburst models have been constructed in this study. First, the turbulent jet model encompasses a free jet at high altitude and a wall jet near the ground surface. Second, the vortex ring model is a combination of a primary and an image vortex ring, with an inviscid -viscous interaction at the central axial and surface regions. An unsteady version of this model is also provided by solving the trajectory equation with the Direct Formal Integration (DFI) method or with the Runge-Kutta method. Third and finally, the complete unsteady microburst model equations (conservation of mass, momentum, and energy), or what has been referred to as the Navier-Stokes model formulation, are solved by the successive over relaxation method. Results show that the microburst can be simulated accurately by impulsive turbulent jet at high altitude and a transient vortex ring in mid-air and near the ground surface. In addition to improved understanding of the physical nature of microbursts, the models presented here can also be used for flight simulation and the pilot training purposes.

Wan, Tung

159

Preliminary Velocity Measurements in the Wake of a Submarine Model  

NASA Astrophysics Data System (ADS)

Preliminary Particle Image Velocimetry (PIV) over a submarine shape has been conducted in a low speed wind tunnel at Princeton University. The model is a 1/67 replica of the USS Albacore, an experimental submarine designed to achieve maximum underwater performance, and based on "bodies of revolution." The model is tested with a sail, and different tail appendages. Velocity vector fields and flow visualizations in the wake region are presented for Reynolds numbers based on model length up to 10^5. The experiments establish the groundwork for future investigations of submarine models in the new High Reynolds Number Test Facility (http://www.princeton.edu/ gasdyn/HRTF.html). Supported by ONR Grants N00014-97-1-0325, N00014-97-1-0340 and N00014-97-1-0618.

Jimenez, J. M.; Reynolds, R.; Smits, A. J.

2000-11-01

160

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

161

Model for nodal quasiparticle scattering in a disordered vortex lattice  

NASA Astrophysics Data System (ADS)

Recent scanning-tunneling experiments on Ca2-xNaxCuO2Cl2 by Hanaguri [Science 323, 923 (2009)] observe field-dependent quasiparticle interference effects which are sensitive to the sign of the d -wave order parameter. Their analysis of spatial fluctuations in the local density of states shows that there is a selective enhancement of quasiparticle scattering events that preserve the gap sign and a selective depression of the quasiparticle scattering events that reverse the gap sign. We introduce a model which accounts for this phenomenon as a consequence of vortex pinning to impurities. Each pinned vortex embeds several impurities in its core. The observations of recent experiments can be accounted for by assuming that the scattering potentials of the impurities inside the vortex cores acquire an additional resonant or Andreev scattering component, both of which induce gap sign preserving scattering events.

Maltseva, Marianna; Coleman, P.

2009-10-01

162

A mathematical model of network dynamics governing sleep-wake patterns in mice  

PubMed Central

Recent advances in the neurophysiology of sleep have identified wake- and sleep-active neuronal populations that selectively promote wake and sleep, respectively; mutual inhibition between these populations has suggested the conceptual model of the sleep-wake switch. To better understand the network dynamics underlying behavioral state control, we modeled the sleep-wake network comprising both of these systems using Morris-Lecar relaxation oscillators. Our model can reproduce features of mouse behavior, including number and duration of bouts and organization of behavioral states. Qualitative differences between brief and sustained wake bouts have been observed experimentally in multiple species. Our model captures the relative differences in incidence and duration between brief and sustained wake bouts, and it suggests that brief awakenings arise due to intrinsic properties of the wake-active population while sustained wake bouts are governed by network dynamics. Therefore, our model provides a novel framework to explore dynamical principles that may underlie normal and pathologic sleep-wake physiology.

Behn, Cecilia Diniz; Brown, Emery N.; Scammell, Thomas E.; Kopell, Nancy

2008-01-01

163

Turbulent near wake of an Ahmed vehicle model  

NASA Astrophysics Data System (ADS)

The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. The simplified Ahmed vehicle model is characterized by controllable flow separation, thus especially suitable for this purpose. In spite of a considerable number of previous investigations, our knowledge of flow around this model remains incomplete. This work aims to revisit turbulent flow structure behind this model. Two rear slant angles, i.e., ? = 25º and 35º, of the model were examined, representing two distinct flow regimes. The Reynolds number was 5.26 × 104 based on the model height ( H) and incident flow velocity. Using particle image velocimetry (PIV), flow was measured with and without a gap ( g/H = 0.174) between the vehicle underside and ground in three orthogonal planes, viz. the x- z, x- y and y- z planes, where x, y, and z are the coordinates along longitudinal, transverse, and spanwise directions, respectively. The flow at g/H = 0 serves as an important reference for the understanding of the highly complicated vehicle wake ( g/H ? 0). While reconfirming the well-documented major characteristics of the mean flow structure, both instantaneous and time-averaged PIV data unveil a number of important features of the flow structure, which have not been previously reported. As such, considerably modified flow structure models are proposed for both regimes. The time-averaged velocities, second moments of fluctuating velocities, and vorticity components are presented and discussed, along with their dependence on g/H in the two distinct flow regimes.

Wang, X. W.; Zhou, Y.; Pin, Y. F.; Chan, T. L.

2013-04-01

164

Center vortex model for the infrared sector of SU(3) Yang-Mills theory: Vortex free energy  

SciTech Connect

The vortex free energy is studied in the random vortex world-surface model of the infrared sector of SU(3) Yang-Mills theory. The free energy of a center vortex extending into two spatial directions, which is introduced into Yang-Mills configurations when acting with the 't Hooft loop operator, is verified to furnish an order parameter for the deconfinement phase transition. It is shown to exhibit a weak discontinuity at the critical temperature, corresponding to the weak first-order character of the transition.

Quandt, M.; Reinhardt, H.; Engelhardt, M. [Institut fuer Theoretische Physik, Universitaet Tuebingen, D-72076 Tuebingen (Germany); Physics Department, New Mexico State University, Las Cruces, New Mexico 88003 (United States)

2005-03-01

165

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

166

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

167

Numerical Modeling of an Axisymmetric Trapped Vortex Combustor  

NASA Astrophysics Data System (ADS)

This paper is concerned with the numerical study of the effect of injection momentum on the flow field, fuel-air distribution and temperature distribution within the cavity of an axisymmetric trapped vortex combustor (TVC) under reacting flow condition. Shear Stress Transport (SST) k-? model was used for turbulence modeling while combustion was simulated using the Eddy Dissipation Model (EDM). The predicted velocity field and temperature profile inside the cavity matched reasonably well with the experimental results available in literature. Reacting flow studies revealed that when the cavity momentum flux ratio, MFR c ? 1, the TVC exhibited similar flow features, irrespective of the relative momentum between the cavity and the mainstream flow. For these cases, a single vortex, rotating in the stream-wise was established within the cavity. The temperature distribution was also found to be similar within the cavity for these cases. However, nonuniform fuel air distribution was observed for these cases. In contrast, MFR c > 1 cases exhibited entirely different flow pattern. For these cases, multiple vortices were observed within the cavity, with a counter streamwise vortex oriented at the bottom wall. Within this counter streamwise vortex, the temperature distribution was observed to be uniform. Hence, MFR c is the dictating factor in determining the flow and temperature field within the TVC cavity. Beside this, by maintaining same aft- and fore-wall dimensions, a counter rotating vortex characterized by better fuel-air mixing within the entire cavity domain can be established for MFR C > 1 cases, which needs further investigation in future.

Ezhil Kumar, P. K.; Mishra, D. P.

2011-04-01

168

Modelling of Offshore Wind Turbine Wakes with the Wind Farm Program FLaP  

Microsoft Academic Search

The wind farm layout program FLaP estimates the wind speed at any point in a wind farm and the power output of the turbines. The ambient flow conditions and the properties of the turbines and the farm are used as input. The core of the program is an axisymmetric wake model describing the wake behind one rotor. Here an approach

Bernhard Lange; Hans-Peter Waldl; Algert Gil Guerrero; Detlev Heinemann; Rebecca J. Barthelmie

2003-01-01

169

A model of the ion wake of Mars  

NASA Astrophysics Data System (ADS)

Observations from the Phobos spacecraft suggest that, for the most part, the Martian magnetotail is induced instead of intrinsic. Test particle techniques are used to determine how the associated ion wake might appear to detectors behind Mars if it is produced primarily by planetary ions picked up by the solar wind in the dayside exosphere. The results suggest that low energy pickup ions populate the inner wake, while a more energetic component is present in the outer magnetosheath and adjacent solar wind.

Luhmann, J. G.

1990-05-01

170

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

171

A high resolution tower shadow model for downwind wind turbines  

Microsoft Academic Search

A high resolution model for tower shadow effects on horizontal axis wind turbines has been developed which involves the use of a prescribed wake vortex model and an efficient near wake dynamic model of the vorticity trailed from the blade. The prescribed wake model is applied at first stage of the modelling strategy to provide basic information on blade aerodynamics.

Tongguang Wang; Frank N. Coton

2001-01-01

172

Vortex Dynamics of Accelerated Inhomogeneous Flows: Visiometrics and Modeling  

NASA Astrophysics Data System (ADS)

Rayleigh-Taylor and Richtmyer-Meshkov instabilities arise in several high-energy fluid enviroments, including: supersonic combustion; laser (inertial confinement) fusion and supernova astrophysics. We use the ``vortex paradigm'' to guide our understanding and modeling of early-to-intermediate time phenomena at perturbed interfaces and bubbles, including reshock and reacceleration processes. We show the emergence of ``vortex projectiles" or VPs ( dipolar or ring-like coherent structures) and comment on their ubiquity. Acknowlegemets: Since 1989 I have benefited from collaborations with : J. Hawley, X. Yang, R. Samtaney, J. Ray, A. Kotelnikov and D. G. Dritschel. The work has been supported by DOE Grant No. DE-FG0293ER25179 under Drs. Daniel Hitchcock and Fred Howes. Computations were done on the C90 and T3E at Pittsburgh Supercomputing Center and the SP2 at Cornell Theory Center. We also acknowledge the support of CAIP, Rutgers University.

Zabusky, Norman

1998-11-01

173

Vortex dynamics for two-dimensional XY models  

NASA Astrophysics Data System (ADS)

Two-dimensional XY models with resistively shunted junction (RSJ) dynamics and time dependent Ginzburg-Landau (TDGL) dynamics are simulated and it is verified that the vortex response is well described by the Minnhagen phenomenology for both types of dynamics. Evidence is presented supporting that the dynamical critical exponent z in the low-temperature phase is given by the scaling prediction (expressed in terms of the Coulomb gas temperature TCG and the vortex renormalization given by the dielectric constant ?~) z=1/?~TCG-2>=2 both for RSJ and TDGL and that the nonlinear IV exponent a is given by a=z+1 in the low-temperature phase. The results are discussed and compared with the results of other recent papers and the importance of the boundary conditions is emphasized.

Kim, Beom Jun; Minnhagen, Petter; Olsson, Peter

1999-05-01

174

Dynamic parameters in models of atmospheric vortex structures  

NASA Astrophysics Data System (ADS)

Vortex simulation and the computation of fields of dynamic parameters of vortex structures (velocity, rotor velocity, and helicity) are carried out with the use of exact hydrodynamic equations in a cylindrical coordinate system. Components of centripetal and Coriolis accelerations are taken into account in the initial equations. Internal and external solutions are defined. Internal solutions ignore the disturbances of the pressure field, but they are considered in external solutions. The simulation is carried out so that the effect of accounting for spatial coordinates on the structure of the above fields is pronounced. It is shown that the initial kinetic energy of rotating motion transforms into the kinetic energy of radial and vertical velocity components in models with centripetal acceleration. In models with Coriolis acceleration, the Rossby effect is clearly pronounced. The method of an "inverse problem" is used for finding external solutions, i.e., reconstruction of the pressure field at specified velocity components. Computations have shown that tangential components mainly contribute to the velocity and helicity vortex moduli at the initial stage.

Dobryshman, E. M.; Kochina, V. G.; Letunova, T. A.

2013-09-01

175

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

176

Modeling for Airframe Noise Prediction Using Vortex Methods  

NASA Astrophysics Data System (ADS)

Various components of the airframe are known to be a significant source of noise. With the advent of technology in quieting modern engines, airframe generated noise competes and, in certain instances, surpasses the engine noise. Airframe noise is most pronounced during aircraft approach when the engines are operating at reduced thrust, and airframe components such as high-lift devices and landing gears are in deployed conditions. Recent experimental studies have reaffirmed that the most significant sources of high-lift noise are from the leading-edge slat and the side edges of flaps. Studies of flow field around these structures have consistently shown that there are complicated unsteady vortical flows such as vortex shedding, secondary vortices and vortex breakdown, which are susceptible to far-field radiated sound. The near-field CFD computational data have been used to calculate the far-field acoustics by employing Ffowcs-Williams/Hawkings equation using Lighthill's analogy. However, because of the limit of current computing capacity, it is very time consuming to generate unsteady Navier-Stokes (N-S) computational data for aeroacoustics. Although the N-S simulations are probably necessary to reveal many complex flow phenomena that are unsteady and fully nonlinear, these simulations are not feasible to be used for parametric design. purposes. The objective of this study is thus to develop theoretical models for airframe noise predictions which have quick turn-around computing time. Since it is known that vorticity is a major mechanism responsible for noise generation on high-lift devices, vortex methods have been chosen as modeling tools. Vortex methods are much faster in comparison with other numerical methods, yet they are able to incorporate nonlinear interactions between vortices. Obviously, as with any theoretical model, assumptions have to be made and justified when such models are used in complex flow. The merit and applicability of the models for aeroacoustics applications will be investigated. Issues related with conservation of vorticity, unsteady Kutta conditions and nonlinear frequency response to vortex shedding will be addressed.

Zheng, Z. Charlie

2002-12-01

177

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

178

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

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

Modeling Development of Sleep–Wake Behaviors. II. Results of Two Cohorts of Preterms  

Microsoft Academic Search

HOLDITCH-DAVIS, D. AND L. J. EDWARDS.Modeling development of sleep–wake behavior. II. Results of two cohorts of preterms.PHYSIOL BEHAV 63(3) 319–328, 1998.—A mixed general linear model analysis of the development of sleep–wake states was conducted on 37 high-risk preterm infants and replicated with a second cohort of 34 infants. Most dependent variables showed significant development over the preterm period: active sleep

Diane Holditch-Davis; Lloyd J. Edwards

1998-01-01

181

Experimental examination of vortex-sound generation in an organ pipe: A proposal of jet vortex-layer formation model  

NASA Astrophysics Data System (ADS)

Aero-dynamical models of sound generation in an organ pipe driven by a thin jet are investigated through an experimental examination of the vortex-sound theory. An important measurement requirement (acoustic cross-flow as an irrotational potential flow reciprocating sinusoidally) from the vortex-sound theory is carefully realized when the pipe is driven with low blowing pressures of about 60 Pa (jet velocities of about 10 m/s). Particle image velocimetry (PIV) is applied to measure the jet velocity and the acoustic cross-flow velocity over the mouth area at the same phase by quickly switching the jet drive and the loudspeaker-horn drive. The vorticity of the jet flow field and the associated acoustic generation term are evaluated from the measurement data. It is recognized that the model of the “jet vortex-layer formation” is more relevant to the sound production than the vortex-shedding model. The acoustic power is dominantly generated by the flow-acoustic interaction near the edge, where the acoustic cross-flow velocity takes larger magnitudes. The acoustic generation formula on the vortex sound cannot deny the conventional acoustical volume-flow model because of the in-phase relation satisfied between the acoustic pressure at the mouth and the acoustic volume flow into the pipe. The vortex layers formed along both sides of the jet act as the source of an accelerating force (through the “acceleration unbalance”) with periodically alternating direction to oscillate the jet flow and to reinforce the acoustic cross-flow at the pipe mouth.

Yoshikawa, Shigeru; Tashiro, Hiromi; Sakamoto, Yumiko

2012-05-01

182

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

183

SOURCE TERM MODEL FOR VORTEX GENERATOR VANES IN A NAVIER-STOKES COMPUTER CODE  

Microsoft Academic Search

A source term model for an array of vortex generators was implemented into a non-proprietary Navier-Stokes computer code, OVERFLOW. The source term models the side force created by a vortex generator vane. The model is obtained by introducing a side force to the momentum and energy equations that can adjust its strength automatically based on the local flow. The model

Kenrick A. Waithe

184

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

185

A Transactional Model of Sleep-Wake Regulation in Infants Born Preterm or Low Birthweight  

Microsoft Academic Search

Objective To test a transactional model of sleep-wake development in infants born preterm or low birthweight (PT LBW), which may inform clinical practice, interventions, and future research in this at risk population. Methods One hundred and twenty-eight mother-infant dyads participated from hospital discharge to 4 months postterm. Assessments of prematurity, infant sleep-wake patterns, maternal interaction quality, depression, feeding route, and

A. J. M. Schwichtenberg; Julie Poehlmann

2008-01-01

186

Aeroelastic analysis of a non-linear airfoil based on unsteady vortex lattice model  

Microsoft Academic Search

Recent years have witnessed a successful model of unsteady vortex lattice to predict the limit cycle oscillations of an airfoil section. The aerodynamic model is usually in the form of discrete time, and hence, is not convenient for the analysis of non-linear aeroelastic systems. In this paper, the aerodynamic model of unsteady vortex lattice is formulated in continuous time domain

Y. H Zhao; H. Y Hu

2004-01-01

187

Vortex model of open channel flows with gravel beds  

NASA Astrophysics Data System (ADS)

Turbulent structures are known to be important physical processes in gravel-bed rivers. A number of limitations exist that prohibit the advancement and prediction of turbulence structures for optimization of civil infrastructure, biological habitats and sediment transport in gravel-bed rivers. This includes measurement limitations that prohibit characterization of size and strength of turbulent structures in the riverine environment for different case studies as well as traditional numerical modeling limitations that prohibit modeling and prediction of turbulent structure for heterogeneous beds under high Reynolds number flows using the Navier-Stokes equations. While these limitations exist, researchers have developed various theories for the structure of turbulence in boundary layer flows including large eddies in gravel-bed rivers. While these theories have varied in details and applicable conditions, a common hypothesis has been a structural organization in the fluid which links eddies formed at the wall to coherent turbulent structures such as large eddies which may be observed vertically across the entire flow depth in an open channel. Recently physics has also seen the advancement of topological fluid mechanical ideas concerned with the study of vortex structures, braids, links and knots in velocity vector fields. In the present study the structural organization hypothesis is investigated with topological fluid mechanics and experimental results which are used to derive a vortex model for gravel-bed flows. Velocity field measurements in gravel-bed flow conditions in the laboratory were used to characterize temporal and spatial structures which may be attributed to vortex motions and reconnection phenomena. Turbulent velocity time series data were measured with ADV and decomposed using statistical decompositions to measure turbulent length scales. PIV was used to measure spatial velocity vector fields which were decomposed with filtering techniques for flow visualization. Under the specific conditions of a turbulent burst the fluid domain is organized as a braided flow of vortices connected by prime knot patterns of thin-cored flux tubes embedded on an abstract vortex surface itself having topology of a Klein bottle. This model explains observed streamline patterns in the vicinity of a strong turbulent burst in a gravel-bed river as a coherent structure in the turbulent velocity field. KEY WORDS: Open channel flow, turbulence, gravel-bed rivers, coherent structures, velocity distributions

Belcher, Brian James

188

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

189

Vortex and gap generation in gauge models of graphene  

SciTech Connect

Effective quantum field theoretical continuum models for graphene are investigated. The models include a complex scalar field and a vector gauge field. Different gauge theories are considered and their gap patterns for the scalar, vector, and fermion excitations are investigated. Different gauge groups lead to different relations between the gaps, which can be used to experimentally distinguish the gauge theories. In this class of models the fermionic gap is a dynamic quantity. The finite-energy vortex solutions of the gauge models have the flux of the ''magnetic field'' quantized, making the Bohm-Aharonov effect active even when external electromagnetic fields are absent. The flux comes proportional to the scalar field angular momentum quantum number. The zero modes of the Dirac equation show that the gauge models considered here are compatible with fractionalization.

Oliveira, O. [Departamento de Fisica, Instituto Tecnologico de Aeronautica, 12.228-900, Sao Jose dos Campos, SP (Brazil); Departamento de Fisica, Universidade de Coimbra, P-3004-516 Coimbra (Portugal); Cordeiro, C.E.; Delfino, A. [Instituto de Fisica, Universidade Federal Fluminense, 24210-3400- Niteroi - RJ (Brazil); Paula, W. de; Frederico, T. [Departamento de Fisica, Instituto Tecnologico de Aeronautica, 12.228-900, Sao Jose dos Campos, SP (Brazil)

2011-04-15

190

Control-oriented modeling and identification of delta wing vortex-coupled roll dynamics  

Microsoft Academic Search

This paper presents the derivation of a control-oriented vortex-based nonlinear state space representation of free-to-roll motion of a delta wing based on a modified nonlinear indicial response method, in conjunction with an internal state-space representation. The relationship among the vortex breakdown location, rolling moment coefficient and roll angle are developed. The proposed model, in fact, integrates the vortex breakdown location

M. Pakmehr; B. W. Gordon; C. A. Rabbath

2005-01-01

191

Numerical Simulation and Wake Modeling of Wind Turbine Rotor as AN Actuator Disk  

NASA Astrophysics Data System (ADS)

Numerical simulations of flow fields around the wind turbine rotor simplified as an actuator disk (AD) with zero thickness have been made to investigate the flow structure and wake development in different operation states. A N-S solver has been used and the energy extracted by the rotor is represented by a discontinuous pressure jump through the actuator disk. Axial pressure and velocity development from far upstream to far downstream is fully described by the simulations, which could never be obtained by the momentum theory. It is showed that there are significant differences in wake development between inviscid and viscous conditions. In inviscid simulations, the axial velocity keeps decreasing along the oncoming flow direction, which is consistent with the momentum theory. In viscous simulations, however, the axial velocity first decreases but then gradually recovers approaching to the undisturbed velocity, due to momentum transport from outer flow to wake flow by viscous shear effect. Based on the numerical analysis, the work of this paper is also focused on wake modeling. A new two-dimensional models based on nonlinear wake development has been developed, which is capable to describe the far wake more accurately.

Shen, Xiang; Wang, Tongguang; Zhong, Wei

192

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

193

Flow curvature and dynamic stall simulated with an aerodynamic free-vortex model for VAWT  

Microsoft Academic Search

The phenomenon of flow curvature has been introduced into the aerodynamic free-vortex model initially developed by Strickland for vertical axis wind turbines with straight blades. By comparing the theoretical results of the modified vortex model with the experimental ones, it is deduced that the consideration of the flow curvature improves the prediction of the transient aerodynamic forces but not that

J. L. Cardona

1984-01-01

194

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

195

Exact vortex solutions in an extended Skyrme-Faddeev model  

NASA Astrophysics Data System (ADS)

We construct exact vortex solutions in 3+1 dimensions to a theory which is an extension, due to Gies, of the Skyrme-Faddeev model, and that is believed to describe some aspects of the low energy limit of the pure SU(2) Yang-Mills theory. Despite the efforts in the last decades those are the first exact analytical solutions to be constructed for such type of theory. The exact vortices appear in a very particular sector of the theory characterized by special values of the coupling constants, and by a constraint that leads to an infinite number of conserved charges. The theory is scale invariant in that sector, and the solutions satisfy Bogomolny type equations. The energy of the static vortex is proportional to its topological charge, and waves can travel with the speed of light along them, adding to the energy a term proportional to a U(1) Noether charge they create. We believe such vortices may play a role in the strong coupling regime of the pure SU(2) Yang-Mills theory.

Ferreira, L. A.

2009-05-01

196

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

197

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

198

An overview of experimental results and dispersion modelling of nanoparticles in the wake of moving vehicles.  

PubMed

Understanding the transformation of nanoparticles emitted from vehicles is essential for developing appropriate methods for treating fine scale particle dynamics in dispersion models. This article provides an overview of significant research work relevant to modelling the dispersion of pollutants, especially nanoparticles, in the wake of vehicles. Literature on vehicle wakes and nanoparticle dispersion is reviewed, taking into account field measurements, wind tunnel experiments and mathematical approaches. Field measurements and modelling studies highlighted the very short time scales associated with nanoparticle transformations in the first stages after the emission. These transformations strongly interact with the flow and turbulence fields immediately behind the vehicle, hence the need of characterising in detail the mixing processes in the vehicle wake. Very few studies have analysed this interaction and more research is needed to build a basis for model development. A possible approach is proposed and areas of further investigation identified. PMID:21193254

Carpentieri, Matteo; Kumar, Prashant; Robins, Alan

2010-12-28

199

From a Desingularized Vortex Sheet Model to a Turbulent Mixing Layer  

NASA Astrophysics Data System (ADS)

The temporal mixing layer is studied using the model of a slightly perturbed vortex sheet which is unstable and tends to roll-up in a spiral. The flow is inviscid and incompressible. A point vortex model tends to evolve into a chaotic cloud of point vortices instead of a smooth double branched spiral. The vortex sheet model is derived (in closed form) from the basic equations of vortex dynamics. The problem of finite time singularity is handled by a technique that invokes longitudinal circulation density diffusion along the sheet at singular points. The present model uses linear segments to interpolate the sheet. Although it is computationally involved compared to point vortices, the vortex sheet does not get distorted and rolls-up into a smooth double branched spiral. The accuracy of such simulations can be independently verified by using the laws of vortex dynamics and conserved quantities. We observe the growth of the two-dimensional shear layer with time and the merger of vortex like structures. The dependence of the mixing layer on the initial conditions is studied in detail and tries to answer the question whether the vortex sheet model yields a turbulent mixing layer.

Paul, Ujjayan; Narasimha, Roddam

2013-03-01

200

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

201

Modeling and simulation of vortex induced vibration on the subsea riser\\/pipeline (GRP pipe)  

Microsoft Academic Search

This paper presents the research work conducted to investigate the dynamics characteristics of the offshore riser pipeline due to vortex flow and to develop a model that could predict its vortex induced responses. Glass-fiber reinforced plastic (GRP) pipe is used for this study. A two-dimensional finite element computational method is implemented to approximately describe the dynamic behavior of the riser.

Raja Nor Fauziah bt Raja Adli; Ir. Idris Ibrahim

2011-01-01

202

Composite vortex model of the electrodynamics of type-II superconductors  

SciTech Connect

A phenomenological model of vortex dynamics is presented in which the vortex is taken as a composite object with two components: the vortex current pattern, which is massless and driven by the Lorentz force, and the vortex core, which is massive and driven by the Magnus force. By combining the characteristics of the Gittleman-Rosenblum model [Phys. Rev. Lett. {bold 16}, 734 (1996)] and Hsu`s theory of vortex dynamics [Physica C {bold 213}, 305 (1993)], the model provides a good description of the magnetoconductivity tensor of superconducting YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films measured over the frequency range from the microwave to 200 cm{sup {minus}1}. {copyright} {ital 1997} {ital The American Physical Society}

Lihn, H.S.; Drew, H.D. [Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, Maryland 20742 (United States)]|[Laboratory for Physical Sciences, College Park, Maryland 20740 (United States)

1997-09-01

203

Applying dynamic wake models to large swirl velocities for optimal propellers  

NASA Astrophysics Data System (ADS)

The dynamic wake model is applied to the optimal propeller systems originally studied by the classic aerodynamicists: Betz, Prandtl and Goldstein. Several modified forms of the model are theoretically developed to extend the applicable range to flight conditions with a large swirl velocity component. Dynamic wake model calculations accurately predict the inflow behavior for helicopter rotors, including axial flow for large tip-speed ratios, (OR/V infinity) ? 20. The swirl velocity is a prominent component for small tip-speed ratios (?5), typical of forward flight for tiltrotor craft such as the V-22 Osprey and the BA609. Dynamic wake calculation results are compared to the closed-form solutions by Prandtl and Goldstein. The exact and approximate solutions correlate strongly for infinite blade cases and finite blade cases with a large tip-speed ratio. The original form of the He-Peters and Morillo-Peters dynamic wake models converge poorly for small tip-speed ratios, due to neglect of the swirl velocity. Derivations are presented for several adaptations of the models to account for the large apparent mass at the inboard blade region. A best modified form is chosen and the associated empirical factor is optimized to correlate well with Prandtl's solution. Error norms for the original and modified forms of the dynamic wake model are presented for propellers of various number of blades and a range of tip-speed ratios. The Goldstein solution is also studied in depth and conclusions are drawn for improving the dynamic wake model.

Makinen, Stephen M.

204

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

205

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

206

Some vortex solutions in the extended Skyrme-Faddeev model  

NASA Astrophysics Data System (ADS)

Analytical and numerical vortex solutions for the extended Skyrme-Faddeev model in a (3 + 1) dimensional Minkowski space-time are investigated. The extension is obtained by adding to the Lagrangian a quartic term, which is the square of the kinetic term, and a potential which breaks the SO(3) symmetry down to SO(2). The construction of the solutions has been done in twofold: one makes use of an axially symmetric ansatz and solves the resulting ODE by an analytical and a numerical way. The analytical vortices are obtained for special form of the potentials, and the numerical ones are computed using the successive over relaxation method for wider choice of the potentials. Another is based on a simulational technique named the simulated annealing method which is available to treat the non-axisymmetric shape of solutions. The crucial thing for determining the structure of vortices is the type of the potential.

Ferreira, L. A.; Hayasaka, M.; Jäykkä, J.; Sawado, N.; Toda, K.

2013-01-01

207

Spiral Galaxy Model with Axial Plasma/Gas Vortex: A Possible Suggestion  

NASA Astrophysics Data System (ADS)

From the measured rotational velocity profiles of spiral galaxies, there appears a central ``core'' with zero central velocity that increases quickly with distance (˜15% of galactic radius) to large constant velocity. This data implies a large centrifugal hydrodynamic force suggesting the ``core'' is a central axial plasma/gas vortex with the following features. (1) MINIMUM central Pressure/Density of Plasma/Gas. (2) Axial Vortex which is manifested as the observed axial Galactic Jets? (3) MAXIMUM central light intensity due to high temperature of energetic central axial vortex? Extensions of the model follow. (1) Matter is sucked into the Galactic Center by the Central Axial Vortex, and that matter is expelled via the central axial galactic jets in dynamic equilibrium over long time scale? (2) No galactic black holes? This vortex model does not address many other important issues such as Dark Matter, galactic evolution, plasma effects, galactic stability, relativistic effects, spiral details, galactic clusters, etc.

Gallo, C. F.

2007-04-01

208

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

209

Influence of the wake model on the thrust of oscillating foil  

Microsoft Academic Search

Trust generation by flapping wing is a complex fluid phenomenon involving unsteady effects. The work discusses a Boundary Element Method (BEM) based computer model for the analysis of hydrodynamic forces on flapping foil. The specific focus is on the wake model and its effects on the generated thrust. An unsteady formulation of the Kutta condition, assuming finite pressure difference at

Marco La Mantia; Peter Dabnichki

2011-01-01

210

Recent advances in modeling of wind turbine wake vortical structure using a differential actuator disk theory  

Microsoft Academic Search

This paper presents the recent developments of a new CFD-based method aimed at predicting wind turbine aerodynamics, where velocity and pressure discontinuities are used to model the vortical system that creates lift on the turbine blades. To illustrate the ability of the present model to predict induced wake effect, the case of the taper wing is thoroughly analyzed and effects

Christophe Sibuet Watters; Christian Masson

2007-01-01

211

PERFORMANCE AND LIMITATIONS OF A GAUSSIAN MODEL FOR CENTERLINE CONCENTRATIONS IN THE WAKE OF BUILDINGS  

EPA Science Inventory

The inherent variability of 10- to 60-min average ground-level plume centerline concentrations in the wake of buildings and inherent limitations on, the performance of a Gaussian plume model are presented. oint comparisons between observed and model-predicted concentrations were ...

212

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.

213

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

214

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

215

PRELIMINARY STUDIES OF VIDEO IMAGES OF SMOKE DISPERSION IN THE NEAR WAKE OF A MODEL BUILDING  

EPA Science Inventory

A scary of analyses of video images of smoke in a wind tunnel study of dispersion in the near wake of a model building is presented. The analyses provide information on both the instantaneous and the time- average patterns of dispersion. ince the images represent vertically-integ...

216

ENDOW (efficient development of offshore wind farms): modelling wake and boundary layer interactions  

Microsoft Academic Search

While experience gained through the offshore wind energy projects currently operating is valuable, a major uncertainty in estimating power production lies in the prediction of the dynamic links between the atmosphere and wind turbines in offshore regimes. The objective of the ENDOW project was to evaluate, enhance and interface wake and boundary layer models for utilization offshore. The project resulted

Rebecca Barthelmie; Gunner Larsen; Sara Pryor; Hans Jørgensen; Hans Bergström; Wolfgang Schlez; Kostas Rados; Bernhard Lange; Per Vølund; Søren Neckelmann; Søren Mogensen; Gerard Schepers; Terry Hegberg; Luuk Folkerts; Mikael Magnusson

2004-01-01

217

Considerations for Modeling Particle Entrainment into the Wake of a Circular Cylinder  

Microsoft Academic Search

The objective of this work is to evaluate the performance of the steady state Reynolds Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) models for estimating concentration of low Stokes number aerosols (Stk = O(10)) in the wake of a bluff body. These simulations are compared with experimental data. In the simulations and experiments, particles are released upstream of the body

J. Richmond-Bryant; A. D. Eisner; M. R. Flynn

2006-01-01

218

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

219

Electronic structure around a vortex core in iron-based superconductors: Numerical studies of a two-orbital model  

NASA Astrophysics Data System (ADS)

Based on a two-orbital model and taking into account the presence of impurities, we study theoretically the electronic structure in the vortex cores of iron-pnictide superconducting materials. The vortex is pinned when the impurity is close to the vortex core. The bound states show up for the unpinned vortex. One strong resonant peak at the negative energy is revealed at the vortex center and the peak splits with the dominant weight at the negative energy. For the pinned vortex, the in-gap low-energy features are wiped out by the impurity so that the gaplike feature for the local density of states persists in the vortex. Our results are in good agreement with recent experiments, presenting a consistent explanation for the different electronic structure of the vortex core revealed by experiments on different materials.

Zhou, Tao; Wang, Z. D.; Gao, Yi; Ting, C. S.

2011-11-01

220

Classical vortex solutions in three-dimensional supersymmetric Abelian-Higgs model.  

National Technical Information Service (NTIS)

Classical vortex solutions in a three-dimensional supersymmetric Abelian-Higgs model are presented. A cylindrically symmetric ansatz for the bosonic and fermionic fields is used, and the asymptotic behaviour of these fields are also obtained. No electric ...

E. R. B. Mello P. R. Peduto S. Frota-Pessoa

1989-01-01

221

Low-dimensional modelling of a transient cylinder wake using double proper orthogonal decomposition  

NASA Astrophysics Data System (ADS)

For the systematic development of feedback flow controllers, a numerical model that captures the dynamic behaviour of the flow field to be controlled is required. This poses a particular challenge for flow fields where the dynamic behaviour is nonlinear, and the governing equations cannot easily be solved in closed form. This has led to many versions of low-dimensional modelling techniques, which we extend in this work to represent better the impact of actuation on the flow. For the benchmark problem of a circular cylinder wake in the laminar regime, we introduce a novel extension to the proper orthogonal decomposition (POD) procedure that facilitates mode construction from transient data sets. We demonstrate the performance of this new decomposition by applying it to a data set from the development of the limit cycle oscillation of a circular cylinder wake simulation as well as an ensemble of transient forced simulation results. The modes obtained from this decomposition, which we refer to as the double POD (DPOD) method, correctly track the changes of the spatial modes both during the evolution of the limit cycle and when forcing is applied by transverse translation of the cylinder. The mode amplitudes, which are obtained by projecting the original data sets onto the truncated DPOD modes, can be used to construct a dynamic mathematical model of the wake that accurately predicts the wake flow dynamics within the lock-in region at low forcing amplitudes. This low-dimensional model, derived using nonlinear artificial neural network based system identification methods, is robust and accurate and can be used to simulate the dynamic behaviour of the wake flow. We demonstrate this ability not just for unforced and open-loop forced data, but also for a feedback-controlled simulation that leads to a 90% reduction in lift fluctuations. This indicates the possibility of constructing accurate dynamic low-dimensional models for feedback control by using unforced and transient forced data only.

Siegel, Stefan G.; Seidel, J.?Rgen; Fagley, Casey; Luchtenburg, D. M.; Cohen, Kelly; McLaughlin, Thomas

222

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

223

Impact of vortex-removal from environmental flow in cyclone track prediction using Lagrangian advection model  

NASA Astrophysics Data System (ADS)

In the present study, a new approach is discussed to find out the residual steering flow from the high-resolution global Numerical Weather Prediction (NWP) model-forecasted wind fields, which have been used in the Lagrangian advection model to determine the track of tropical cyclones formed in the Indian Ocean. The Lagrangian advection model is newly developed model and conceptually closer to the dynamical models, which utilizes environmental steering flow and the effect due to earth's rotation (the beta-effect) to determine the motion of cyclone. In this approach, the effect of environmental flow on the cyclone track is examined by removing the existing cyclone vortex from the steering flow which is determined by potential vorticity approach. A new approach based on vortex pattern matching has been used to identify the cyclone vortex and to remove it from the steering flow. The tracks of five tropical cyclones (viz., Nargis, Khai_Muk, Nisha, Aila and Phyan) which were formed in the North Indian basin during the period 2008-2009 have been generated by the Lagrangian advection model using the proposed scheme. The position errors were computed with respect to the Joint Typhoon Warning Center (JTWC) best track analysis positions and compared with that of without-vortex-removal scheme. The results show that the mean track errors for five cyclones are reduced by 6-35 % for 12-72 h forecast in case of vortex-removal scheme as compared to the without-vortex-removal scheme.

Singh, Sanjeev Kumar; Kishtawal, C. M.; Jaiswal, Neeru; Singh, Randhir; Pal, P. K.

2012-08-01

224

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

225

Theories, Models, and Frameworks Related to Sleep-Wake Disturbances in the Context of Cancer  

PubMed Central

The purpose of this article was to review theories, models, and frameworks of sleep disturbances referenced in the cancer literature. Sleep-wake disturbances in cancer are a significant problem that negatively affects quality of life. There is no previously published review of the theories, models, or frameworks used to study sleep-wake disturbances in the context of cancer. Describing existing theories or models and their application in cancer is important to advance knowledge in this area. Two theories and 9 models were identified for review. These have been used to further understand the problem of sleep-wake disturbances as a primary or secondary symptom within the cancer literature. Searches were conducted from January 1, 1970, to July 31, 2008, to find relevant articles using 4 electronic databases: MEDLINE, CINAHL, PubMed, and PsychINFO. On the basis of the search, 73 descriptive or intervention studies were identified and reviewed. Most research was atheoretical, with no identified theory, model, or framework. In studies that did use theory or models, few were applied in more than one study. Although several commonalities across models did emerge, a more comprehensive and widely used model could help guide nursing research to facilitate effective symptom management for this prominent problem in cancer.

Otte, Julie L.; Carpenter, Janet S.

2010-01-01

226

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

227

Dynamical properties of the two-process model for sleep-wake cycles in infantile autism  

Microsoft Academic Search

The two-process model is a scheme for the timing of sleep that consists of homeostatic (Process S) and circadian (Process\\u000a C) variables. The two-process model exhibits abnormal sleep patterns such as internal desynchronization or sleep fragmentation.\\u000a Early infants with autism often experience sleep difficulties. Large day-by-day changes are found in the sleep onset and waking\\u000a times in autistic children. Frequent

Hirotsune Matsuura; Katsumi Tateno; Shuji Aou

2008-01-01

228

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

229

Application of a stochastic vehicular wake erosion model to determine PM2.5 exposure  

NASA Astrophysics Data System (ADS)

A stochastic model based on empirical probability distribution functions has been applied to estimate human exposure to emissions from the resuspension of road dust due to isolated wakes from moving vehicles. The model incorporates the turbulent nature of resuspension due to vehicular wake erosion phenomena, and is tested by in situ measurements. It statistically estimates exposure as a function of ambient temperature, traffic activity, human height, and breathe intake rates as a function of age and activity. The model is applied to evaluate whether communities in the Molango region in Mexico are affected by vehicular resuspension of Manganese (Mn) contaminated soil from on-road activities related to mining. Results showed that six and 10 year old children are at higher risk to vehicular resuspended dust exposure than adults, due to their lower height and higher respiratory intake rates during periods of light and moderate activity.

Jazcilevich, Aron; Wellens, Ann; Siebe, Christina; Rosas, Irma; Bornstein, Robert D.; Riojas-Rodríguez, Horacio

2012-06-01

230

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

231

Center vortex model for the infrared sector of SU(3) Yang-Mills theory: Topological susceptibility  

SciTech Connect

The topological susceptibility of the SU(3) random vortex world-surface ensemble, an effective model of infrared Yang-Mills dynamics, is investigated. The model is implemented by composing vortex world surfaces of elementary squares on a hypercubic lattice, supplemented by an appropriate specification of vortex color structure on the world surfaces. Topological charge is generated in this picture by writhe and self-intersection of the vortex world surfaces. Systematic uncertainties in the evaluation of the topological charge, engendered by the hypercubic construction, are discussed. Results for the topological susceptibility are reported as a function of temperature and compared to corresponding measurements in SU(3) lattice Yang-Mills theory. In the confined phase, the topological susceptibility of the random vortex world-surface ensemble appears quantitatively consistent with Yang-Mills theory. As the temperature is raised into the deconfined regime, the topological susceptibility falls off rapidly, but significantly less so than in SU(3) lattice Yang-Mills theory. Possible causes of this deviation, ranging from artefacts of the hypercubic description to more physical sources, such as the adopted vortex dynamics, are discussed.

Engelhardt, M. [Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003 (United States)

2011-01-15

232

Wind-Tunnel Air Wake Survey of a 1/144-Scale Model Aircraft Carrier to Evaluate Several Carrier Configurations.  

National Technical Information Service (NTIS)

This document presents a wind tunnel investigation of the air distrubances along the flight approach path behind aircraft carriers. The air wake behind a model of the proposed CVA 67 aircraft carrier with various deck and island modifications was surveyed...

M. L. Cook D. R. Chaddock

1964-01-01

233

Investigation of Model Wake Blockage Effects at High Angles of Attack in Low-Speed Wind Tunnel  

Microsoft Academic Search

To improve the fidelity of measured aerodynamic characteristics at high angle of attack for modern jet fighters, this paper examines the model wake blockage effect. The wake blockage effect in a 2.2×3.1 m low-speed wind tunnel is investigated by analyzing drag and wall pressure measurements. Circular flat plates of different sizes are used to simulate a test model at high

Lih-Shyng Shyu; Shu-Hao Chuang

2008-01-01

234

Effective dipole moment for the mode coupling instability: Mapping of self-consistent wake models  

SciTech Connect

The theory of the mode coupling instability operating in two-dimensional plasma crystals is generalized, by employing the linear plasma response formalism to describe the interparticle interactions self-consistently. In this approach, the underlying ion distribution function is calculated from first principles. Subthermal and suprathermal regimes of the ion flow are considered. A mapping procedure is proposed, which relates the self-consistent coupling coefficients to the effective dipole moment of the wake-the parameter which characterizes the mode coupling in the framework of the conventionally used Yukawa/point-wake model. The importance of the self-consistent approach is demonstrated by comparing the theoretically obtained dipole moments with the values deduced from experiments.

Roecker, T. B.; Zhdanov, S. K.; Ivlev, A. V.; Morfill, G. E. [Max Planck Institute for Extraterrestrial Physics, 85741 Garching (Germany); Lampe, M. [Department of Astronomy, University of Maryland, College Park, Maryland 20740 (United States); Joyce, G. [Icarus Research, Inc., Bethesda, Maryland 20814 (United States)

2012-07-15

235

Sites of action of sleep and wake drugs: insights from model organisms.  

PubMed

Small molecules have been used since antiquity to regulate our sleep. Despite the explosion of diverse drugs to treat problems of too much or too little sleep, the detailed mechanisms of action and especially the neuronal targets by which these compounds alter human behavioural states are not well understood. Research efforts in model systems such as mouse, zebrafish and fruit fly are combining conditional genetics and optogenetics with pharmacology to map the effects of sleep-promoting drugs onto neural circuits. Recent studies raise the possibility that many small molecules alter sleep and wake via specific sets of critical neurons rather than through the global modulation of multiple brain targets. These findings also uncover novel brain areas as sleep/wake regulators and indicate that the development of circuit-selective drugs might alleviate sleep disorders with fewer side effects. PMID:23706898

Rihel, Jason; Schier, Alexander F

2013-05-23

236

Influence of Wake Models on Calculated Tiltrotor Aerodynamics  

Microsoft Academic Search

Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 1\\/4-scale V- 22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons

Wayne Johnson

2002-01-01

237

A simple model for the origin of quasiperiodic ultradian rhythms in sleep-wake state in the rat  

PubMed Central

In a recent study,1 ultradian rhythms of rat sleep-wake behavior were found, using several methods of time series analysis, to be “quasiperiodic.” That is, ultradian period varied apparently randomly around a mean of approximately 4 h, with no relationship between ultradian period and time of day. Here it is proposed that a simple two-oscillator model can explain the quasiperiodic characteristic of these rhythms. Specifically, in this model a periodic oscillator interacts with a stochastic oscillator to generate a behavioral pattern in which the period and amplitude of the simulated ultradian waves vary randomly around an average value. Preliminary simulations support the plausibility of the model; simulated waveforms were closely similar to behavior patterns observed in adult male rats. It is hypothesized that ultradian rhythms in sleep-wake behavior may arise from a periodic feedback loop (e.g., the sleep-wake homeostat) coupled to a stochastic sleep-wake “flip-flop” switch.

Stephenson, Richard

2013-01-01

238

Analytical Model of Rotor Wake Aerodynamics in Ground Effect.  

National Technical Information Service (NTIS)

The model and the computer program developed provides the velocity, location, and circulation of the tip vortices of a two-blade helicopter in and out of the ground effect. Comparison of the theoretical results with some experimental measurements for the ...

H. A. Saberi

1983-01-01

239

Artifacts in the Wake: Leadership via an Oriented Compass Model  

ERIC Educational Resources Information Center

|Although inextricable, the act of leading, the leader, and outcome of leadership are unique entities. Lack of such differentiation may ensnare novice leaders in broad suppositions. This conceptual article introduces a tool for analyzing leadership. Leaders can leverage the model to evaluate the act of leading, in route, via a measurable…

Fallon, Paul D.

2013-01-01

240

ENDOW: Improvement of Wake Models within Offshore Wind Farms  

Microsoft Academic Search

T he partners in the E NDOW (Ef ficient Developm ent of Of fshore W indfarms) project are validating , testing, desig ning and improving wind farm design tools for the ef ficient design of of fshore wind farm s. T he dif ferent meteorological conditions of fshore constitute a challeng e for the current design tools and models because

W. Schlez; A. Umaña; R. Barthelmie; G. Larsen; K. Rados; B. Lange; G. Schepers; T. Hegberg

2001-01-01

241

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

242

Mean velocity and moments of turbulent velocity fluctuations in the wake of a model ship propulsor  

NASA Astrophysics Data System (ADS)

Pod drives are modern outboard ship propulsion systems with a motor encapsulated in a watertight pod, whose shaft is connected directly to one or two propellers. The whole unit hangs from the stern of the ship and rotates azimuthally, thus providing thrust and steering without the need of a rudder. Force/momentum and phase-resolved laser Doppler anemometry (LDA) measurements were performed for in line co-rotating and contra-rotating propellers pod drive models. The measurements permitted to characterize these ship propulsion systems in terms of their hydrodynamic characteristics. The torque delivered to the propellers and the thrust of the system were measured for different operation conditions of the propellers. These measurements lead to the hydrodynamic optimization of the ship propulsion system. The parameters under focus revealed the influence of distance between propeller planes, propeller frequency of rotation ratio and type of propellers (co- or contra-rotating) on the overall efficiency of the system. Two of the ship propulsion systems under consideration were chosen, based on their hydrodynamic characteristics, for a detailed study of the swirling wake flow by means of laser Doppler anemometry. A two-component laser Doppler system was employed for the velocity measurements. A light barrier mounted on the axle of the rear propeller motor supplied a TTL signal to mark the beginning of each period, thus providing angle information for the LDA measurements. Measurements were conducted for four axial positions in the slipstream of the pod drive models. The results show that the wake of contra-rotating propeller is more homogeneous than when they co-rotate. In agreement with the results of the force/momentum measurements and with hypotheses put forward in the literature (see e.g. Poehls in Entwurfsgrundlagen für Schraubenpropeller, 1984; Schneekluth in Hydromechanik zum Schiffsentwurf, 1988; Breslin and Andersen in Hydrodynamics of ship propellers, 1996; Schneekluth and Bertram in Ship design for efficiency and economy, 1998), the co-rotating propellers model showed a much stronger swirl in the wake of the propulsor. The anisotropy of turbulence was analyzed using the anisotropy tensor introduced by Lumley and Newman (J Fluid Mech 82(1):161-178, 1977). The invariants of the anisotropy tensor of the wake flow were computed and were plotted in the Lumley-Newman-diagram. These measurements revealed that the anisotropy tensor in the wake of ship propellers is located near to the borders of the invariant map, showing a large degree of anisotropy. They will be presented and will be discussed with respect to applications of turbulence models to predict swirling flows.

Pêgo, J. P.; Lienhart, H.; Durst, F.

2007-08-01

243

Modelling of dynamics of vortex reversal in nanodisc of cobalt  

NASA Astrophysics Data System (ADS)

By micromagnetic simulations, the dynamic of vortex-core-polarization reversal under the influence of an in-plane oriented magnetic field pulse has been examined for a 3 nm thick cobalt disc. The results are summarized in a diagram showing the range of both pulse strength and pulse width, which should be used in the aim to force a controlled toggle switch of the core of vortex in discs of diameter varying from 90 nm to 180 nm. Typical values of these parameters are the following: pulse width is in the range of one-tenth of nanosecond and pulse strength is in the range of hundred mT. The smaller disc diameter, the more right-side-limited range of pulse width. The obtained results are qualitatively similar to previously reported ones for a 200 nm diameter and 20 nm thick Permalloy discs (R. Hertel, S. Gliga, M. Fa¨hnle, C. M. Schneider, Physical Review Letters 98 (2007) 117201).

Dzienisiuk, U.; Kisielewski, M.; Maziewski, A.

2013-11-01

244

Near-field tip vortex behind a swept wing model  

Microsoft Academic Search

The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared\\u000a 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\\u000a was about 3% of the root chord c\\u000a r, regardless of the

P. Gerontakos; T. Lee

2006-01-01

245

Vortex dynamics in the two-fluid model  

NASA Astrophysics Data System (ADS)

We have used two-fluid dynamics to study the discrepancy between the work of Thouless, Ao, and Niu (TAN) and that of Iordanskii. In TAN no transverse force on a vortex due to normal fluid flow was found, whereas the earlier work found a transverse force proportional to normal fluid velocity un and normal fluid density ?n. We have linearized the time-independent two-fluid equations about the exact solution for a vortex, and find three solutions that are important in the region far from the vortex. Uniform superfluid flow gives rise to the usual superfluid Magnus force. Uniform normal fluid flow gives rise to no forces in the linear region, but does not satisfy reasonable boundary conditions at short distances. A logarithmically increasing normal fluid flow gives a viscous force. As in classical hydrodynamics, and as in the early work of Hall and Vinen, this logarithmic increase must be cut off by nonlinear effects at large distances; this gives a viscous force proportional to un/ln un, and a transverse contribution that goes like un/(ln un)2, even in the absence of an explicit Iordanskii force. In the limit un-->0 the TAN result is obtained, but at nonzero un there are important corrections that were not found in TAN. We argue that the Magnus force in a superfluid at nonzero temperature is an example of a topological relation for which finite-size corrections may be large.

Thouless, D. J.; Geller, M. R.; Vinen, W. F.; Fortin, J.-Y.; Rhee, S. W.

2001-06-01

246

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

247

Using vortex corelines to analyze the hemodynamics of patient specific cerebral aneurysm models  

NASA Astrophysics Data System (ADS)

We construct one-dimensional sets known as vortex corelines for computational fluid dynamic (CFD) simulations of blood flow in patient specific cerebral aneurysm models. These sets identify centers of swirling blood flow that may play an important role in the biological mechanisms causing aneurysm growth, rupture, and thrombosis. We highlight three specific applications in which vortex corelines are used to assess flow complexity and stability in cerebral aneurysms, validate numerical models against PIV-based experimental data, and analyze the effects of flow diverting devices used to treat intracranial aneurysms.

Byrne, Greg; Mut, Fernando; Cebral, Juan

2012-02-01

248

Flow curvature and dynamic stall simulated with an aerodynamic free-vortex model for VAWT  

NASA Astrophysics Data System (ADS)

The phenomenon of flow curvature has been introduced into the aerodynamic free-vortex model initially developed by Strickland for vertical axis wind turbines with straight blades. By comparing the theoretical results of the modified vortex model with the experimental ones, it is deduced that the consideration of the flow curvature improves the prediction of the transient aerodynamic forces but not that of power coefficients. On the other hand, some modifications in the application to vertical axis wind turbines of a semiempirical dynamic stall method are tried, that can improve the prediction of instantaneous aerodynamic forces as well as of power coefficients.

Cardona, J. L.

249

A model of the lateral line of fish for vortex sensing  

Microsoft Academic Search

In this paper, the lateral line trunk canal (LLTC) of a fish is modeled to investigate how it is affected by an external flow field. Potential flow theory is adopted to model the flow field around a fish's body in the presence of a Karman vortex street. Karman and reverse Karman streets represent the flow patterns behind a bluff body

Zheng Ren; Kamran Mohseni

2012-01-01

250

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

251

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

252

Experimental characterization of rotating flow field in a model vortex burner  

SciTech Connect

Acoustic techniques, high speed filming and LDA were employed to characterize swirling jet flow in a model vortex burner. The isothermal flow conditions studied correspond to Re = 16,000 and swirl number S = 1, resulting in onset of the swirling jet breakdown. The breakdown zone exhibited distinct flow unsteadiness in the form of a precessing vortex core (PVC). Phase-averaged analysis of the LDA data was used to reveal an ''instantaneous'' flow field spatial distribution and to determine the precessing vortex characteristics. These results were compared against the time mean data to reveal the PVC's footprint in the time-averaged flow structure. In particular, this approach was shown to provide access to the precessing structure parameters making use of conventional flow field diagnostics. (author)

Shtork, S.I.; Cala, C.E.; Fernandes, E.C. [Laboratory of Thermofluids, Combustion and Energy Systems, Center for Innovation, Technology and Policy Research, IN+, Department of Mechanical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisbon (Portugal)

2007-07-15

253

Active twist rotor blade modelling using particle-wake aerodynamics and geometrically exact beam structural dynamics  

NASA Astrophysics Data System (ADS)

An active aeroelastic and aeroacoustic analysis of helicopter rotor systems is presented in this paper. It is a tightly coupled computational aeroelastic code that interfaces a particle-wake panel method code with an active nonlinear mixed variational intrinsic beam element code. In addition, a Ffowcs-Williams-Hawkings equation-based acoustic component is incorporated to complete the numerical implementation. The theory behind each component is summarized here as well as the method for coupling the aerodynamic and structural components. Sample acoustic and aeroelastic results are given for different model-scale rotors. Comparisons with available (passive) results show very good agreement. Preliminary study with an active twist rotor is also shown.

Cesnik, C. E. S.; Opoku, D. G.; Nitzsche, F.; Cheng, T.

2004-06-01

254

Integration of approaches in David Wake's model-taxon research platform for evolutionary morphology.  

PubMed

What gets integrated in integrative scientific practices has been a topic of much discussion. Traditional views focus on theories and explanations, with ideas of reduction and unification dominating the conversation. More recent ideas focus on disciplines, fields, or specialties; models, mechanisms, or methods; phenomena, problems. How integration works looks different on each of these views since the objects of integration are ontologically and epistemically various: statements, boundary conditions, practices, protocols, methods, variables, parameters, domains, laboratories, and questions all have their own structures, functions and logics. I focus on one particular kind of scientific practice, integration of "approaches" in the context of a research system operating on a special kind of "platform." Rather than trace a network of interactions among people, practices, and theoretical entities to be integrated, in this essay I focus on the work of a single investigator, David Wake. I describe Wake's practice of integrative evolutionary biology and how his integration of approaches among biological specialties worked in tandem with his development of the salamanders as a model taxon, which he used as a platform to solve, re-work and update problems that would not have been solved so well by non-integrative approaches. The larger goal of the project to which this paper contributes is a counter-narrative to the story of 20th century life sciences as the rise and march of the model organisms and decline of natural history. PMID:23588059

Griesemer, James

2013-04-12

255

Calculation of velocity structure functions for vortex models of isotropic turbulence  

SciTech Connect

Velocity structure functions ({ital u}{sub {ital p}}{sup {prime}}{minus}{ital u}{sub {ital p}}){sup {ital m}} are calculated for vortex models of isotropic turbulence. An integral operator is introduced which defines an isotropic two-point field from a volume-orientation average for a specific solution of the Navier{endash}Stokes equations. Applying this to positive integer powers of the longitudinal velocity difference then gives explicit formulas for ({ital u}{sub {ital p}}{sup {prime}}{minus}{ital u}{sub {ital p}}){sup {ital m}} as a function of order {ital m} and of the scalar separation {ital r}. Special forms of the operator are then obtained for rectilinear stretched vortex models of the Townsend{endash}Lundgren type. Numerical results are given for the Burgers vortex and also for a realization of the Lundgren-strained spiral vortex, and comparison with experimental measurement is made. In an Appendix, we calculate values of the velocity-derivative moments for the Townsend{endash}Burgers model. {copyright} {ital 1996 American Institute of Physics.}

Saffman, P.G. [Applied Mathematics 217-50, California Institute of Technology, Pasadena, California 91125 (United States); Pullin, D.I. [Graduate Aeronautical Laboratories 105-50, California Institute of Technology, Pasadena, California 91125 (United States)

1996-11-01

256

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

257

A generalized model for assessment of vortex-induced vibrations of flexible structures  

Microsoft Academic Search

The present paper proposes a one degree of freedom (1DOF) non-linear model of self limiting cyclic wind loads for application in finite element method analyses of light structures subjected to vortex shedding excitation under lock-in conditions. Being empirical by nature, the model includes three independent parameters to be determined from response tests with representative aero-elastic wind tunnel models or prototypes.

Allan Larsen

1995-01-01

258

Formation Mechanism and Characteristics Research of Ball Lightning Based on Vortex Model  

NASA Astrophysics Data System (ADS)

The strange characteristics of ball lightning are considered as a question hard to explain. In order to solve the problem, in this paper a complete model of plasma vortex is presented for the ball lightning. By ideal MHD equations, through imposing disturbance to plasma column, the possibility of sausage and kink instability of the lightning channel is analyzed from the perspective the minimum potential energy. The conclusion is that the kink instability (m = 1) is most prone to occur. And when instability occurs, because of the difference of the magnetic field in the twisted area, the magnetic pressure makes the trend further and therefore forming the plasma vortex that may eventually turn into ball lightning if the energy of the vortex is large enough. The existence of the vortex makes ball lightning have a short period of time stability. By the proposed model, the ball lightning features that are hard to understand in the past are explained. In this paper, the reason for bead lightning is also explained from the perspective of the sausage instability.

Li, Zi-Cheng; Yang, Guo-Hua

2011-05-01

259

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

260

Point Vortex Models and the Dynamics of Strong Vortices in the Atmosphere and Oceans  

NASA Astrophysics Data System (ADS)

The models to be considered are the simplest imaginable: The flow is assumed two-dimensional and inviscid, and each strong vortex is represented as a deltafunction singularity of the vorticity field. This simple Ansatz leads to what is called the point vortex model. It was introduced already by Helmholtz in his seminal 1858 paper on vortex dynamics [9] (English translation by Tait [21]) and ha s been a mainstay of fluid mechanics modeling ever since. J. G. Charney extolled the virtues of this approximation in the context of numerical simulations of atmospheric flows. He wrote [8]: ...the continuous vorticity distribution in two-dimensional flow may be approximated by a finite set of parallel rectilinear vortex filaments of infinitesimal cross-section and finite strength, whose motion is governed by a set of ordinary differential equations. This is analogous to replacing a continuous mass distribution by a set of gravitating mass points. It has the virtue that mass, energy, linear and angular momentum continue to be conserved, and that the motions represented are those of conceivable, though idealized, physical systems. It is, in a sense, the dual or complement of [the] functional representation, the Green's function being the dual of the eigenfunction, or the 'particle' the dual of the 'wave'. Which representation is the more suitable depends on the nature of the field of motion to be approximated. Fields with wave-like properties are more amenable to functional representation, whereas those with discontinuities or vortex-like properties are more naturally represented by discrete vortices.

Aref, Hassan; Stremler, Mark A.

261

A summary of recent refinements to the WAKE dispersion model, a component of the HGSYSTEM/UF{sub 6} model suite  

SciTech Connect

The original WAKE dispersion model a component of the HGSYSTEM/UF{sub 6} model suite, is based on Shell Research Ltd.`s HGSYSTEM Version 3.0 and was developed by the US Department of Energy for use in estimating downwind dispersion of materials due to accidental releases from gaseous diffusion plant (GDP) process buildings. The model is applicable to scenarios involving both ground-level and elevated releases into building wake cavities of non-reactive plumes that are either neutrally or positively buoyant. Over the 2-year period since its creation, the WAKE model has been used to perform consequence analyses for Safety Analysis Reports (SARs) associated with gaseous diffusion plants in Portsmouth (PORTS), Paducah (PGDP), and Oak Ridge. These applications have identified the need for additional model capabilities (such as the treatment of complex terrain and time-variant releases) not present in the original utilities which, in turn, has resulted in numerous modifications to these codes as well as the development of additional, stand-alone postprocessing utilities. Consequently, application of the model has become increasingly complex as the number of executable, input, and output files associated with a single model run has steadily grown. In response to these problems, a streamlined version of the WAKE model has been developed which integrates all calculations that are currently performed by the existing WAKE, and the various post-processing utilities. This report summarizes the efforts involved in developing this revised version of the WAKE model.

Yambert, M.W.; Lombardi, D.A.; Goode, W.D. Jr.; Bloom, S.G.

1998-08-01

262

Laminar flow and turbulence modeling to trace wake pattern around building clusters for domestic scale wind turbine siting  

Microsoft Academic Search

Some of the primary concerns in integrating domestic scale renewable energy systems in city buildings is siting optimization and size scaling of domestic wind turbine models on building roof tops discounting the zone of wake influence. This is a Case study scenario of a regional wind resources distributions and predominant wind directions computation for CFD modeling to identify laminar and

G. A. Chittaranjan

2009-01-01

263

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

264

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

265

Modeling and simulation of vortex induced vibration on the subsea riser/pipeline (GRP pipe)  

NASA Astrophysics Data System (ADS)

This paper presents the research work conducted to investigate the dynamics characteristics of the offshore riser pipeline due to vortex flow and to develop a model that could predict its vortex induced responses. Glass-fiber reinforced plastic (GRP) pipe is used for this study which has smaller density from the steel. A two-dimensional finite element computational method is implemented to describe the dynamic behavior of the riser. The governing equation of motion was based on Hamilton's principle, consists of the strain energy due to bending and axial deformation, kinetic energy due to both riser and internal fluid movement and also external force from currents and waves. A direct integration method namely Newmark integration scheme is proposed to solve the equation of motion. A MATLAB program code was developed to obtain the simulation results. The natural frequency and damping ratio are presented for each mode. Dynamic response of riser is shown in time-domain and the numerical results are discussed. Several parameter effects are used to investigate dynamic responses and the results show an agreement with the theory. Vortex shedding phenomenon also has been discussed in this paper. As a conclusion, the simulation results have successfully shown the vortex induced vibration responses for GRP pipeline.

Raja Adli, Raja Nor Fauziah bt; Ibrahim, Idris

2012-06-01

266

Recent advances in modeling of wind turbine wake vortical structure using a differential actuator disk theory  

NASA Astrophysics Data System (ADS)

This paper presents the recent developments of a new CFD-based method aimed at predicting wind turbine aerodynamics, where velocity and pressure discontinuities are used to model the vortical system that creates lift on the turbine blades. To illustrate the ability of the present model to predict induced wake effect, the case of the taper wing is thoroughly analyzed and effects of both domain discretization and convection scheme are presented. Results are mitigated regarding predicted performance of induced drag, but accurate induced and upstream flow angles values are obtained. The method is even shown to be a useful calculator for the relationship between inflow angle measured upstream and effective angle of attack of a wing section. Interesting results for the NREL phase VI rotor have been obtained showing improvement of the method upon actuator-disk approach in handling tip vortices effect on the blade aerodynamics.

Sibuet Watters, Christophe; Masson, Christian

2007-07-01

267

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

268

Direct numerical simulations and modeling of a spatially-evolving turbulent wake  

Microsoft Academic Search

Understanding of turbulent free shear flows (wakes, jets, and mixing layers) is important, not only for scientific interest, but also because of their appearance in numerous practical applications. Turbulent wakes, in particular, have recently received increased attention by researchers at NASA Langley. The turbulent wake generated by a two-dimensional airfoil has been selected as the test-case for detailed high-resolution particle

John M. Cimbala

1994-01-01

269

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

270

A model of the lateral line of fish for vortex sensing.  

PubMed

In this paper, the lateral line trunk canal (LLTC) of a fish is modeled to investigate how it is affected by an external flow field. Potential flow theory is adopted to model the flow field around a fish's body in the presence of a Karman vortex street. Karman and reverse Karman streets represent the flow patterns behind a bluff body and a traveling fish, respectively. An analytical solution is obtained for a flat body, while a fish-like body is modeled using a Joukowski transformation and the corresponding equations are solved numerically. The pressure distribution on the body surface is then computed employing Bernoulli's equation. For a known external flow, the flow inside the LLTC is driven by the pressure gradient between a pair of consecutive pores, which can be solved analytically. Governing dimensionless parameters are obtained from this analytical solution, and the effects of these numbers on the amplitude or features of the velocity distribution inside the canal are studied. The results show that the main characteristics of a vortex street including the magnitude of vortices, their translational speed, their spacing, their distance from the fish's body and the angle of the vortex street axis can all be recovered by measuring the velocity distribution along the canal and its changes with time. To this end, the proposed LLTC model could explain how a fish identifies the characteristics of a Karman vortex street shed by a nearby object or a traveling fish. It is also demonstrated that while this model captures the ac (alternating current) component of the external velocity signal, the dc (direct current) component of the signal is filtered out. Based on the results of our model, the role of the LLTC in a fish's schooling and its evolutionary impact on fish sensing are discussed. PMID:22585366

Ren, Zheng; Mohseni, Kamran

2012-05-15

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

Wind tunnel study of wake downwash behind A 6% scale model B1B aircraft  

Microsoft Academic Search

Parachute system performance issues such a turnover and wake recontact may be strongly influenced by velocities induced by the wake of the delivering aircraft, especially if the aircraft is maneuvering at the time of parachute deployment. The effect of the aircraft on the parachute system is a function of the aircraft size, weight, and flight path. In order to provide

J. H. Strickland; E. L. Tadios; D. A. Powers

1990-01-01

279

Slab model for Rayleigh--Taylor stabilization by vortex shedding, compressibility, thermal conduction, and ablation  

SciTech Connect

Analytical models are derived for stabilization of the Rayleigh--Taylor instability of an ablatively accelerated slab. The procedure is to conserve mass, momentum, and energy flux across a perturbed interface through which plasma flows. In various regimes, the stabilization mechanism can be described as vortex shedding, compressibility, thermal conduction, and ablation. Comparisons are made with recent fluid simulations, and simple laws relevant to laser-driven ablative acceleration are derived.

Manheimer, W.M.; Colombant, D.G.

1984-04-01

280

Development of a vortex combustor (VC) for space\\/water heating applications (cold flow modeling)  

Microsoft Academic Search

This report focuses on the discussion of cold flow measurements and mathematical modeling of the vortex combustor (VC). A parallel research project on the combustion tests of a 2-4 MB\\/H proof-of- concept VC has been conducted by the Naval Civil Engineering Laboratory. This final report is divided into seven chapters: Chapter 1 discusses the concept and advantages of the VC,

Nieh

1990-01-01

281

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

282

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

283

Possible first-order transition in the two-dimensional Ginzburgh-Landau model induced by thermally fluctuating vortex cores  

SciTech Connect

We study the two-dimensional Ginzburg-Landau model of a neutral superfluid in the vicinity of the vortex unbinding transition. The model is mapped onto an effective interacting vortex gas by a systematic perturbative elimination of all fluctuating degrees of freedom (amplitude and phase of the order parameter field) except the vortex positions. In the Coulomb gas descriptions derived previously in the literature, thermal amplitude fluctuations were neglected altogether. We argue that if one includes the latter, the vortices still form a two-dimensional Coulomb gas, but the vortex fugacity can be substantially raised. Under the assumption that Minnhagen`s generic phase diagram of the two-dimensional Coulomb gas is correct, our results then point to a first-order transition rather than a Kosterlitz-Thouless transition, provided the Ginzburg-Landau correlation length is large enough in units of a microscopic cutoff length for fluctuations. The experimental relevance of these results is briefly discussed.

Bormann, D. [Universitaet Augsburg (Germany); Beck, H. [Universite de Neuchatel (Switzerland)

1994-07-01

284

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

285

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

286

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

287

Closed-loop approaches to control of a wake flow modeled by the Ginzburg–Landau equation  

Microsoft Academic Search

A short computational program was undertaken to evaluate the effectiveness of a closed-loop control strategy for the stabilization of an unstable bluff-body flow. In this effort, the non-linear one-dimensional Ginzburg–Landau wake model at 20% above the critical Reynolds number was studied. The numerical model, which is a non-linear partial differential equation with complex coefficients, was solved using the FEMLAB®\\/MATLAB® software

Kelly Cohen; Stefan Siegel; Thomas McLaughlin; Eric Gillies; James Myatt

2005-01-01

288

A Semi-Automated Wake Survey and Analysis System Applied to Small Models.  

National Technical Information Service (NTIS)

A semi-automated wake survey and data analysis system is described. This system was developed to satisfy the modern needs of the propeller specialist, overcoming the shortcomings of the traditional methods. The salient features of this system are electro-...

N. A. Brown J. P. Tjoenneland

1970-01-01

289

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

290

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

291

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

292

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

293

Bimodality, regime shifts, and decadal variability in the QBO and the NH stratospheric winter vortex as seen in model ensembles.  

NASA Astrophysics Data System (ADS)

The stratospheric Northern Hemisphere (NH) winter mean vortex alternates between a strong and a weak state which is manifested in a statistically significant bimodal distribution. In the end of the 1970s a regime change took place increasing the probability of the strong phase relative to the weak phase (Christiansen 2003). Christiansen (2010) found a strong coincidence between strong (weak) vortex winters and the westerly (easterly) QBO phase. This work also demonstrated that the change of the vortex in the late 1970s can be related to a change in the QBO. However, this change in the QBO can be random process simply related to the annual sampling of the QBO. In this paper we investigate the connection between the decadal variability of the vortex and the QBO in historical CMIP5 and CCMVal2 experiments. The CMIP5 archive contains both models with and without a spontaneously generated QBO. Additionally, the CCMVal2 archive includes models with prescribed QBOs. Preliminary results indicate that models with prescribed or spontaneously generated QBOs do show realistic bimodal behaviour of the QBO. While the strength of the variability of the vortex in the models is realistic there is in general little bimodality. We also find that the connection between QBO and the NH vortex is weaker in the models than in the reanalysis. References: Christiansen, B., Evidence for nonlinear climate change: Two stratospheric regimes and a regime shift. J. Climate, 16, 3681-3689, 2003. Christiansen, B., Stratospheric bimodality: Can the equatorial QBO explain the regime behavior of the NH winter vortex? J. Climate, 23, 3953-3966, 2010.

Christiansen, Bo

2013-04-01

294

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

295

Discrete vortex model of a Helmholtz resonator subjected to high-intensity sound and grazing flow.  

PubMed

In this paper, a theoretical model is developed to study the acoustical response of a Helmholtz resonator as a duct-branched acoustic absorber subjected to both high-intensity sound and grazing flow. The present model is comprised of a discrete vortex model in combination with a one-dimensional duct sound propagation model. The present work is to study the overall effect of incident sound interacting with grazing flow but putting emphasis on the nonlinear or intermediate regime where the sound intensity has a marked or non-negligible influence on the acoustic behavior of the Helmholtz resonator. The numerical results reveal that the flow field around the orifice is dominated by the evolution of the vortex sheet and the flow pattern is influenced by the ratio of the orifice flow velocity to the grazing flow velocity. When the incident sound pressure is high or the resonance occurs, the resonator shows nonlinearity, i.e., the acoustic impedance and absorption coefficient vary not only with duct flow Mach number buy also with incident frequency and incident sound pressure level. PMID:23145586

Dai, Xiwen; Jing, Xiaodong; Sun, Xiaofeng

2012-11-01

296

Evaluation of dynamic subgrid-scale models and wind-turbine models in large-eddy simulations of wind-turbine wakes in boundary layer flows  

NASA Astrophysics Data System (ADS)

Large-eddy simulation (LES), coupled with a wind turbine model, is used to investigate the characteristics of wind-turbine wakes in a neutrally stratified boundary layer flow. Three different subgrid-scale (SGS) models for the SGS stresses are tested: (1) the Smagorinsky model, (2) the Lagrangian dynamic model, and (3) the scale-dependent Lagrangian dynamic model (Stoll and Porté-Agel, 2006). The turbine-induced forces (lift and drag) are parameterized using blade element momentum theory. Three wind-turbine models, using different force integration over temporal and spatial resolutions, are applied: (a) the standard actuator-disk model without rotation (ADM-NR), (b) the actuator-disk model with rotation (ADM-R), and (c) the actuator line model (ALM). Simulation results obtained with all SGS models together with wind turbine models are compared to wind-tunnel measurements collected with hot-wire and cold-wire anemometry in the wake of a miniature 3-blade wind turbine at the St. Anthony Falls Laboratory atmospheric boundary layer wind tunnel. In general, the scale-dependent Lagrangian dynamic model is able to account (without tuning) for the local changes in the eddy- viscosity model coefficient at different positions in the wake. It can also capture the scale dependence of this coefficient associated with flow anisotropy in regions of the flow with strong mean shear. The characteristics of the wakes simulated with the proposed LES framework using the scale-dependent Lagrangian dynamic model together with the ADM-R and the ALM are in good agreement with the measurements. However, the ALM is better able to capture vortical structures induced by the blades in the near-wake region.

Wu, Yu Ting; Porte-Agel, Fernando

2010-05-01

297

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

298

MODEL-BASED COHERENT-STRUCTURE CONTROL OF TURBULENT SHEAR FLOWS USING LOW-DIMENSIONAL VORTEX MODELS  

Microsoft Academic Search

In this study, a flow control strategy is presented for manipulating coherent shear-flow structures. As a benchmark problem, the transitional flow around a backward-facing step with local acoustic actuation at the upper edge is chosen. The strategy is based on a hierarchy of low-dimensional vortex models and targets the use of control-theory methods for con- trol design. The hierarchy ranges

Mark Pastoor; Rudibert King; Bernd R. Noack; Andreas Dillmann; Gilead Tadmor

299

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

300

A minimal physiologically based model of the HPA axis under influence of the sleep-wake cycles.  

PubMed

The hypothalamic-pituitary-adrenal axis (also called the HPA or stress axis) exhibits distinct circadian and ultradian rhythms in cortisol release that cannot be explained solely by the feedback loops from cortisol to the control systems in the paraventricular nucleus (PVN) and pituitary gland. The HPA axis is intimately connected with other brain functions. In particular, it is strongly affected by the sleep-wake cycles via direct and indirect effects of the circadian and homeostatic mechanisms. For example, the HPA axis has direct inputs from the master circadian clock in the suprachiasmatic nuclei (SCN), and from the various sleep-wake related neuronal populations, which themselves are under the effects of the circadian and homeostatic processes. In this paper a first step towards a physiologically based mathematical model of the HPA-axis under effects of the sleep-wake cycles is presented. This model accounts for 3 major characteristics of daily cortisol profile in the blood: i) abrupt increase of cortisol concentration in response to awakening, the so-called cortisol-awakening response (CAR); ii) reduced cortisol levels during daytime with underlying ultradian oscillations; and iii) suppression of cortisol release during sleep. PMID:23599244

Postnova, S; Fulcher, R; Braun, H A; Robinson, P A

2013-04-18

301

Quantification of the transport of chemical constituents from the polar vortex to midlatitudes in the lower stratosphere using the high-resolution advection model MIMOSA and effective diffusivity  

NASA Astrophysics Data System (ADS)

The observed decrease of ozone in the northern midlatitude lower stratosphere is only partially reproduced by chemical models. The transport of ozone-depleted air from the polar vortex is one of the proposed mechanisms to explain the discrepancy. Here we present a study on the quantification of the air mass transported from the polar vortex to midlatitude during the four winters 1996-1997 to 1999-2000, in relation with vortex filamentation and break up, using the high-resolution advection model MIMOSA on isentropic surfaces. Sensitivity tests show that the advection model is able to predict the location of polar filaments with accuracy better than 100 km, limited by uncertainties in meteorological advecting wind fields. The effective diffusivity diagnostic is used to evaluate the intensity of the vortex edge barrier and to quantify the transport of air from the polar vortex to midlatitude. The intensity of the polar barrier is increasing with height from 400 to 550 K and is nearly constant above. During periods with a cold and undisturbed vortex, favorable to chlorine activation, the transport is very weak. This suggests that the midwinter vortex filamentation plays a minor role in the midlatitude ozone decline. In the opposite limit, during a stratospheric warming up to 30% of the polar vortex air is transported to midlatitudes. The cumulative transport from early January to end of April across the polar edge varies from almost 5% of the polar vortex air at 675 K in 1997 to 50% at 435 K in 1999.

Hauchecorne, Alain; Godin, Sophie; Marchand, Marion; Heese, Birgit; Souprayen, Claude

2002-10-01

302

Numerical Modeling of Electric Arcs with Water Vortex and Hybrid Stabilizations  

NASA Astrophysics Data System (ADS)

In this paper we deal with numerical investigation of properties and processes occurring in the electric arcs with tangential stabilization of electric arc by water vortex (Gerdien arc) and with the combined stabilization of arc by axial gas flow and water vortex. The net emission coefficient and the partial characteristics method for radiation loss from these arcs are employed. Results carried out for the water arc for 150-600 A proved that typical outlet velocities are 0.7-8 km s-1, temperatures 14 000 K-26 000 K, the voltage drop 110-200 V, the pressure drop 0.02-0.4 atm. and the Mach numbers range from 0.1 to 0.8. The partial characteristics model gives a lower value of radiation loss from the arc than the net emission model, implying higher outlet velocities and temperatures, closer to experimental values. The hybrid arc exhibits higher outlet velocities under the practically unchanged plasma enthalpy compared to Gerdien arc. The contribution of O2, H2 and OH molecular bands to the amount of reabsorbed radiation has been also discussed. Comparison between present calculation and available experiments carried out at the Institute shows a good agreement.

Jeništa, J.; Bartlová, M.; Aubrecht, V.

2008-02-01

303

Numerical Modeling of Electric Arcs with Water Vortex and Hybrid Stabilizations  

SciTech Connect

In this paper we deal with numerical investigation of properties and processes occurring in the electric arcs with tangential stabilization of electric arc by water vortex (Gerdien arc) and with the combined stabilization of arc by axial gas flow and water vortex. The net emission coefficient and the partial characteristics method for radiation loss from these arcs are employed. Results carried out for the water arc for 150-600 A proved that typical outlet velocities are 0.7-8 km s{sup -1}, temperatures 14 000 K-26 000 K, the voltage drop 110-200 V, the pressure drop 0.02-0.4 atm. and the Mach numbers range from 0.1 to 0.8. The partial characteristics model gives a lower value of radiation loss from the arc than the net emission model, implying higher outlet velocities and temperatures, closer to experimental values. The hybrid arc exhibits higher outlet velocities under the practically unchanged plasma enthalpy compared to Gerdien arc. The contribution of O{sub 2}, H{sub 2} and OH molecular bands to the amount of reabsorbed radiation has been also discussed. Comparison between present calculation and available experiments carried out at the Institute shows a good agreement.

Jenista, J.; Bartlova, M. [Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 3, 182 00 Prague 82 (Czech Republic); Aubrecht, V. [Brno University of Technology, Technicka 8, 616 00 Brno (Czech Republic)

2008-02-21

304

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

305

A Model Study on Understanding the Influence of Arabian Sea Mini Warm Pool on Monsoon Onset Vortex Formation  

NASA Astrophysics Data System (ADS)

The Arabian Sea Mini Warm Pool (MWP) is a zone of anomalously high Sea Surface Temperature (SST) in the Arabian Sea over which the monsoon onset vortex (OV) is believed to form. In the present study it is shown that this MWP is a key parameter in the development of the onset vortex. Atmospheric model experiments are carried out with and without MWP to understand the mechanisms for the formation of the OV. The model failed to simulate the OV with the cold SST advocating the importance of the MWP for the formation of the OV. The MWP is found to favor the formation of the onset vortex in the east central Arabian Sea by increasing the horizontal shear and decreasing the vertical wind shear.

Deepa, R.; Gnanaseelan, C.; Deshpande, M.; Salvekar, P. S.

2012-09-01

306

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

307

A model for the formation of ``optimal'' vortex rings taking into account viscosity  

NASA Astrophysics Data System (ADS)

The evolution of a viscous vortex ring from thin to thick-cored form is considered using an improved asymptotic solution, which is obtained after impressing a spatially uniform drift on the first-order solution of the Navier-Stokes equations. The obtained class of rings can be considered as the viscous analog solution to the Norbury vortices and classified in terms of the ratio of their initial outer radius to the core radius. The model agrees with the reported theoretical and experimental results referring to the post-formation and the formation stages. By using the matching procedure suggested earlier and the obtained properties of the viscous vortex ring, it is found that when the length-to-diameter aspect ratio L/D reaches the limiting value 4.0 (``formation number''), the appropriate values of the normalized energy and circulation become around 0.3 and 2.0, respectively. An approach that enables to predict the ``formation number'' is proposed.

Kaplanski, F. B.; Rudi, Y. A.

2005-08-01

308

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.

309

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

310

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

311

Flux noise resulting from vortex avalanches using a simple kinetic model  

SciTech Connect

We have carried out a model calculation of the flux noise produced by vortex avalanches in a type-II superconductor, using a simple kinetic model proposed by Bassler and Paczuski. Over a broad range of frequencies, we find that the flux noise S{sub {Phi}}({omega}) has a power-law dependence on frequency, S{sub {Phi}}({omega}){approximately}{omega}{sup {minus}s}, with s{approximately}1.4 in reasonable agreement with experiment. In addition, for small lattices, the calculated S{sub {Phi}}({omega}) has a high-frequency knee, which is seen in some experiments, and which is due to the finite lattice size. Deviations between calculation and experiment are attributed mostly to uncertainties in the measured critical current densities and pinning strengths of the experimental samples. {copyright} {ital 1999} {ital The American Physical Society}

Mohler, G.; Stroud, D. [Department of Physics, The Ohio State University, Columbus, Ohio 43210 (United States)

1999-10-01

312

Quantification of the transport of chemical constituents from the polar vortex to midlatitudes in the lower stratosphere using the high-resolution advection model MIMOSA and effective diffusivity  

Microsoft Academic Search

The observed decrease of ozone in the northern midlatitude lower stratosphere is only partially reproduced by chemical models. The transport of ozone-depleted air from the polar vortex is one of the proposed mechanisms to explain the discrepancy. Here we present a study on the quantification of the air mass transported from the polar vortex to midlatitude during the four winters

Alain Hauchecorne; Sophie Godin; Marion Marchand; Birgit Heese; Claude Souprayen

2002-01-01

313

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

314

Effect of unsteady stretch rate on OH chemistry during a flame-vortex interaction: To assess flamelet models  

Microsoft Academic Search

Some basic assumptions of flamelet models are assessed by comparing profiles of OH mole fraction measured during an unsteady flame-vortex interaction to the OH profiles computed for a steady, planar counterflow flame (SPCF) with full chemistry. It is important to make such comparisons for both the same local three-dimensional stretch rate, which is measured instantaneously at locations along the flame

C. J. Mueller; J. F. Driscoll; D. J. Sutkus; W. L. Roberts; M. C. Drake; M. D. Smooke

1995-01-01

315

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

316

Modeling turbine wakes and power losses within the Horns Rev offshore wind farm using large-eddy simulation  

NASA Astrophysics Data System (ADS)

A recently-developed large-eddy simulations (LES) framework is implemented to predict multiple wake flows and the associated power losses within the Horns Rev offshore wind farm under near-neutral stability conditions. A tuning-free Lagrangian scale-dependent dynamic subgrid-scale (SGS) model is used for the parametrization of the SGS stresses. The turbine-generated power outputs and the turbine-induced forces (e.g., thrust, lift, drag) are parameterized using two models: (a) the traditional actuator-disk model without rotation (ADM-NR), which uses the 1D momentum theory to relate the power output and the thrust force with a representative velocity over the rotor (e.g., the disk-averaged velocity); and (b) the actuator-disk model with rotation (ADM-R), which adopts blade element theory to calculate the lift and drag forces (that produce thrust, rotor shaft torque, and power) based on the local blade and flow characteristics. In general, the predicted power outputs obtained using the ADM-R are in good agreement with observed power data from the Horns Rev wind farm. The ADM-NR tends to underestimate the power output. A similar under-prediction is obtained using industry-standard wind-farm models such as the Wind Atlas Analysis and Application Program (WAsP). Simulations using different inflow conditions show that the mean wind direction has a strong effect on the spatial distributions of the time-averaged velocity and the turbulence intensity within the farm. These, in turn, affect the power output and the fatigue loads on the turbines. When the prevailing wind direction is parallel to the turbine rows (i.e., a full wake condition), the velocity deficit and the power losses are largest, and the turbulence intensity levels are highest and have a symmetric pattern (dual-peak at hub height) on both sides of the turbine wakes. A detailed analysis of the turbulence kinetic energy budget in the full wake condition shows an important effect of the increased turbulence level on the magnitude and spatial distribution of the shear production and transport terms.

Wu, Yu-Ting; Porte-Agel, Fernando

2013-04-01

317

Three-dimensional island wakes in the field, laboratory experiments and numerical models  

Microsoft Academic Search

Results of field, laboratory and numerical studies are used to describe the three-dimensional circulation in a barotropic island wake in shallow waters. Bottom friction generates a closed circulation characterized by a strong upwelling (typically 10–20 m h?1) in the bulk of the eddy and an even larger downwelling velocity in a narrow zone along the edges of the eddy. This

Eric Wolanski; Takashi Asaeda; Akihiro Tanaka; Eric Deleersnijder

1996-01-01

318

Hot jet\\/wake turbulent structure and laser propagation. Part 3: Laser propagation measurements and modeling  

Microsoft Academic Search

The use of lasers aboard aircraft is affected by the perturbed airflow in its vicinity. Therefore, an ability to predict the structure of aircraft-induced turbulence would be useful in system and performance analysis. Pacific-Sierra Research Corp. performed temperature and velocity fluctuation measurements in the wakes of an NRA-3B and a B-1B at trail distances of 50 to 5500 m, at

Alan R. Shapiro; J. H. Churnside

1994-01-01

319

Experimental investigation of vortex control with an axial jet in the draft tube of a model pump-turbine  

NASA Astrophysics Data System (ADS)

The operation of hydropower plants, especially of pump-storage plants, changes since the deregulation of the energy market. They are increasingly operating at off-design conditions in order to follow the demand in the electrical grid. Therefore the ability of hydropower plants handling the operation in a wide range of off-design conditions has become more important. In this context one problem is the vortex rope in the draft tube, especially for Francis turbines and pump-turbines running in part load. An experimental investigation in mitigation of the vortex rope phenomenon by injecting water axially in the centre of the draft tube on a pump-turbine model was carried out. Also the mitigation by additionally injected air in the centre of the draft tube was analysed. The results of the experimental investigation are focused on the reduction of the pressure fluctuations in the draft tube. In this paper two different part-load operating points were investigated. One of these operating points is a high part load operating point where a vortex rope exists. The other one is a low part load operating point, where the pressure fluctuation is not caused by a vortex rope. The results of the investigation show, that the injection of stabilizing water can mitigate the pressure fluctuation caused by a vortex rope. But the investigation of operating points where the pressure fluctuation is not caused by a vortex rope shows, that there is no significant reduction in the pressure fluctuation by this method. In these operating points the method of injecting additionally air reduces the pressure fluctuation better.

Kirschner, O.; Schmidt, H.; Ruprecht, A.; Mader, R.; Meusburger, P.

2010-08-01

320

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

321

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

322

Blade-vortex interaction data obtained from a pressure-instrumented model rotor at the DNW  

NASA Astrophysics Data System (ADS)

Blade surface pressure data obtained during a model helicopter rotor test are examined to determine the locations and strengths of impulsive loading events caused by blade vortex interactions (BVI). Data from 43 descent conditions are used to identify, locate, and characterize the BVI events. The combination of azimuthal, radial, and chordwise resolution has generated a detailed picture of the aerodynamic response of a rotor blade to BVI. The paper examines both global and local features of the interaction. Global features include the dependence of the impulsive airload strength and position on three independent parameters: rotor tip path plane angle, rotor thrust, and advance ratio. Local features are examined by using the unsteady pressure distributions to determine the strength and the chordwise dependence of the amplitude and phase of a representative BVI event.

Lorber, Peter F.

323

Modeling of Vortex Flows in Direct Current (DC) Electric Arc Furnace with Different Bottom Electrode Positions  

NASA Astrophysics Data System (ADS)

This article is devoted to the numerical modeling of electrovortex and convection flows in the direct current (DC) electric arc furnace with a different position of the bottom electrode. The electromagnetic, temperature, and hydrodynamic distribution parameters are given. The shear stress on the fettle area is offered as a criterion for the estimation of vortex flow influence on the increased wearing of the fettle. It is shown that lifting the bottom electrode above the fettle surface at the electrode radius leads to the decrease of shear stress on the fettle area by 30 pct. Putting the bottom electrode lower than the fettle surface by the distance equal to the electrode radius and its expanding by the same distance reduces the stress by 10 pct.

Kazak, Oleg

2013-10-01

324

One-loop corrections to the string tension of the vortex in the Abelian Higgs model  

SciTech Connect

We present an exact numerical computation of the one-loop correction of the string tension for the Nielsen-Olesen vortex in the Abelian Higgs model. The computation proceeds via the computation of the Euclidean Green's function for the gauge, Higgs, and Faddeev-Popov fields using mode functions, and taking the appropriate trace. Renormalization is an essential part of this computation. It is done by removing leading order contributions from the numerical results so as to make these finite, and to add the divergent parts back, after suitable regularization and renormalization. We encounter and solve some problems which are specific to gauge theories and topological solutions. The corrections to the energy are found to be sizable, but still smaller than the classical energy as long as g{sup 2} is smaller than unity.

Baacke, Jurgen; Kevlishvili, Nina [Fachbereich Physik, Technische Universitaet Dortmund. D-44221 Dortmund (Germany); Dipartimento di Fisica, Universita degli studi di Ferrara, I-44100 Ferrara (Italy); INFN, Sezione di Ferrara, I-44100 Ferrara, Italy, (Italy); and Andronikashvili Institute of Physics, GAS, 0177 Tbilisi (Georgia)

2008-10-15

325

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.

326

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

Microsoft Academic Search

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

John Allen Rule

1997-01-01

327

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

328

Numerical study of energy separation in a vortex tube with different RANS models  

Microsoft Academic Search

The aim of the present paper is to investigate numerically the energy separation mechanism and flow phenomena within a vortex tube. A 3D computational domain has been generated considering the quarter of the geometry and assuming periodicity in the Azimuthal direction which was found to exhibit correctly the general behaviour expected from a vortex tube. Air is selected as the

Ahmed Ouadha; Omar Imine; Yacine Addad

2011-01-01

329

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

330

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

331

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

332

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

333

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

334

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

335

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

336

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.

337

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

338

Helicopter model rotor-blade vortex interaction impulsive noise - Scalability and parametric variations  

NASA Astrophysics Data System (ADS)

Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions. At low advance ratios (mu = 0.164 to 0.194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise. At moderate advance ratios (mu = 0.224 to 0.270), the scalig deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists. Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation.

Boxwell, D. A.; Schmitz, F. H.; Splettstoesser, W. R.; Schultz, K. J.

339

Vortex Ring Dyons of the SU(2) Yang-Mills-Higgs Model  

SciTech Connect

We present an axially symmetric vortex ring dyons solutions of the SU(2) Yang-Mills-Higgs theory. These vortex rings carry electric charges that are determined by a parameter, -1{<=}{eta}{<=}1. They possess vanishing magnetic charge and are located at a ring centered around the z-axis where the Higgs field vanishes. These stationary vortex ring dyon solutions possess finite energy but they do not satisfy the first order Bogomol'nyi equations. In the Bogomol'nyi-Prasad-Sommerfield (BPS) limit where the Higgs field potential is zero, the time component of the gauge field is parallel to the Higgs field in isospace. The total energy, net electric charge and diameter of the vortex ring increase exponentially to infinity when {eta} approaches {+-}1. On the contrary, when {lambda} = 1, all these three values reach their critical value as {eta} approaches {+-}1.

Lim, Kok-Geng; Teh, Rosy; Wong, Khai-Ming [School of Physics, Universiti Sains Malaysia 11800 USM Penang (Malaysia)

2011-03-30

340

Low-energy vortex dynamics in the self-dual Chern-Simons-Higgs model  

NASA Astrophysics Data System (ADS)

The relativistic Chern-Simons-Higgs theory finds application in anyonic superconductivity and contains topological vortices whose dynamics are poorly understood. The gauge fields are defined by a set of nonlinear constraint equations that can be accurately solved with effective Green's functions, spectral methods, and a discretization scheme using lattice gauge techniques. Simulations show that low-energy two-vortex interactions are elastic with final scattering angles sensitive to vortex velocity; furthermore, vortex pairs form rotating breather states for certain impact parameters. In this study, a function that reproduces scattering angles in the adiabatic limit for nontangential collisions is presented. Simulation results are discussed in the context of analytical methods that extract vortex dynamics from low-energy effective Lagrangians, and a numerical method to calculate the effective Lagrangian is suggested. The numerical techniques used can be applied to the study of other Chern-Simon theories.

Strilka, Richard J.

2012-10-01

341

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

342

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

343

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

344

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

345

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

346

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

347

The Three-Dimensional Leading-Edge Vortex of a "Hovering" Model Hawkmoth  

NASA Astrophysics Data System (ADS)

Recent flow visualization experiments with the hawkmoth, Manduca sexta, revealed a small but clear leading-edge vortex and a pronounced three-dimensional flow. Details of this flow pattern were studied with a scaled-up, robotic insect ('the flapper') that accurately mimicked the wing movements of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing confirmed the existence of a small, strong and stable leading-edge vortex, increasing in size from wingbase to wingtip. Between 25 and 75% of the wing length, its diameter increased approximately from 10 to 50% of the wing chord. The leading-edge vortex had a strong axial flow velocity, which stabilized it and reduced its diameter. The vortex separated from the wing at approximately 75% of the wing length and thus fed vorticity into a large, tangled tip vortex. If the circulation of the leading-edge vortex were fully used for lift generation, it could support up to two-thirds of the hawkmoth's weight during the downstroke. The growth of this circulation with time and spanwise position clearly identify dynamic stall as the unsteady aerodynamic mechanism responsible for high lift production by hovering hawkmoths and possibly also by many other insect species.

van den Berg, Coen; Ellington, Charles P.

1997-03-01

348

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

349

Acoustic results of the blade-vortex interaction acoustic test of a 40 percent model rotor in the DNW  

Microsoft Academic Search

Acoustic data of a 40 percent scale model of the 4-bladed B0-105 main rotor, measured in a large test section aeroacoustic wind tunnel, are presented. Rotor blade-vortex interaction (BVI) noise data were acquired using a traversing in-flow microphone array in the low to moderate speed flight range. Results are presented which document the flight conditions for maximum BVI impulsive noise;

Ruth M. Martin; Wolf R. Splettstoesser

1987-01-01

350

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

351

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

352

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

353

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

354

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

355

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

356

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

357

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

358

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.

359

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

360

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

361

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

362

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

363

Numerical modeling of precessing vortex core in the presence of local heat sources  

NASA Astrophysics Data System (ADS)

Based on the results of numerical simulation of a nonstationary, nonaxisymmetric turbulent swirling gas flow in a tube with local sources of heat release, it is shown that a precessing vortex core (PVC) appears at supercritical values of the swirl parameter as a result of the development of instability of a left-handed bending mode. The dependence of the PVC frequency on the mass flow rate of the gas and the heat-source power has been studied. As the heat-source power increases, the frequency of precession grows while the amplitude of vortex core oscillations drops.

Zavershinskii, I. P.; Kogan, E. Ya.; Makaryan, V. G.; Molevich, N. E.; Porfir'ev, D. P.; Sugak, S. S.

2013-04-01

364

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

365

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

366

Vortex condensation in the Chern-Simons Higgs model: An existence theorem  

Microsoft Academic Search

It is shown that there is a critical value of the Chern-Simons coupling parameter so that, below the value, there exists self-dual doubly periodic vortex solutions, and, above the value, the vortices are absent. Solutions of such a nature indicate the existence of dyon condensates carrying quantized electric and magnetic charges.

Luis A. Caffarelli; Yisong Yang

1995-01-01

367

The phenomenology of the self and its objects in waking and dreaming: implications for a model of dreaming.  

PubMed

The current psychoanalytic emphasis on self psychology allows attention to be drawn to the nature of the self, internal and external object representations, and possibilities involving the self in relationship to aspects of self and objects in dreams. An emphasis on the self, applied to dreams, appears to fit well with current concepts of REM sleep that involve this sleep stage's importance for the individual in learning, adaptation, and assimilation of emotionally meaningful information. Examples have been provided to indicate possible contributions of self psychological perspectives to an understanding of dreams. A model describing changes in the self and its objects in dream mentation compared to the self and its objects in waking mentation has been described. The model suggests that the self of the dream relates to separated off, changed, or newly cathected attitudes, beliefs, and object representations in a manner that describes the state of the individual and possibilities for change available to the individual. This model, having historical roots in Jungian psychology, adaptive/learning theories of REM sleep, and current Kohutian initiated self-state dream theory, provides an opportunity to review with the dreamer the relationships of self and objects as these representations are concerned with problematic developmental or emotional situations. PMID:8226178

Gabel, S

1993-01-01

368

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

369

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

370

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

371

Dynamics of vortex core switching in ferromagnetic nanodisks  

NASA Astrophysics Data System (ADS)

Dynamics of magnetic vortex core switching in nanometer-scale Permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular and an antivortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core.

Xiao, Q. F.; Rudge, J.; Choi, B. C.; Hong, Y. K.; Donohoe, G.

2006-12-01

372

Correlation study of a rotor in descending flight using DYMORE with a freewake model  

Microsoft Academic Search

In this work, the Blade Vortex Interaction (BVI) airloads characteristic for a rotor in descending flight condition is investigated\\u000a using a nonlinear flexible multibody dynamics analysis code DYMORE. A free vortex wake model is incorporated into the comprehensive\\u000a analysis system and improvement of airloads prediction as well as collective and cyclic pitch control settings is obtained\\u000a over the finite-state dynamic

Jae-Sang Park; Sung Nam Jung; Soo Hyung Park; Yung Hoon Yu

2010-01-01

373

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

374

Uncovering Physiologic Mechanisms of Circadian Rhythms and Sleep/Wake Regulation Through Mathematical Modeling.  

National Technical Information Service (NTIS)

Mathematical models of neurobehavioral function are useful both for understanding the underlying physiology and for predicting the effects of rest- activity-work schedules and interventions on neurobehavioral function. In a symposium titled Modeling Human...

E. B. Klerman G. Gunzelmann H. P. Van Dongen I. F. Doyle R. E. Kronauer

2007-01-01

375

An investigation of counterrotating tip vortex interaction  

NASA Astrophysics Data System (ADS)

A tip vortex interaction model originally developed for compressors has been extended and adapted for use with counterrotating open rotors. Comparison of available acoustic data with predictions (made with and without the tip vortex model included) illustrate the importance of this interaction effect. This report documents the analytical modeling, a limited experimental verification, and certain key parametric studies pertaining to the tip vortex as a noise source mechanism for the unsteady loading noise of counterrotating properllers.

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

1989-10-01

376

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

377

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

378

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

379

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.

380

Influence of adaptive control on vortex-driven instabilities in a scaled model of solid propellant motors  

NASA Astrophysics Data System (ADS)

Aeroacoustic instabilities occur in many applications of technological interest and have undesirable effects on the steady operation of the system. Passive and active means are sought to reduce the level of oscillation and eliminate the instability. In the case of segmented solid rocket motors, observations indicate that low-frequency oscillations are generated by a coupling between vortex shedding in shear regions established in the flow and the acoustic eigenmodes of the system. This process is investigated in this article on a model-scale configuration representing the geometry of the motor. An active control loop is exploited to obtain resonant and non-resonant conditions for the same operating point. Adaptive techniques are used to stabilize the flow and the experiment serves as a testbed for active control. It is shown that an adaptive system may be applied to essentially suppress the pressure oscillations. The instability mechanism is then studied by analyzing the flow field with particle image velocimetry. It is found that control noticeably modifies the mean flow structure. Detailed studies of the vortex pattern in the shedding region indicate that the concentrated vorticity and the corresponding circulation values remain in the same range but that vorticity is shed more randomly when the resonance is eliminated by the controller. This indicates that control is achieved by reducing the level of organization in the vortex pattern. Under resonant conditions the level of pressure fluctuations results from coherent interactions between vortices and the downstream nozzle. This process feeds energy in one of the acoustic modes of the system enhancing the pressure level. It is made less effective by the control loop.

Anthoine, J.; Mettenleiter, M.; Repellin, O.; Buchlin, J.-M.; Candel, S.

2003-05-01

381

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

382

The eect of blade aerodynamic modelling on the prediction of high-frequency rotor airloads  

Microsoft Academic Search

Interactions between the blades and vortical structures within the wake of a helicopter rotor are a significant source of impulsive loading and noise, particularly in descending flight. Brown's Vorticity Transport Model has been used to investigate the influence of the fidelity of the local blade aerodynamic model on the accuracy with which the high-frequency airloads associated with blade-vortex interactions can

Mary E. Kelly; Richard E. Brown

2009-01-01

383

EFFICIENT HIGH-RESOLUTION WAKE MODELLING USING THE VORTICITY TRANSPORT EQUATION  

Microsoft Academic Search

Computational Fluid Dynamic models, through their fundamental treatment of the flow physics, present a unique and powerful tool for analysing the aerodynamics of helicopter rotors. Their effectiveness is limited though by their intensive use of computational resources and by their tendency to dissipate vorticity. The Vorticity Transport Model already addresses the problem of vorticity diffusion by solving the fundamental fluid

A. J. Line; R. E. Brown

384

DNS and RANS Modeling of Dispersion in the Wake of an Obstacle  

NASA Astrophysics Data System (ADS)

We present a numerical study of the dispersion of a passive scalar in turbulent separated flows to establish the predictive capabilities of algebraic flux models against the standard eddy-diffusivity representation. The scalar dispersion from a point source over a wavy wall is initially investigated to carefully evaluate scalar flux models through comparisons with DNS data. The roof-top release of a passive plume from a wall-mounted cube in a turbulent boundary layer is then presented to demonstrate that algebraic models can also be applied successfully to atmospheric dispersion at street-scale. Despite the questionable validity of local-equilibrium conditions, the numerical experiments show that algebraic models provide a significant improvement for scalar dispersion simulations of complex flows with respect to the standard eddy-diffusivity model.

Philips, David; Rossi, Riccardo; Iaccarino, Gianluca

2009-11-01

385

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

386

Development of a building wake/stack height numerical modeling capability  

SciTech Connect

We are developing state-of-the-art numerical tools which can be used to provide reliable estimates of potential emissions at various LLNL sites. In particular we have focused our efforts in generating models which can simulate the wind flow and dispersion of airborne pollutants around surface-mounted structures such as buildings or building complexes. To achieve this goal, we have adopted two different but complementary approaches in the modeling of this complex problem. The first approach employs a Reynolds-averaged set of equations whose solution results in a description of the mean flow and concentration pattern. In the second approach, we are developing a more advanced model based on the large eddy simulation (LES) concept. In this report, we describe the progress in the development of the two approaches. We begin by discussing the calculational procedure which has been chosen for the Reynolds-averaged model, namely: prediction of the mean flow via a turbulent flow model, and; employment of the calculated flow field to drive a particle-in-cell transport and diffusion model (ADPIC). The performance of this model is benchmarked against experimental data obtained for flow over a backward-facing step. The backward-facing step problem can be viewed as a simplification of a rectangular-shaped surface mounted obstacle. We next include a brief description of the LES method, the continuum and discretized LES equations, the numerical methodology, and some preliminary flow calculations. As with the Reynolds-averaged model, the backward- facing step is used to benchmark the LES model development. The results are in agreement with the calculations of other researchers. We conclude by discussing several improvements which will be considered as we continue the development of both the Reynolds-averaged and the LES models. 40 refs., 15 figs., 1 tab.

Lee, R.L.; McCallen, R.C.

1991-09-06

387

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

388

Review of a model to assess stranding of juvenile salmon by ship wakes along the Lower Columbia River, Oregon and Washington  

USGS Publications Warehouse

Long period wake waves from deep draft vessels have been shown to strand small fish, particularly juvenile Chinook salmon Oncorhynchus tschawytcha, in the lower Columbia River (LCR). The U.S. Army Corps of Engineers is responsible for maintaining the shipping channel in the LCR and recently conducted dredging operations to deepen the shipping channel from an authorized depth of 40 feet(ft) to an authorized depth of 43 ft (in areas where rapid shoaling was expected, dredging operations were used to increase the channel depth to 48 ft). A model was developed to estimate stranding probabilities for juvenile salmon under the 40- and 43-ft channel scenarios, to determine if channel deepening was going to affect wake stranding (Assessment of potential stranding of juvenile salmon by ship wakes along the Lower Columbia River under scenarios of ship traffic and channel depth: Report prepared for the Portland District U.S. Army Corps of Engineers, Portland, Oregon). The U.S. Army Corps of Engineers funded the U.S. Geological Survey to review this model. A total of 30 review questions were provided to guide the review process, and these questions are addressed in this report. In general, we determined that the analyses by Pearson (2011) were appropriate given the data available. We did identify two areas where additional information could have been provided: (1) a more thorough description of model diagnostics and model selection would have been useful for the reader to better understand the model framework; and (2) model uncertainty should have been explicitly described and reported in the document. Stranding probability estimates between the 40- and 43-ft channel depths were minimally different under most of the scenarios that were examined by Pearson (2011), and a discussion of the effects of uncertainty given these minimal differences would have been useful. Ultimately, however, a stochastic (or simulation) model would provide the best opportunity to illustrate uncertainty within a given set of model predictions, but such an approach would require a substantial amount of additional data collection. Several review questions focused on the accuracy and precision of the model estimates, but we were unable to address these questions because of the limited data that currently exists regarding wake stranding in the LCR. Additional field studies will be required to validate findings from Pearson (2011), if concerns regarding accuracy and precision remain a priority. Although the Pearson (2011) model provided a useful examination of stranding under pre-construction and post-construction conditions, future research will be required to better understand the effects of wake stranding on juvenile salmonids throughout the entire LCR. If additional information on wake stranding is desired in the future, the following topics may be of interest: (1) spatial examination of wake stranding throughout the entire LCR; (2) additional evaluation of juvenile salmonid behavior and population dynamics; (3) assessing and integrating predicted changes in ship development; and (4) assessing and integrating predicted changes in climate on environmental factors known to cause stranding.

Kock, Tobias J.; Plumb, John M.; Adams, Noah S.

2013-01-01

389

AIRLOADS AND WAKE GEOMETRY CALCULATIONS FOR AN ISOLATED TILTROTOR MODEL IN A WIND TUNNEL  

Microsoft Academic Search

Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 0.25-scale V-22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of

Wayne Johnson

390

A hierarchy of low-dimensional models for the transient and post-transient cylinder wake  

Microsoft Academic Search

A hierarchy of low-dimensional Galerkin models is proposed for the viscous, incompressible flow around a circular cylinder building on the pioneering works of Stuart (1958), Deane et al. (1991), and Ma & Karniadakis (2002). The empirical Galerkin model is based on an eight-dimensional Karhunen Loève decomposition of a numerical simulation and incorporates a new `shift-mode' representing the mean-field correction. The

Bernd R. Noack; Konstantin Afanasiev; Marek Morzynski; Gilead Tadmor; Frank Thiele

2003-01-01

391

Opportunities for improvements in simple models for estimating runway capacity  

Microsoft Academic Search

Arrival rates may be misestimated by Monte Carlo simulation models that assume uniform distributions for landing speeds (Vg) and the inter-arrival distance spacings (?) between successive arrivals, based on the wake vortex weight classes of the paired aircraft. Analysis of arrival data measured at Memphis International Airport (MEM) shows that the Vg and ? values are not uniformly distributed. Over

B. Levy; J. Legge; M. Romano

2004-01-01

392

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

393

Interactions between Wavemaking and the Boundary Layer and Wake of a Ship Model.  

National Technical Information Service (NTIS)

A detailed experimental study has been conducted in a towing tank on a 3.048 meter long mathematical model, known as the Wigley hull, to study the effects of viscosity on the wavemaking resistance of the ship form. The measurements included total resistan...

A. Shahshahan L. Landweber

1986-01-01

394

Hierarchical Galerkin and non-linear Galerkin models for laminar and turbulent wakes  

Microsoft Academic Search

In this thesis we present a hierarchy of differential models for simulating unsteady laminar and turbulent flows in complex-geometry domains. They include high- resolution spectral methods (DNS), large-eddy simulations (LES), and proper orthogonal decomposition (POD) based on Galerkin and nonlinear Galerkin projections. While the approach we develop is general, here we focus on the flow past a circular cylinder as

Xia Ma

2001-01-01

395

The role of shape and relative submergence on the structure of wakes of low-aspect-ratio wall-mounted bodies  

NASA Astrophysics Data System (ADS)

The effects of shape and relative submergence (the ratio of flow depth to obstacle height, d/ H) are investigated on the wakes around four different low-aspect-ratio wall-mounted obstacles at Re H = 17,800: semi-ellipsoids with the major axes of the base ellipses aligned in the streamwise and transverse directions, and two cylinders with aspect ratios matching the ellipsoids ( H/ D = 0.89 and 0.67, where D is the maximum transverse dimension). Particle Image Velocimetry was used to interrogate the flow. Streamwise features observed in the mean wake include counter-rotating distributions of vorticity inducing downwash (tip structures), upwash (base structures), and horseshoe vortices. In particular, the relatively subtle change in geometry produced by the rotation of the ellipsoid from the streamwise to the transverse orientation results in a striking modification of the mean streamwise vorticity distribution in the wake. Tip structures are dominant in the former case, while base structures are dominant in the latter. A vortex skeleton model of the wake is proposed in which arch vortex structures, shed from the obstacle, are deformed by the competing mechanisms of Biot-Savart self-induction and the external shear flow. The selection of tip or base structures in the ellipsoid wakes is caused by tilting of the arch structures either upstream or downstream, respectively, which is governed by ellipsoid curvature. An inverse relationship was observed between the relative submergence and the strength of the base structures for the ellipsoids, with a dominant base structure observed for d/ H = 1 in both cases. These results demonstrate a means by which to achieve significant modifications to flow structure and thereby also to transport mechanisms in the flow. Therefore, this work provides insight into the modeling and control of flow over wall-mounted bodies.

Hajimirzaie, Seyed M.; Wojcik, Craig J.; Buchholz, James H. J.

2012-12-01

396

Advancing-side directivity and retreating-side interactions of model rotor blade-vortex interaction noise  

NASA Astrophysics Data System (ADS)

Acoustic data are presented from a 40 percent scale model of the four-bladed BO-105 helicopter main rotor, tested in a large aerodynamic wind tunnel. Rotor blade-vortex interaction (BVI) noise data in the low-speed flight range were acquired using a traversing in-flow microphone array. Acoustic results presented are used to assess the acoustic far field of BVI noise, to map the directivity and temporal characteristics of BVI impulsive noise, and to show the existence of retreating-side BVI signals. The characterics of the acoustic radiation patterns, which can often be strongly focused, are found to be very dependent on rotor operating condition. The acoustic signals exhibit multiple blade-vortex interactions per blade with broad impulsive content at lower speeds, while at higher speeds, they exhibit fewer interactions per blade, with much sharper, higher amplitude acoustic signals. Moderate-amplitude BVI acoustic signals measured under the aft retreating quadrant of the rotor are shown to originate from the retreating side of the rotor.

Martin, R. M.; Splettstoesser, W. R.; Elliott, J. W.; Schultz, K.-J.

1988-05-01

397

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

398

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

399

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

400

An investigation of counterrotating tip vortex interaction  

Microsoft Academic Search

A tip vortex interaction model originally developed for compressors has been extended and adapted for use with counterrotating open rotors. Comparison of available acoustic data with predictions (made with and without the tip vortex model included) illustrate the importance of this interaction effect. This report documents the analytical modeling, a limited experimental verification, and certain key parametric studies pertaining to

R. K. Majjigi; K. Uenishi; P. R. Gliebe

1989-01-01

401

Modelling of the effect of the foregoing wake on the bypass transition on the airfoil  

NASA Astrophysics Data System (ADS)

A modified algebraic model of the bypass transition was used for the simulation of the flow around the symmetrical airfoil NACA 0012 in the tandem configuration. The transition model is based on local variables only to enable its application for complex flow geometry using unstructured computational grids. The attention was especially focused on the flow near the leading edge where the boundary layer is not yet fully developed while used empirical correlations were established for the boundary-layer flow. The numerical simulation was compared with experiments of Lee and Kang [1] carried out for the Reynolds number Rec = (2÷6)×105, the zero angle of attack and various distance between both airfoils. The agreement of numerical simulation with experimental data is quite satisfactory.

Straka, Petr; P?íhoda, Jaromír; Šimurda, David

2012-04-01

402

A study on wake-galloping employing full aeroelastic twin cable model  

Microsoft Academic Search

The basic aerodynamic characteristics of twin cable were investigated through a series of wind tunnel tests employing full aeroelastic twin cable model. The cable arrangement employed here is characterized by W\\/D ratio that varies from 4.3 to 8.7 (where W is the spacing of centers of cables, D is the diameter of the cable), namely corresponds to the value between

S Tokoro; H Komatsu; M Nakasu; K Mizuguchi; A Kasuga

2000-01-01

403

Analysis of Array Motion in the Wake of a Submarine Model  

Microsoft Academic Search

High resolution stereo-PIV measurements were made on a long, small diameter cylinder towed from the control surface of a 1\\/18^th scale submarine model. The experiments were performed in the Deep Water Tow Basin at NSWCCD at 5 kts. Three-dimensional velocity fields over ten body lengths downstream were obtained. The cylinders were approximately neutrally buoyant and towed through a stationary laser

Damien Bretall; Deborah Furey; Paisan Atsavapranee; Kimberly Cipolla

2006-01-01

404

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

405

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

SciTech Connect

The wakes behind turbine blade trailing edges are characterized by large-scale periodic vortex patterns known as the von Karman vortex street. The failure of steady-state Navier-Stokes calculations in modeling wake flows appears to be mainly due to ignoring this type of flow instabilities. In an effort to contribute to a better understanding of the time-varying wake flow characteristics behind turbine blades, VKI has performed large-scale turbine cascade tests to obtain very detailed information about the steady and unsteady pressure distribution around the trailing edge of a nozzle guide vane. Tests are run at an outlet Mach number of M{sub 2,is} = 0.4 and a Reynolds number of Re{sub C} = 2 {times} 10{sup 6}. The key to the high spatial resolution of the pressure distribution around the trailing edge is a rotatable trailing edge with an embedded miniature pressure transducer underneath the surface and a pressure slot opening of about 1.5 deg of the trailing edge circle. Signal processing allowed differentiation between random and periodic pressure fluctuations. Ultrashort schlieren pictures help in understanding the physics behind the pressure distribution.

Cicatelli, G.; Sieverding, C.H. [von Karman Inst., Rhode St. Genese (Belgium)

1997-10-01

406

Wind-Tunnel Free-Flight Investigation of a 0.15-Scale Model of the F-106B Airplane with Vortex Flaps.  

National Technical Information Service (NTIS)

An investigation to determine the effects of vortex flaps on the flight dynamic characteristics of the F-106B in the area of low-speed, high-angle-of-attack flight was undertaken on a 0.15-scale model of the airplane in the Langley 30- by 60-Foot Tunnel. ...

L. P. Yip

1987-01-01

407

Implementation and assessment of turbine wake models in the Weather Research and Forecasting model for both mesoscale and large-eddy simulation  

Microsoft Academic Search

Flow dynamics in large wind projects are influenced by the turbines located within. The turbine wakes, regions characterized by lower wind speeds and higher levels of turbulence than the surrounding free stream flow, can extend several rotor diameters downstream, and may meander and widen with increasing distance from the turbine. Turbine wakes can also reduce the power generated by downstream

M Singer; J Mirocha; J Lundquist; J Cleve

2010-01-01

408

Noise generated by low pressure axial flow fans. I - Modeling of the turbulent noise  

Microsoft Academic Search

A new analytical treatment is proposed for estimating the sound pressure level of turbulent noise radiated from low pressure axial flow fans. In the analysis, a physical model which is very simple but reasonable is introduced to explain the turbulent noise generation originating from the vortex shedding from rotor blades. The analytical result shows that the wake width, which is

T. Fukano; Y. Kodama; Y. Senoo

1977-01-01

409

Effect of Varying the Angle of Attack of the Scales on a Biomimetic Shark Skin Model on Embedded Vortex Formation  

NASA Astrophysics Data System (ADS)

The skin of fast-swimming sharks is proposed to have mechanisms to reduce drag and delay flow separation. The skin of fast-swimming sharks is covered with small denticles, on the order of 0.2 mm, that if bristled create cavities. It has been shown that for an angle of attack of 90 degrees, vortices form within these cavities and impose a partial slip condition at the surface of the cavity. This experiment focuses on smaller angles of attack for denticle bristling, closer to the range thought to be achieved on real shark skin. A 3-D bristled shark skin model with varying angle of attack, embedded below a boundary layer, was used to study the formation of cavity vortices through fluorescent dye visualization and Digital Particle Image Velocimetry (DPIV). The effect of varying angle of attack on vortex formation will be discussed.

Wheelus, Jennifer; Lang, Amy

2009-11-01

410

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

411

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

412

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

413

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

414

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

415

Interaction of a vortex ring with a non-premixed methane flame  

NASA Astrophysics Data System (ADS)

Direct numerical simulation (DNS) is used to study the non-equilibrium characteristics of non-premixed methane flames in an unsteady strain rate field generated by a vortex ring. Two canonical flame-vortex ring configurations are used. In the first configuration the vortex ring interacts with an initially unstrained non-premixed flame. Two stages of interaction are identified. The first stage corresponds to the head-on collision between the flame and the vortex ring, and lasts until the flame is quenched near the centerline. The unsteady effects are dominant and result in local flame extinction. The second stage of the interaction corresponds to the passage of the ring through the flame and its interaction with the flame from the oxidizer side. During this stage, the vortex ring losses its strength and, in addition to the unsteady effects, curvature effects can also become important. In the second configuration, the ambient contains only oxidizer. The high oxidizer temperature leads to the auto-ignition of the flame surrounding the vortex ring. Three flame regions, front, top, and wake are identified. Detailed (GRI-Mech) and augmented reduced (11-step, 12-step) kinetic mechanisms are used to model the methane combustion. The methane flame ignition characteristics and combustion regimes are examined in this configuration. For the range of parameters accessible, unsteady, curvature and thickening effects on the flame structure are observed. The contributions of time varying straining, fuel temperature and concentration to the unsteady effects on the flame structure are separated through comparisons with unsteady counterflow diffusion flame simulations. The capability of the current augmented reduced kinetic models to capture the ignition and flame structure is assessed through comparisons with detailed kinetic model results. The quasi steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times. The 11-step model predicts well the ignition delay time for all flame regions. At later times the fuel rich side of the flame predicted by this reduced mechanism exhibit differences compared to the detailed models. The ability of a conserved scalar approach to capture the characteristics of flame-vortex ring interactions is also assessed through comparisons with the DNS results.

Safta, Cosmin

416

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

417

The Far Wake After A Body Towing In The Thermocline: The Laboratory Experiment and A Physical Model  

Microsoft Academic Search

Experimental study of the far wake after a body (8cm radius sphere) towing in the thermocline at a large velocity (90 cm\\/s) was carried out in the large thermostratified tank with overal sizes 20m 4m 2m. It was revealed that at early times (less than 30- 60\\/Nmax, where Nmax being the maximum buoyancy frequensy in the thermocline) the mean value

V. I. Kazakov; V. A. Kostrov; D. P. Korotkov; B. V. Serin; D. A. Sergeev; Yu. I. Troitskaya

2002-01-01

418

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

419

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

420

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

421

Wind Turbine Aerodynamics Prediction Using Free-Wake Method in Axial Flow  

NASA Astrophysics Data System (ADS)

Wind turbine aerodynamics remains a particularly challenging and crucial research for wind energy industry. The blade element momentum theory is the most widely used in predicting the performance of wind turbine, since the method is simple and fast numerical algorithm. The flow field generated by rotary wing is considerably important and complicated, however, the BEM method has some limitations to model the unsteady effects. To overcome these limitations, the aerodynamic analysis using a time-marching free-vortex wake method was performed in this paper. Moreover, the inboard region of the blade experience a delay in stall and enhanced values of the normal force coefficient because of rotational boundary layer augmentation and three-dimensional effects. For this reason, Raj-Selig stall delay model was applied in this research. The numerical results were compared with experimental data, and the present results show excellent agreement with experiment.

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

422

Guiding-center dynamics of vortex dipoles in Bose-Einstein condensates  

SciTech Connect

A quantized vortex dipole is the simplest vortex molecule, comprising two countercirculating vortex lines in a superfluid. Although vortex dipoles are endemic in two-dimensional superfluids, the precise details of their dynamics have remained largely unexplored. We present here several striking observations of vortex dipoles in dilute-gas Bose-Einstein condensates, and develop a vortex-particle model that generates vortex line trajectories that are in good agreement with the experimental data. Interestingly, these diverse trajectories exhibit essentially identical quasiperiodic behavior, in which the vortex lines undergo stable epicyclic orbits.

Middelkamp, S.; Schmelcher, P. [Zentrum fuer Optische Quantentechnologien, Universitaet Hamburg, Luruper Chaussee 149, DE-22761 Hamburg (Germany); Torres, P. J. [Departamento de Matematica Aplicada, Universidad de Granada, ES-18071 Granada (Spain); Kevrekidis, P. G. [Department of Mathematics and Statistics, University of Massachusetts, Amherst, Massachusetts 01003-4515 (United States); Frantzeskakis, D. J. [Department of Physics, University of Athens, Panepistimiopolis, Zografos, Athens 157 84 (Greece); Carretero-Gonzalez, R. [Nonlinear Dynamical System Group, Computational Science Research Center and Department of Mathematics and Statistics, San Diego State University, San Diego, California 92182-7720 (United States); Freilich, D. V.; Hall, D. S. [Department of Physics, Amherst College, Amherst, Massachusetts 01002-5000 (United States)

2011-07-15

423

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

424

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

425

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

426

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

427

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

428

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

429

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 th