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1

Wake field acceleration experiments  

SciTech Connect

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

Simpson, J.D.

1988-01-01

2

Vorticity Field from Successive Wake Vortices  

NASA Technical Reports Server (NTRS)

The two-dimensional version of the Terminal Area Simulation System (TASS) was used to numerically simulate the interaction of wake vortices from closely separated aircraft. The aircraft parameters and separations are taken from observed data at an actual airport. The wake vortices are generated near the runway threshold for four successive aircraft. The ambient conditions are characterized by light crosswinds and stable stratification. This movie shows the time sequence of the vorticity field from the successive wake vortices. Apparent are the interactions between each pair of successive wake vortices and the ground.

1997-01-01

3

DIELECTRIC WAKE FIELD RESONATOR ACCELERATOR MODULE  

SciTech Connect

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

Hirshfield, Jay L.

2013-11-06

4

Wake fields and energy spread for the ERHIC ERL  

SciTech Connect

Wake fields in high-current ERLs can cause significant beam quality degradations. Here we summarize effects of coherent synchrotron radiation, resistive wall, accelerating cavities and wall roughness for ERL parameters of the eRHIC project. A possibility of compensation of such correlated energy spread is also presented. An emphasis in the discussion is made on the suppression of coherent synchrotron radiation due to shielding and a possible reduction of wall roughness effects for realistic surfaces.

Fedotov A. V.; Kayran& #44; D.

2011-10-16

5

Suppression of short sea waves in ship wakes: Measurements and observations  

NASA Astrophysics Data System (ADS)

Measurements of short wave energy, surface tension distributions, and radar imagery of centerline ship wakes are presented. A direct association between reduced radar return and reduced scattering wave energy is demonstrated. The dominant influences of ship-generated turbulence and surface film distributions upon reduced short wave energy in ship wakes are shown. The effect of turbulence is emphasized by the very slow regrowth of attenuated wave energy in the especially turbulent wake of a towed barge. The effect of surface film distributions is emphasized by the suppression of short wave growth during a wind puff in a wake that is about 1 hour old.

Milgram, J. H.; Peltzer, R. D.; Griffin, O. M.

1993-01-01

6

Wake field effects in APT linac  

SciTech Connect

The 1.7-GeV 100-mA CW proton linac is now under design for the Accelerator Production of Tritium (APT) Project. The high current leads to stringent restrictions on allowable beam losses (<1 nA/m), that requires analyzing carefully all possible loss sources. While wake-field effects are usually considered negligible in proton linacs, the author studies these effects for the APT to exclude potential problems at such a high current. Loss factors and resonance frequency spectra of various discontinuities of the vacuum chamber are investigated, both analytically and using 2-D and 3-D simulation codes with a single bunch as well as with many bunches. Here he concentrates on two features specific to the APT linac: loss factors for the design {beta} < 1 and CW beam structure.

Kurennoy, S.S.

1998-12-31

7

Multiple-fluid models for plasma wake-field phenomena  

SciTech Connect

In this paper we present various treatments of plasma wake-field phenomena which employ multiple-fluid models. These models generalize the one-dimensional, nonlinear, relativistic single-fluid model which has been used extensively in previous plasma wake-field calculations. Using a two-fluid model, we discuss the interaction of a low-energy continuous electron beam with wake-field-generated plasma waves. The phenomena of continuous-beam modulation and wave period shortening are discussed. The relationship between these effects and the two-stream instability is also examined. Also, using a three-fluid model, effects due to plasma electron temperature in nonlinear plasma wake-fields are examined and compared to previous work. Finally, the consequences of ion motion induced by large-amplitude electron plasma waves are calculated by including the fluid behavior of the ions.

Rosenzweig, J.B. (Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (US))

1989-11-01

8

Wake fields and energy spread for the eRHIC ERL  

SciTech Connect

Wake fields in high-current ERLs can cause significant beam quality degradations. Here we summarize effects of coherent synchrotron radiation, resistive wall, accelerating cavities and wall roughness for ERL parameters of the eRHIC project. A possibility of compensation of such correlated energy spread is also presented. An emphasis in the discussion is made on the suppression of coherent synchrotron radiation due to shielding and a possible reduction of wall roughness effects for realistic surfaces. In this report we discuss the wake fields with a focus on their effect on the energy spread of the beam. Other effects of wake fields are addressed elsewhere. An energy spread builds up during a pass though a very long beam transport in the eRHIC ERL under design. Such energy spread become important when beam is decelerated to low energy, and needs to be corrected. Several effects, such as Coherent Synchrotron Radiation (CSR), Resistive Wall (RW), accelerating RF cavities (RF) and Wall Roughness (WR) were considered. In this paper, we briefly summarize major contributions to energy spread from the wake fields for eRHIC parameters, and present possible energy spread compensation for decelerated beam. In the rest of the report we discuss effects which we believe are suppressed for the eRHIC parameters.

Fedotov, A.; Kayran, D.

2011-10-16

9

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

2013-09-01

10

Field-aligned currents in Io's plasma wake  

NASA Astrophysics Data System (ADS)

In Io's reference frame, the downstream distribution of various physical quantities in Io's plasma wake can be regarded as unchangeable with respect to time. A magnetic flux tube in a specific position in the wake can be related to a state of its evolution after been perturbed by Io. Thus the investigation of the wake can be transferred to the study of an Io-perturbed flux tube in the Jovian corotational frame. A magnetohydrodynamics approach called "The theory of a thin filament motion" is employed here. Our simulations suggest that the underlying physics possesses the characteristics typical for both Alfvén wave and corotational lag models. An upstream-coming flux tube must be in contact with Io for approximately 500 seconds, until a tilt angle of about 4 degrees has been developed, before it is released downstream. A perturbed flux tube would undergo a subcorotational motion in Io's plasma wake. This motion is inevitably modulated by Alfvén wave bouncing back and forth inside the Io plasma torus. The scale of the subcorotation region is in the order of one Jovian radius. The distribution of the simulated field-aligned currents downstream is consistent with the observed wake aurora brightness profile; in particular, the periodic structure in the current distribution is in agreement with recent infrared and FUV observations showing the presence of secondary spots in the auroral emissions.

Chen, Chuxin

2008-09-01

11

ARTEMIS observations of extreme diamagnetic fields in the lunar wake  

NASA Astrophysics Data System (ADS)

We present two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) observations of diamagnetic fields in the lunar wake at strengths exceeding twice the ambient magnetic field during high plasma beta conditions. The first observation was 350 km from the lunar surface while the Moon was located in the terrestrial magnetosheath with elevated particle temperatures. The second observation was in the solar wind ranging from 500 to 2000 km downstream, with a relatively low magnetic field strength of approximately 1.6 nT. In both cases, the plasma beta exceeded 10. We discuss the observations and compare the data to hybrid plasma simulations in order to validate the model under such extreme conditions and to elucidate the global structure of the lunar wake during these observations. The extreme nature of the diamagnetic field in the lunar wake provides an important end-member test case for theoretical and modeling studies of the various plasma processes operating in the lunar wake. could not parse page for

Poppe, A. R.; Fatemi, S.; Halekas, J. S.; Holmström, M.; Delory, G. T.

2014-06-01

12

Experimental studies of plasma wake-field acceleration and focusing  

SciTech Connect

More than four years after the initial proposal of the Plasma Wake-field Accelerator (PWFA), it continues to be the object of much investigation, due to the promise of the ultra-high accelerating gradients that can exist in relativistic plasma waves driven in the wake of charged particle beams. These large amplitude plasma wake-fields are of interest in the laboratory, both for the wealth of basic nonlinear plasma wave phenomena which can be studied, as well as for the applications of acceleration of focusing of electrons and positrons in future linear colliders. Plasma wake-field waves are also of importance in nature, due to their possible role in direct cosmic ray acceleration. The purpose of the present work is to review the recent experimental advances made in PWFA research at Argonne National Laboratory, in which many interesting beam and plasma phenomena have been observed. Emphasis is given to discussion of the nonlinear aspects of the PWFA beam-plasma interaction. 29 refs., 13 figs.

Rosenzweig, J.B.; Cole, B.; Ho, C.; Gai, W.; Konecny, R.; Mtingwa, S.; Norem, J.; Rosing, M.; Schoessow, P.; Simpson, J.

1989-07-18

13

Field measurements in the wake of a model wind turbine  

NASA Astrophysics Data System (ADS)

As a first step to study the dynamics of a wind farm' we experimentally explored the flow field behind a single wind turbine of diameter 1.17 m at a hub height of 6.25 m. A 10 m tower upstream of the wind farm characterizes the atmospheric conditions and its influence on the wake evolution. A vertical rake of sonic anemometers is clustered around the hub height on a second tower' 6D downstream of the turbine. We present preliminary observations from a 1- hour block of data recorded in near-neutral atmospheric conditions. The ratio of the standard deviation of power to the inflow velocity is greater than three' revealing adverse effects of inflow turbulence on the power and load fluctuations. Furthermore' the wake defect and Reynolds stress and its gradient are pronounced at 6D. The flux of energy due to Reynolds stresses is similar to that reported in wind tunnel studies. The swirl and mixing produces a constant temperature wake which results in a density jump across the wake interface. Further field measurements will explore the dynamics of a model wind farm' including the effects of atmospheric variability.

Pol, Suhas; Taylor, Amelia; Bilbao, Argenis; Doostalab, Ali; Novoa, Santiago; Westergaard, Carsten; Hussain, Fazle; Sheng, Jian; Ren, Beibei; Giesselmann, Michael; Glauser, Mark; Castillo, Luciano

2014-06-01

14

Simulation of ultrashort electron pulse generation from optical injection into wake-field plasma waves  

E-print Network

Simulation of ultrashort electron pulse generation from optical injection into wake-field plasma are focused in a plasma, one exciting a wake-field electron plasma wave while another locally alters some was proposed [1]. These wake-field accelerators seek to take advantage of the ultra-high electron acceleration

Umstadter, Donald

15

Wake-field generation by the ponderomotive memory effect  

NASA Astrophysics Data System (ADS)

An analytical and numerical investigation of the plasma response to an imposed high frequency wave packet with a slow explicit time-dependent envelope is presented. An underlying picture of ponderomotive effects is developed, which shows that the explicit time dependence forces us to treat the problem kinetically, and furthermore, that a wake field is generated by the ponderomotive memory effect. The latter supplements the well-known ponderomotive force and fake heating effect. Several perturbation schemes are compared showing that the influence of resonant particles, treated by the method of characteristics, has to be taken into account for Langmuir wave packets with k?d>=0.2, where k is the wave number and ?d the Debye length. A self-consistent Vlasov simulation shows the disappearance of the density depression in the case of immobile ions, whereas the wake-field pattern survives self-consistency.

Wolf, U.; Schamel, H.

1997-10-01

16

Modulation of continuous electron beams in plasma wake-fields  

SciTech Connect

In this paper we discuss the interaction of a continuous electron beam with wake-field generated plasma waves. Using a one-dimensional two fluid model, a fully nonlinear analytical description of the interaction is obtained. The phenomena of continuous beam modulation and wave period shortening are discussed. The relationship between these effects and the two-stream instability is also examined. 12 refs., 1 fig.

Rosenzweig, J.B.

1988-09-08

17

Direct simulation of single bubble motion under vertical magnetic field: Paths and wakes  

NASA Astrophysics Data System (ADS)

Motion of single Ar bubbles rising in GaInSn under vertical magnetic fields is studied numerically using a volume-of-fluid method and adaptive mesh refinement technique for two-phase interface treatment; a consistent and conservative scheme calculates induced current density and Lorentz force. Numerical results are compared with published experimental data [C. Zhang, S. Eckert, and G. Gerbeth, "Experimental study of single bubble motion in a liquid metal column exposed to a DC magnetic field," Int. J. Multiphase Flow 31, 824-842 (2005)], where bubble diameters range from 2.5 to 6.4 mm, producing Reynolds numbers that vary between 2000 and 4000. Maximum experimental magnetic field strength was set to 0.3 T because of experimental restrictions, although we increased it to 0.5 T for firm conclusions. Apart from terminal rising velocity comparisons, we focused on variations in bubble motion paths and wake structures under magnetic fields, which cannot be observed experimentally because liquid metal is opaque. Magnetic field effects on bubble trajectory are exerted through vortex structure modification, which reinforced the conjecture that path instability is mainly attributed to wake instability. In bubble motion without magnetic fields, vortex threads in the bubble wake wrap around each other while vortex filaments incline parallel to the field with increasing magnetic intensity. Additionally, high magnetic fields will induce secondary bubble path instabilities, which contribute to the high Reynolds number flow that instabilities develop around the bubble, producing an asymmetrical Lorentz force distribution. This instability vanishes under higher magnetic intensities because flow instability is suppressed. Rising bubble aspect ratios decrease considerably under magnetic fields and may also contribute to smaller vorticities at the bubble surface. A close relationship between fluctuations in rising velocity and shape variations is found.

Zhang, Jie; Ni, Ming-Jiu

2014-10-01

18

Field-aligned Currents in Io's Plasma Wake  

NASA Astrophysics Data System (ADS)

Since the discovery of Io-controlled decametric radio emissions, the interaction between Io and Jovian magnetosphere has been studied intensively. Two types of interaction have been proposed so far. One is electric circuit model, in which the induced currents flow between Io and the Jovian ionosphere along the magnetic flux tube threading Io. The other is Alfvén wing model. A wing forms in the perturbed magnetic field lines behind Io, the Alfvénic currents develop in the wing rather than along the magnetic flux tubes. More recently, auroral emission associated with Io's footprint and its trailing emission were observed. Such auroral arc may extend longitudinally westward for more than 100 degrees. This trail of aurora is brightest near Io and dims with increasing downstream distance. There is no clear theoretical understanding of the physics that generates this downstream aurora. However it is generally believed that Io's plasma wake is associated with this phenomenon and field-aligned currents lead to downstream emissions. Along with the above two types of the interaction between Io and its surrounding medium, there are also two theoretical frameworks in which these downstream emissions can be interpreted. The first one is corotational lag. When an Io-perturbed (mass loading and/or Io's conductivity) magnetic flux tube moves slowly relative to Jovian magnetosphere, an electric field would be induced at the equatorial plane of the flux tube, which in turn causes a current perpendicular to the field lines that is connected by field-aligned currents. The Lorentz force due to the perpendicular current would play the role of bring the lagged plasma up to corotation. The second is Alfvén wave, in which the Io-perturbed Alfvén wave is reflected between the Jovian ionosphere and the torus edge, driving particles into loss cone. Our present study attempts to use a MHD method to solve the above problem. MHD simulations of Io-Jupiter interaction has been carried out by several groups and yielded some suggestive results, but these studies concentrated primarily on the vicinity of Io and did not treat the Jovian ionospheric effect realistically. To investigate the mechanism for emissions in the trailing tail, a model extending longitudinally more than 100 degrees and latitudinally from the southern Jovian ionosphere to the northern ionosphere is needed. In particular, such a model should reflect both the non-uniform magnetic field and the non-uniform plasma distributions together with realistic boundary conditions. To tackle this problem with available computer resources, we provide instead an equivalent approach "theory of thin filament motion". Our model is indeed a one-dimensional MHD simulation that satisfies all the above requirements and has the advantage of using much less computer resources than the earlier MHD models, which in turn allows us to try various physical conditions within limited computing time. We assume Io's plasma wake can be regarded as a tail of thin magnetic flux tubes perturbed by Io successively. In this assumption, a flux tube is considered as thin if the pressure variations across the flux tube are negligible compared to the total external pressure (gas plus magnetic pressure) representing the effects of the enveloping magnetized plasma (Jovian magnetosphere). Furthermore we assume that in Io's reference frame the variations of the physical quantities along the downstream distance do not change with time. After converting to the corotating frame, the study of Io's plasma wake can be simplified to investigate the evolution of a magnetic flux tube in Io's wake with appropriate initial conditions. Our simulations suggest that the mechanism for producing wake aurora could not be explained by either Alfvén wave or electric circuit alone, rather, the underlying physics possesses the characteristics typical for both Alfvén wave and corotational lag models. An upstream-coming flux tube must be in contact with Io for approximately 500 seconds, until a tilt angle of about 4 degrees has been develo

Chen, Chuxin

2008-09-01

19

Two-Channel Rectangular Dielectric Wake Field Accelerator Structure Experiment  

SciTech Connect

A design is presented for a two-channel 30-GHz rectangular dielectric wake field accelerator structure being built for experimental tests at Argonne National Laboratory (ANL). This structure allows for a transformer ratio T much greater than two, and permits continuous coupling of energy from drive bunches to accelerated bunches. It consists of three planar slabs of cordierite ceramic ({epsilon} = 4.7) supported within a rectangular copper block, forming a drive channel 12 mmx6 mm, and an accelerator channel 2 mmx6 mm. When driven by a 50 nC, 14 MeV single bunch available at ANL, theory predicts an acceleration field of 6 MeV/m, and T = 12.6. Inherent transverse wake forces introduce deflections and some distortion of bunch profiles during transit through the structure that are estimated to be tolerable. Additionally, a cylindrical two-channel DWFA is introduced which shares many advantages of the rectangular structure including high T, and the added virtue of axisymmetry that eliminates lowest-order transverse deflecting forces.

Sotnikov, G. V. [Omega-P, Inc., New Haven Connecticut (United States); NSC Kharkov Institute of Physics and Technology, Kharkov (Ukraine); Marshall, T. C. [Omega-P, Inc., New Haven Connecticut (United States); Columbia University, New York, New York (United States); Shchelkunov, S. V. [Yale University, New Haven, Connecticut (United States); Didenko, A. [Fermi National Accelerator Laboratory, Batavia, IL (United States); Hirshfield, J. L. [NSC Kharkov Institute of Physics and Technology, Kharkov (Ukraine); Yale University, New Haven, Connecticut (United States)

2009-01-22

20

Wake fields in HCS accelerator and CTS power line of the CLIC Test Facility (CTF II) simulation with program wake  

E-print Network

One of the two parallel beam lines of the CTF II, (the 'Drive Beam') is providing the other (the accelerator) with 30 GHz power. Experimentation was performed in 98 and later with a CTF layout to study acceleration of a train of bunches with beam loading compensation, bunch length compression and 30 GHz conversion of the Drive Beam power [1]. This conversion is limited by the difficulty of transmitting the beam through the structures extracting the beam power (CTS). A large transverse wake loss factor is associated with the necessary high longitudinal wake loss factor in CTS. Therefore the limitation of transmission should come mainly from transverse wakes in CTS. Dynamics in HCS and in the bunch compression device was studied with codes GPT [2] and PARMELA [3], [4] using beam parameters input derived from calculations of the beam in the RF gun with code MAFIA. Code WAKE is used to verify that the influence of the wake-fields in HCS is small, to follow the beam along the 4 CTS of the drive linac, and to give ...

Riche, A

2000-01-01

21

COAXIAL TWO-CHANNEL DIELECTRIC WAKE FIELD ACCELERATOR  

SciTech Connect

Theory, computations, and experimental apparatus are presented that describe and are intended to confirm novel properties of a coaxial two-channel dielectric wake field accelerator. In this configuration, an annular drive beam in the outer coaxial channel excites multimode wakefields which, in the inner channel, can accelerate a test beam to an energy much higher than the energy of the drive beam. This high transformer ratio is the result of judicious choice of the dielectric structure parameters, and of the phase separation between drive bunches and test bunches. A structure with cm-scale wakefields has been build for tests at the Argonne Wakefield Accelerator Laboratory, and a structure with mm-scale wakefields has been built for tests at the SLAC FACET facility. Both tests await scheduling by the respective facilities.

Hirshfield, Jay L. [Omega-P, Inc.

2013-04-30

22

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

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

23

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

NASA Technical Reports Server (NTRS)

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

Booth, Earl R., Jr.

1987-01-01

24

Nonthermal Lorentzian wake-field effects on collision processes in complex dusty plasmas  

NASA Astrophysics Data System (ADS)

The influence of nonthermal Lorentzian wake-field on the electron-dust grain collision is investigated in complex dusty plasmas. The Eikonal method and the effective interaction potential are applied to obtain the Eikonal scattering phase shift, the differential Eikonal collision cross section, and the total Eikonal collision cross section as functions of the collision energy, the impact parameter, the Mach number, and the spectral index of Lorentzian plasma. It is found that the nonthermal effect enhances the Eikonal scattering phase shift and, however, suppresses the Eikonal collision cross section for the electron-dust grain in Lorentzian complex dusty plasmas. It is also found that the Eikonal scattering phase shift decreases with increasing Mach number and spectral index. In addition, the Eikonal collision cross section increases with an increase of the spectral index and Mach number in Lorentzian complex dusty plasmas.

Hong, Woo-Pyo; Jung, Young-Dae

2014-10-01

25

Suppression of short sea waves in ship wakes: Measurements and observations  

Microsoft Academic Search

Measurements of short wave energy, surface tension distributions, and radar imagery of centerline ship wakes are presented. A direct association between reduced radar return and reduced scattering wave energy is demonstrated. The dominant influences of ship-generated turbulence and surface film distributions upon reduced short wave energy in ship wakes are shown. The effect of turbulence is emphasized by the very

J. H. Milgram; R. D. Peltzer; O. M. Griffin

1993-01-01

26

Clutter suppression and classification using twin inverted pulse sonar in ship wakes.  

PubMed

Twin inverted pulse sonar (TWIPS) is here deployed in the wake of a moored rigid inflatable boat (RIB) with propeller turning, and then in the wake of a moving tanker of 4580 dry weight tonnage (the Whitchallenger). This is done first to test its ability to distinguish between scatter from the wake and scatter from the seabed, and second to test its ability to improve detectability of the seabed through the wake, compared to conventional sonar processing techniques. TWIPS does this by distinguishing between linear and nonlinear scatterers and has the further property of distinguishing those nonlinear targets which scatter energy at the even-powered harmonics from those which scatter in the odd-powered harmonics. TWIPS can also, in some manifestations, require no range correction (and therefore does not require the a priori environment knowledge necessary for most remote detection technologies). PMID:22088017

Leighton, T G; Finfer, D C; Chua, G H; White, P R; Dix, J K

2011-11-01

27

Correction to "Electrons and magnetic fields in the lunar plasma wake"  

E-print Network

2011 by the American Geophysical Union. 01480227/11/2011JA016929 JOURNAL OF GEOPHYSICAL RESEARCH, VOL and magnetic fields in the lunar plasma wake" by J. S. Halekas et al. (Journal of Geophysical Research, 110, A

California at Berkeley, University of

28

Effect of nonlinear chirped Gaussian laser pulse on plasma wake field generation  

NASA Astrophysics Data System (ADS)

An ultrashort laser pulse propagating in plasma can excite a nonlinear plasma wake field which can accelerate charged particles up to GeV energies within a compact space compared to the conventional accelerator devices. In this paper, the effect of different kinds of nonlinear chirped Gaussian laser pulse on wake field generation is investigated. The numerical analysis of our results depicts that the excitation of plasma wave with large and highly amplitude can be accomplished by nonlinear chirped pulses. The maximum amplitude of excited wake in nonlinear chirped pulse is approximately three times more than that of linear chirped pulse. In order to achieve high wake field generation, chirp parameters and functions should be set to optimal values.

Afhami, Saeedeh; Eslami, Esmaeil

2014-08-01

29

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

NASA Technical Reports Server (NTRS)

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

Proctor, Fred H.

1996-01-01

30

Decoherence suppression in a resonant driving field  

NASA Astrophysics Data System (ADS)

Resonant radio frequency (rf) control fields have been employed to suppress decoherence in single quantum bits (qubits) encoded in the probability amplitudes of np fine-structure states in Li Rydberg atoms. As described previously [1], static electric-field tuning of the spin and orbital angular momentum composition of the fine-structure eigenstates enables qubit storage in an approximate decoherence-free subspace in which phase errors due to small stray electric and magnetic fields are strongly suppressed. In addition, it was found that sequences of short electric field pulses could be utilized in a 'bang-bang' dynamic decoupling scheme to improve coherence times. We now show that a continuous resonant rf field can also suppress decoherence in this system. The rf-dressed fine-structure states form a more robust basis in which the energy splitting between the component qubit levels is locked to the drive frequency, and decoherence is essentially eliminated. Measurements of the operational range of rf frequency and field strength required to achieve decoherence suppression are in agreement with the predictions of a two-level model.

Minns, R. S.; Kutteruf, M. R.; Commisso, M. A.; Jones, R. R.

2008-04-01

31

3D Analysis of Wake Field Excitation in a Dielectric Loaded Rectangular Resonator  

SciTech Connect

The results of a three-dimensional analysis of wake field excitation in a slab-symmetric dielectric-loaded resonator by rigid electron bunches are presented. The complete set of solutions, including the solenoidal and potential parts of the electromagnetic field, consists of LSM and LSE modes. Each of the LSM and LSE modes contains odd and even waves. A numerical analysis of wake field excitation by symmetric electron bunches is carried out. The three-dimensional spatial structure of the longitudinal electric field is investigated. The influence of the drift vacuum channel on the wake field amplitude and on the coherent summation of wakefields for a regular sequence of bunches is studied.

Sotnikov, Gennadij V.; Onishchenko, Ivan N. [NSC 'Kharkov Institute of Physics and Technology', Academic Str. 1, Kharkov 61108 (Ukraine); Marshall, Thomas C. [Columbia University, New York City 10027 (United States)

2006-11-27

32

3D Analysis of Wake Field Excitation in a Dielectric Loaded Rectangular Resonator  

NASA Astrophysics Data System (ADS)

The results of a three-dimensional analysis of wake field excitation in a slab-symmetric dielectric-loaded resonator by rigid electron bunches are presented. The complete set of solutions, including the solenoidal and potential parts of the electromagnetic field, consists of LSM and LSE modes. Each of the LSM and LSE modes contains odd and even waves. A numerical analysis of wake field excitation by symmetric electron bunches is carried out. The three-dimensional spatial structure of the longitudinal electric field is investigated. The influence of the drift vacuum channel on the wake field amplitude and on the coherent summation of wakefields for a regular sequence of bunches is studied.

Sotnikov, Gennadij V.; Onishchenko, Ivan N.; Marshall, Thomas C.

2006-11-01

33

Blunt body near wake flow field at Mach 6  

NASA Technical Reports Server (NTRS)

Tests were conducted in a Mach 6 flow to examine the reattachment process of an axisymmetric free shear layer associated with the near wake of a 70 deg. half angle, spherically blunted cone with a cylindrical after body. Model angle of incidence was fixed at 0 deg. and free-stream Reynolds numbers based on body diameter ranged from 0.5 x 10(exp 6) to 4 x 10(exp 6). The sensitivity of wake shear layer transition on reattachment heating was investigated. The present perfect gas study was designed to compliment results obtained previously in facilities capable of producing real gas effects. The instrumented blunted cone model was designed primarily for testing in high enthalpy hypervelocity shock tunnels in both this country and abroad but was amenable for testing in conventional hypersonic blowdown wind tunnels as well. Surface heating rates were inferred from temperature - time histories from coaxial surface thermocouples on the model forebody and thin film resistance gages along the model base and cylindrical after body. General flow feature (bow shock, wake shear layer, and recompression shock) locations were visually identified by schlieren photography. Mean shear layer position and growth were determined from intrusive pitot pressure surveys. In addition, wake surveys with a constant temperature hot-wire anemometer were utilized to qualitatively characterize the state of the shear layer prior to reattachment. Experimental results were compared to laminar perfect gas predictions provided by a 3-D Navier Stokes code (NSHYP). Shear layer impingement on the instrumented cylindrical after body resulted in a localized heating maximum that was 21 to 29 percent of the forebody stagnation point heating. Peak heating resulting from the reattaching shear layer was found to be a factor of 2 higher than laminar predictions, which suggested a transitional shear layer. Schlieren flow visualization and fluctuating voltage time histories and spectra from the hot wire surveys across the shear layer substantiate this observation. The sensitivity of surface heating to forebody roughness was characterized for a reattaching shear layer. For example, at R(sub infinity), d = 4 x 10(exp 6), when the shear layer was transitional, the magnitude of peak heating from shear layer impingement was reduced by approximately 24 percent when transition grit was applied to the forebody. The spatial location of the local peak, however, remained unchanged.

Horvath, Thomas J.; McGinley, Catherine B.; Hannemann, Klaus

1996-01-01

34

Wake-field and fast head-tail instability caused by an electron cloud.  

PubMed

In positron and proton storage rings, electrons produced by photoemission, ionization, and secondary emission accumulate in the vacuum chamber during multibunch operation with close spacing. A positron or proton bunch passing through this "electron cloud" experiences a force similar to a short-range wake field. This effective wake field can cause a transverse-mode-coupling instability, if the electron-cloud density exceeds a threshold value. In this report, we compute the electron-cloud induced wake in a region without external magnetic field both analytically and via computer simulation, for parameters representing the low-energy positron ring of KEKB and the LHC proton beam in the CERN SPS. We study the linearity and time dependence of the wake function and its variation with the size of the electron cloud. Using a broadband resonator model for the electron-cloud wake field, we then evaluate theoretical expressions for the transverse-mode-coupling instability based on the linearized Vlasov equation, and for the instability threshold of fast transverse blow up including its dependence on chromaticity. PMID:11800799

Ohmi, K; Zimmermann, F; Perevedentsev, E

2002-01-01

35

Magnetic Fields in the Lunar Wake and Its Responses to the External Solar Wind Conditions  

NASA Astrophysics Data System (ADS)

The Moon has no thick atmosphere and no global magnetic field. When the solar wind plasma impacts with the Moon, particles can be mostly absorbed by the lunar surface, so it leaves a plasma void downstream, i.e., the lunar wake. Considering the pressure balance, people could expect that the magnetic field may have an enhancement in the central lunar wake. Such an enhancement has been detected earlier [Ogilvie et al., 1996; Owen et al., 1996; Halekas et al., 2005]. Besides the observational analysis, lots of theoretical studies and numerical modeling have also been used to investigate this process, e.g., in MHD [Xie et al., 2012], 3D hybrid model [Wiehle et al., 2011; Wang et al., 2011; Holmström et al., 2012] as well as PIC model [Birch and Chapman, 2011]. In the present study, we find that the enhancement of magnetic field in the central part is larger in the deep lunar wake than that in the far downtail region with the observations from the two ARTEMIS probes. However, in the wake boundary, there are usually two depletion dips on the two sides. As the distances from the lunar center increase, the slope of the enhancement of magnetic field strength becomes smoother. It means that the enhancement of magnitude in the deep lunar wake is sharpest from the boundary to the center. Another signature observed is that the magnitude of magnetic field decreases in the wake center as the distance from the body increases. So the distributions of magnetic field strength across the cross section as a function of distances from the lunar center are different. We have also tried to find the responses of the magnetic field distributions in the lunar wake to the angle of the IMF with respect to the direction of the solar wind flow. In the near wake, the dependence of field distributions on the angle is not obvious in the observational data. However, in the far downstream region from the lunar body, as the angle decreases, the amplitude of the magnetic field fluctuations becomes higher. And the effects of IMF directions to magnetic field disturbance across different lunar distances are very distinct. All these indicate that the magnetic field distributions are depending on the orientation of IMF in the far downtail region. The detailed mechanism implied in this refilling process of plasma cavity along the tailward distance needs further research in the future.

Wong, H.; Ma, Y.; Ip, W.; Xu, X.

2013-12-01

36

Venera-9 magnetic field measurements in the Venus wake - Evidence for an earth-like interaction  

NASA Technical Reports Server (NTRS)

Venera-9 magnetic field measurements in the Venus wake provide additional support for the hypothesis that Venus has an intrinsic planetary field. The observed field is in the direction expected for a northward moment, and is similar to that observed in equivalent locations in the terrestrial magnetosphere, both in its temporal and spatial behavior. In particular, Venera-9 appears to have observed a plasma sheet expansion, field-aligned currents, and tail-field dipolarization.

Russell, C. T.

1976-01-01

37

Analyzing the vortex dynamics in bluff-body wakes by Helmholtz decomposition of the velocity field  

Microsoft Academic Search

The velocity field in the wake of a bluff body is calculated by a novel procedure for the Navier–Stokes equations in the vorticity–velocity formulation. The time evolution of the vorticity is solved as an ODE problem on each node of the spatial discretization, using at each step of the time discretization the spatial solution for the velocity field provided by

F. L. Ponta

2006-01-01

38

Analyzing the vortex dynamics in bluff-body wakes by Helmholtz decomposition of the velocity field  

Microsoft Academic Search

The velocity field in the wake of a bluff body is calculated by a novel procedure for the Navier-Stokes equations in the vorticity-velocity formulation. The time evolution of the vorticity is solved as an ODE problem on each node of the spatial discretization, using at each step of the time discretization the spatial solution for the velocity field provided by

F. L. Ponta

2006-01-01

39

Blunt Body Near-Wake Flow Field at Mach 10  

NASA Technical Reports Server (NTRS)

Tests were conducted in a Mach 10 air flow to examine the reattachment process of a free shear layer associated with the near wake of a 70 deg half angle, spherically blunted cone having a cylindrical after body. The nominal free-stream Reynolds number based on model diameter ranged from 0.25 x l0(exp 6) to 1 x l0(exp 6) and the angle of incidence set at 0 and +/- 20 deg. The present study was designed to complement previously reported Mach 6 perfect air tests as well as results obtained in several hypervelocity facilities capable of producing real gas effects. Surface heating rates were inferred from temperature time histories from coaxial surface thermocouples on the model forebody and thin film resistance gages along the model base and cylindrical after body. Limited forebody, base, and support sting surface pressures were obtained with piezoresistive Experimental results are compared to laminar perfect gas predictions provided by a 3-0 Navier Stokes code (NSHYP). Shear layer impingement on the instrumented cylindrical after body resulted in a localized heating maximum that was 16 to 18percent of the forebody stagnation point and a factor of 2 higher than laminar predictions, suggesting a transitional or turbulent shear layer. transducers.

Horvath, Thomas; Hannemann, Klaus

1997-01-01

40

Application of the wide-field shadowgraph technique to rotor wake visualization  

NASA Technical Reports Server (NTRS)

The wide field shadowgraph technique is reviewed along with its application to the visualization of rotor wakes. In particular, current experimental methods and data reduction requirements are discussed. Sample shadowgraphs are presented. These include shadowgraphs of model-scale helicopter main rotors and tilt rotors, and full scale tail rotors, both in hover and in forward flight.

Norman, Thomas R.; Light, Jeffrey S.

1989-01-01

41

Modelling of the transverse mode suppressor for dielectric wake-field accelerator  

Microsoft Academic Search

The wake fields in the dielectric waveguide with deflection-mode damping are calculated. In addition, numerical results are presented and compared with experiment. An interesting property of the structure is that the axisymmetric mode is unaffected by the lined wire boundary and all higher order modes can be damped very quickly (in a few cycles). This has important implications for the

W. Gai; C.-H. Ho

1991-01-01

42

Amplitudes and Spectra of Wake Fields in a Planar Dielectric Resonator with Finite Q-Factor  

NASA Astrophysics Data System (ADS)

The influence of dielectric losses upon the amplitude and spectrum of the wake fields excited by a train of electron bunches in a planar dielectric resonator is investigated. Analytical expressions for the wakefields in a planar dielectric resonator with finite value of Q-factor are obtained. Numerical calculations using various values of the dielectric resonator Q-factor are carried out.

Onishchenko, Nikolay I.; Sotnikov, Gennadij V.; Marshall, Thomas C.

2006-11-01

43

Amplitudes and Spectra of Wake Fields in a Planar Dielectric Resonator with Finite Q-Factor  

SciTech Connect

The influence of dielectric losses upon the amplitude and spectrum of the wake fields excited by a train of electron bunches in a planar dielectric resonator is investigated. Analytical expressions for the wakefields in a planar dielectric resonator with finite value of Q-factor are obtained. Numerical calculations using various values of the dielectric resonator Q-factor are carried out.

Onishchenko, Nikolay I.; Sotnikov, Gennadij V. [NSC 'Kharkov Institute of Physics and Technology', Academic Str. 1, Kharkov 61108 (Ukraine); Marshall, Thomas C. [Columbia University, New York City 10027 (United States)

2006-11-27

44

Exposure to pulsed high-frequency electromagnetic field during waking affects human sleep EEG  

Microsoft Academic Search

The aim of the study was to investigate whether the electro- magnetic field (EMF) emitted by digital radiotelephone handsets affects brain physiology. Healthy, young male subjects were exposed for 30 min to EMF (900 MHz; spatial peak specific absorption rate 1 W\\/kg) during the waking period preceding sleep. Compared with the control condition with sham exposure, spectral power of the

Reto Huber; Thomas Graf; Kimberly A. Cote; Lutz Wittmann; Eva Gallmann; Daniel Matter; Jürgen Schuderer; Niels Kuster; Alexander A. Borbély; Peter Achermann

2000-01-01

45

Suppressive Surrounds of Receptive Fields In Monkey Frontal Eye Field  

PubMed Central

A critical step in determining how a neuron contributes to visual processing is determining its visual receptive field (RF). While recording from neurons in frontal eye field (FEF) of awake monkeys (Macaca mulatta), we probed the visual field with small spots of light and found excitatory RFs that decreased in strength from RF center to periphery. However, presenting stimuli with different diameters centered on the RF revealed suppressive surrounds that overlapped the previously determined excitatory RF, and reduced responses 84% on average. Consequently, in that overlap area, stimulation produced excitation or suppression, depending on the stimulus. Strong stimulation of the RF periphery with annular stimuli allowed us to quantify this effect. A modified Difference of Gaussians (DoG) model that independently varied center and surround activation accounted for the nonlinear activity in the overlap area. Our results suggest that: 1) the suppressive surrounds found in FEF are fundamentally the same as those in V1 except for the size and strength of excitatory and suppressive mechanisms, 2) methodically assaying suppressive surrounds in FEF is essential for correctly interpreting responses to large and/or peripheral stimuli and therefore understanding the effects of stimulus context, 3) regulating the relative strength of the surround clearly changes neuronal responses, and may therefore play a significant part in the neuronal changes resulting from visual attention and stimulus salience. PMID:22933810

Cavanaugh, James; Joiner, Wilsaan M.; Wurtz, Robert H.

2012-01-01

46

On the Production of Flat Electron Bunches for Laser Wake Field Acceleration  

SciTech Connect

We suggest a novel method for injection of electrons into the acceleration phase of particle accelerators, producing low emittance beams appropriate even for the demanding high energy Linear Collider specifications. In this paper we work out the injection into the acceleration phase of the wake field in a plasma behind a high intensity laser pulse, taking advantage of the laser polarization and focusing. With the aid of catastrophe theory we categorize the injection dynamics. The scheme uses the structurally stable regime of transverse wake wave breaking, when electron trajectory self-intersection leads to the formation of a flat electron bunch. As shown in three-dimensional particle-in-cell simulations of the interaction of a laser pulse in a line-focus with an underdense plasma, the electrons, injected via the transverse wake wave breaking and accelerated by the wake wave, perform betatron oscillations with different amplitudes and frequencies along the two transverse coordinates. The polarization and focusing geometry lead to a way to produce relativistic electron bunches with asymmetric emittance (flat beam). An approach for generating flat laser accelerated ion beams is briefly discussed.

Kando, M.; Fukuda, Y.; Kotaki, H.; Koga, J.; Bulanov, S.V.; Tajima, T.; /JAERI, Kyoto; Chao, A.; Pitthan, R.; /SLAC; Schuler, K.-P.; /DESY; Zhidkov, A.G.; /CRIEPI, Tokyo; Nemoto, K.; /CRIEPI, Tokyo

2006-06-27

47

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

48

Teaching Biology Field Courses in the Wake of Environmental Disasters.  

ERIC Educational Resources Information Center

A biology field course organized to study the effects of the June 1979 Mexican oil spill on the marine biology of the shores of south Texas and Mexico is described, demonstrating how to effectively couple a biology classroom course with a natural or human caused environmental disaster. (Author/DC)

Baca, Bart J.

1982-01-01

49

Matched Field Noise Suppression based on Matrix Filter  

Microsoft Academic Search

The tow-ship noise suppression is of the key of towed line array sonar system. Referencing to the novel concept matched field noise suppression (MFNS), the matched field noise suppression based on matrix filter, called MF-CBF in this paper, is proposed to suppress the tow-ship noise. The response of MF- CBF to tow-ship noise is set to be zero and unit

Bo Lei; Kunde Yang; Yuanliang Ma

2007-01-01

50

Theta activity in local field potential of the ventral tegmental area in sleeping and waking rats.  

PubMed

Hippocampal theta rhythm appears in two vigilance states: active waking and paradoxical sleep. The ventral tegmental area (VTA) is active in sleep and waking and is connected to the hippocampus. We assessed the relationship between local field potential (LFP) of the VTA and sleep-waking stages in freely moving rats. Electrical activity of the VTA was divided into: quiet waking (W), waking with theta (WT), slow wave sleep (SWS) and paradoxical sleep (PS), depending on the hippocampal signal and the animal's behavior. We analyzed total power in the VTA signal and we also extracted peak power (Pmax) and corresponding frequency (Fmax) in theta and delta bands from both the VTA and hippocampal recording. In the VTA the 6-9 Hz band had the highest power during PS, and the ratio of the 6-9 to 3-6 Hz power was highest during both PS and WT, which accentuated Pmax of this particular theta sub-band. During W, a very slight increase (or plateau) in signal power was seen in theta range. Pmax and Fmax of theta were higher in PS than in both WT and W, and these parameters did not differ between W and WT. During WT and PS, Fmax in the 6-9 Hz band was greatly correlated between the VTA and hippocampus signal. We also detected high cross-correlation in power spectra between the hippocampus and the VTA (for delta and theta, during WT and PS). The results suggest that the VTA may belong to the broad network involved in theta rhythm induction. PMID:24569012

Orze?-Gryglewska, Jolanta; Matulewicz, Pawe?; Jurkowlaniec, Edyta

2014-05-15

51

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

SciTech Connect

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

John Rhoads, Eric Edlund and Hantao Ji

2013-04-17

52

Wave-breaking injection of electrons to a laser wake field in plasma channels at the strong focusing regime  

SciTech Connect

Efficient and fast self-injection of plasma electrons into the wake-field acceleration phase can be procured during the transverse wake-wave breaking when the wake-wave is generated by the high-intensity laser pulse propagating in a narrow plasma channel. In the strong focusing regime, when the laser pulse power exceeds critical for the self-focusing power threshold, the injected electron bunch length becomes comparable with the plasma wavelength and the bunch has the femtosecond duration. The total charge of self-injected electrons depends strongly on the laser pulse amplitude.

Ohkubo, T.; Bulanov, S. V.; Zhidkov, A. G.; Esirkepov, T.; Koga, J.; Uesaka, M.; Tajima, T. [Nuclear Professional School, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188 (Japan); Kansai Photon Science Institute, Advanced Photon Research Centre, Japan Atomic Energy Agency, 8-1 Umemidai, Kizu, Souraku, Kyoto 619-0215 (Japan) and A. M. Prokhorov General Physics Institute of Russian Academy of Sciences, Moscow 119991 (Russian Federation); National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage, Chiba 263-8555 (Japan) and A. M. Prokhorov General Physics Institute of Russian Academy of Sciences, Moscow 119991 (Russian Federation); Kansai Photon Science Institute, Advanced Photon Research Centre, Japan Atomic Energy Agency, 8-1 Umemidai, Kizu, Souraku, Kyoto 619-0215 (Japan); Nuclear Professional School, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188 (Japan); Kansai Photon Science Institute, Advanced Photon Research Centre, Japan Atomic Energy Agency, 8-1 Umemidai, Kizu, Souraku, Kyoto 619-0215 (Japan)

2006-10-15

53

Velocity field in the wake of a hydropower farm equipped with Achard turbines  

NASA Astrophysics Data System (ADS)

The study consists of experimental and numerical investigations related to the water flow in the wake of a hydropower farm, equipped with three Achard turbines. The Achard turbine is a French concept of vertical axis cross-flow marine current turbine, with three vertical delta-blades, which operates irrespective of the water flow direction. A farm model built at 1:5 scale has been tested in a water channel. The Achard turbines run in stabilized current, so the flow can be assumed to be almost unchanged in horizontal planes along the vertical z-axis, thus allowing 2D numerical modelling, for different farm configurations: the computational domain is a cross-section of all turbines at a certain z-level. The two-dimensional numerical model of that farm has been used to depict the velocity field in the wake of the farm, with COMSOL Multiphysics and FLUENT software, to compute numerically the overall farm efficiency. The validation of the numerical models with experimental results is performed via the measurement of velocity distribution, by Acoustic Doppler Velocimetry, in the wake of the middle turbine within the farm. Three basic configurations were studied experimentally and numerically, namely: with all turbines aligned on a row across the upstream flow direction; with turbines in an isosceles triangular arrangement pointing downstream; with turbines in an isosceles triangular arrangement pointing upstream. As long as the numerical flow in the wake fits the experiments, the numerical results for the power coefficient (turbine efficiency) are trustworthy. The farm configuration with all turbines aligned on a same row leads to lower values of the experimental velocities than the numerical ones, while the farm configurations where the turbines are in isosceles triangular arrangement, pointing downstream or upstream, present a better match between numerical and experimental data.

Georgescu, A.-M.; Georgescu, S. C.; Cosoiu, C. I.; Alboiu, N.; Hamzu, Al

2010-08-01

54

Field investigation of a wake structure downwind of a VAWT in a windfarm array  

SciTech Connect

The effects of upwind turbine wakes on the performance of a FloWind 17-m VAWT were investigated through a series of field experiments conducted at the FloWind windfarm on Cameron Ridge, Tehachapi, California. The field experiment was conducted within a VAWT array consisting of more than nine VAWTs with separations 3D crosswised by 8D downwind (where D is the turbine diameter) in a staggered configuration. The array is the upwind three rows of VAWTS in a total of six rows that are on top of the Cameron Ridge plateau. The terrain features in the vicinity are reasonably regular, with an upslope of 7 deg on the average; however, several local irregularities are present. The annual hourly averaged wind speed exceeds 8 m/s at the site. The wind field and the power-outputs of nine turbines within the array were measured with wind sensors and power transducers. Nine Gill propeller and 18 Maximum cup anemometers and one direction sensor were mounted on portable and stack-up towers installed upwind and within the turbine array. From the field measurements, the velocity and power/energy deficits were derived under various turbine on/off configurations. Much information was provided to characterize the structure of VAWT wakes and to assess their effects on the performance of downwind turbines. Recommendations are made for optimizing windfarm design and operations as well as for wind energy management.

Liu, H.T.; Buck, J.W.; Germain, A.C.; Hinchee, M.E.; Solt, T.S.; LeRoy, G.M.; Srnsky, R.A.

1987-10-01

55

Positron acceleration by plasma wake fields driven by a hollow electron beam  

E-print Network

A scheme of wake field generation for positron acceleration using hollow or donut shaped electron driver beams is studied. An annular shaped, electron free region forms around a hollow driver beam creating a favorable region (longitudinal field is accelerating and transverse field is focusing and radially linear) for positron acceleration. Accelerating gradients of the order of 10 GV/m are produced by a hollow electron beam driver with FACET like parameters. The peak accelerating field increases linearly with the total charge in the beam driver while the axial size of the favorable region ($\\sim$ one plasma wavelength) remains approximately fixed. The radial size drops with the total charge but remains large enough for the placement of a witness positron beam. We simulate an efficient acceleration of a 23 GeV positron beam to 35.4 GeV with a maximum energy spread of 0.4\\% and very small emittance over a plasma length of 140 cm.

Jain, Neeraj; Palastro, J P

2014-01-01

56

Structure of the Velocity and Vorticity Field in a Confined Forced Wake  

NASA Astrophysics Data System (ADS)

Previous results have shown that forcing a low Reynolds number 2D wake inside of a confining channel can lead to large increases in the amount of molecular mixing. The current work is part of an investigation to understand the behavior of this flow based on the structure of its vorticity field. Molecular Tagging Velocimetry (MTV) is used to map the u and v components of the velocity field at several streamwise and spanwise locations within the test section. Approximately 500 measurements are made in each region, which extends about two forcing wavelengths. These measurements allow the examination of the structure of the spanwise vorticity field, which is computed from the velocity data by a 2nd order finite difference scheme. Results show a spatial periodicity in the velocity rms, which can be explained by the existence of a phase difference between the shed spanwise vorticity and the free-stream perturbation. This is different from that usually observed in forced wakes with a steady free-stream. Data also show that the lateral spacing between vortices is highly dependent upon the forcing amplitude.

Cohn, R. K.; Koochesfahani, M. M.

1997-11-01

57

GV/m Wake Fields Generated by a Train of pC Femtosecond Bunches in a Planar Dielectric Microstructure  

SciTech Connect

A tall, dielectric-lined rectangular wake field microstructure is analyzed as a possible element of an advanced linear wake field accelerator. This accelerator would be driven by a train of fs 500-MeV electron microbunches that would be chopped out of a longer bunch using e.g. a powerful CO2 laser, and then formed into a train of rectangular-profile bunches using a quadrupole. The bunches set up a periodic wake field in the microstructure, using a train of bunches spaced by the period of the wake fields in the structure. Stability is examined for drive and accelerated bunches using computations of test particle orbits in the longitudinal and transverse wake fields excited by the drive bunches. It is found that nearly all test electrons in the drive bunches are confined within the structure for a travel distance of 15 cm or more, while test electrons located in an accelerated bunch can have stable motion over the same distance in a gradient of 0.4 GV/m without passing through the structure walls. A single 25-pC bunch will set up gradients of 1.16 GV/m and will permit stable acceleration over a 10-cm distance with very low energy spread in the same structure.

Marshall, T.C. [Applied Physics Department, Columbia University, New York, NY 10027 (United States); Wang Changbiao [Omega-P, Inc., 199 Whitney Ave., New Haven, CT 06511 (United States); Hirshfield, J.L. [Omega-P, Inc., 199 Whitney Ave., New Haven, CT 06511 (United States); Beam Physics Laboratory, Yale University, 272 Whitney Ave., New Haven, CT 06511 (United States)

2004-12-07

58

Modal analysis of wake fields and its application to elliptical pill-box cavity with finite aperture  

SciTech Connect

The potential of the wake-field produced by a bunch of relativistic charged particles passing through a pill-box cavity is expressed by using Floquet's theorem, and an obvious requirement that the energy gain over all acceleration cavity of many pill boxes must be proportional to the number of pill boxes, based on the previous modal approach (BWW theory). It is found that the wake-field is consisted of two classes of modes: the longitudinal modes which are independent of the aperture and the pill-box gap, the hybrid (pill-box) modes which are dependent of the pill-box gap. The wake field is predominated by the fundamental longitudinal mode whose wavelength is on the order of the effective diameter of the cavity, and its magnitude is inversely proportional to the cross sectional area of the cavity for practical cavities with small apertures. Both longitudinal and transverse wake fields due to the longitudinal modes in an elliptical pill box cavity are expressed analytically in a closed series form by solving exactly the longitudinal eigenmode equation in the elliptical cylindrical coordinates in terms of Mathieu functions. It is found that both longitudinal and transverse wake fields whose amplitudes per driving charge are greater than 100 MV/m/{mu}C can be generated in an elliptical cavity.

Kim, S.H. (Department of Physics, University of Texas at Arlington, Arlington, TX (USA)); Chen, K.W.; Yang, J.S. (Center for Accelerator Science and Technology, University of Texas at Arlington, Arlington, TX (USA))

1990-11-15

59

Wake Fields Excited in a Semi-Infinite Rectangular Dielectric Waveguide by a Train of Electron Bunches  

SciTech Connect

Wake fields set up in a dielectric having planar rectangular geometry by the passage of relativistic electron bunches (Vavilov-Cherenkov radiation) can be described by a summation of a 2D set of equally-spaced radial harmonics. This differs from the case of cylindrical geometry where the harmonics are not equally-spaced. We study peculiarities of excitation at the entrance boundary, namely the appearance of a 'quenching wave' and transition radiation. The former cancels Vavilov-Cherenkov radiation in the region between the entrance and a front moving with the group velocity, while the latter distorts the wake field. Exact expressions for transition and Vavilov-Cherenkov radiation are obtained, and the spatial profile of the field excited by an electron bunch of finite size is computed numerically. Wake field excitation by a train of equally-spaced short bunch is investigated.

Marshall, Thomas C. [Columbia University, New York City 1002 (United States); Onishchenko, Nikolay I.; Sotnikov, Gennadij V. [NSC 'Kharkov Institute of Physics and Technology', Academic Str. 1, Kharkov 61108 (Ukraine)

2004-12-07

60

The velocity and vorticity fields of the turbulent near wake of a circular cylinder  

NASA Technical Reports Server (NTRS)

The purpose of this research is to provide a detailed experimental database of velocity and vorticity statistics in the very near wake (x/d less than 10) of a circular cylinder at Reynolds number of 3900. This study has determined that estimations of the streamwise velocity component in flow fields with large nonzero cross-stream components are not accurate. Similarly, X-wire measurements of the u and v velocity components in flows containing large w are also subject to the errors due to binormal cooling. Using the look-up table (LUT) technique, and by calibrating the X-wire probe used here to include the range of expected angles of attack (+/- 40 deg), accurate X-wire measurements of instantaneous u and v velocity components in the very near wake region of a circular cylinder has been accomplished. The approximate two-dimensionality of the present flow field was verified with four-wire probe measurements, and to some extent the spanwise correlation measurements with the multisensor rake. Hence, binormal cooling errors in the present X-wire measurements are small.

Wallace, James; Ong, Lawrence; Moin, Parviz

1995-01-01

61

A Concept of Plasma Wake Field Acceleration Linear Collider (PWFA-LC)  

SciTech Connect

Plasma Wake-Field Acceleration (PWFA) has demonstrated acceleration gradients above 50 GeV/m. Simulations have shown drive/witness bunch configurations that yield small energy spreads in the accelerated witness bunch and high energy transfer efficiency from the drive bunch to the witness bunch, ranging from 30% for a Gaussian drive bunch to 95% for a shaped longitudinal profile. These results open the opportunity for a linear collider that could be compact, efficient and more cost effective that the present microwave technologies. A concept of a PWFA-based Linear Collider (PWFA-LC) has been developed and is described in this paper. The drive beam generation and distribution, requirements on the plasma cells, and optimization of the interaction region parameters are described in detail. The R&D steps needed for further development of the concept are also outlined.

Seryi, Andrei; Hogan, Mark; Pei, Shilun; Raubenheimer, Tor; Tenenbaum, Peter; /SLAC; Katsouleas, Tom; /Duke U.; Huang, Chengkun; Joshi, Chan; Mori, Warren; /UCLA; Muggli, Patric; /Southern California U.

2009-10-30

62

Comparison of observed plasma and magnetic field structures in the wakes of Mars and Venus  

SciTech Connect

Plasma and magnetic field observations from the Phobos 2 spacecraft at Mars and the Pioneer Venus orbiter (PVO) at Venus show that there are some notable similarities in the structure of the low-altitude magnetotails at both of these weakly magnetized planets. In particular, it is found that when conditions in the interplanetary medium are steady and the orbit sampling geometry is appropriate, two magnetic tail lobes, with an intervening plasma sheet or central tail ray in the approximate location of the dividing current sheet, are present. This behavior is seen in both the Phobos 2 ASPERA plasma analyzer data and in the PVO Langmuir probe data. In the Phobos 2 data, the tail ray is found to be composed primarily of antisunward streaming oxygen (O{sup +}) plasma which has a bulk velocity consistent with an energy close to that of the upstream solar wind plasma. The PVO Langmuir probe experiment also detected two (or more) additional cold plasma structures flanking the central figure; Phobos 2 data, on the other hand, show a proton plasma boundary layer flanking the central (mostly O{sup +}) tail ray or plasma sheet, with sporadic fluxes or rays of O{sup +} ions. If the latter considered is to be the magnetosheath (solar wind plasma) at the tail boundary, it is mainly the common central tail O{sup +} features that suggest that there are common planetary ion acceleration and magnetotail formation processes at work in the low-altitude wakes of Mars and Venus. On the other hand, an important contribution from picked-up exospheric hydrogen in the wake at Mars cannot be ruled out.

Dubinin, E. (Space Research Inst., Moscow (USSR)); Lundin, R. (Swedish Inst. of Space Physics, Kiruna (Sweden)); Riedler, W.; Schwingenschuh, K. (Space Research Inst., Graz (Austria)); Luhmann, J.G.; Russell, C.T. (Univ. of California, Los Angeles (USA)); Brace, L.H. (Univ. of Michigan, Ann Arbor (USA))

1991-07-01

63

Generation of parallel electric fields in the Jupiter-Io torus wake region  

NASA Astrophysics Data System (ADS)

Infrared and ultraviolet images have established that auroral emissions at Jupiter caused by the electromagnetic interaction with Io not only produce a bright spot, but an emission trail that extends in longitude from Io's magnetic footprint. Electron acceleration that produces the bright spot is believed to be dominated by Alfvén waves whereas we argue that the trail or wake aurora results from quasi-static parallel electric fields associated with large-scale, field-aligned currents between the Io torus and Jupiter's ionosphere. These currents ultimately transfer angular momentum from Jupiter to the Io torus. We examine the generation and the impact of the quasi-static parallel electric fields in the Io trail aurora. A critical component to our analysis is a current-voltage relation that accounts for the low-density plasma along the magnetic flux tubes that connect the Io torus and Jupiter. This low-density region, ˜ 2 R J from Jupiter's center, can significantly limit the field-aligned current, essentially acting as a “high-latitude current choke.” Once parallel electric fields are introduced, the governing equations that couple Jupiter's ionosphere to the Io torus become nonlinear and, while the large-scale behavior is similar to that expected with no parallel electric field, there are substantial deviations on smaller scales. The solutions, bound by properties of the Io torus and Jupiter's ionosphere, indicate that the parallel potentials are on the order of 1 kV when constrained by peak energy fluxes of a few milliwatts per square meter. The parallel potentials that we predict are significantly lower than earlier reports.

Ergun, R. E.; Ray, L.; Delamere, P. A.; Bagenal, F.; Dols, V.; Su, Y.-J.

2009-05-01

64

Generation of Parallel Electric Fields in the Jupiter-Io Torus Wake Region  

NASA Astrophysics Data System (ADS)

Infrared and ultraviolet images have established that auroral emissions at Jupiter caused by the electromagnetic interaction with Io not only produce a bright spot, but an emission trail that extends in longitude from Io's magnetic footprint. Electron acceleration that produces the bright spot is believed to be dominated by Alfvén waves whereas we argue that the trail or wake aurora results from quasi-static parallel electric fields associated with large-scale, field-aligned currents between the Io torus and Jupiter's ionosphere. These currents ultimately transfer angular momentum from Jupiter to the Io torus. We examine the generation and the impact of the quasi-static parallel electric fields in the Io trail aurora. A critical component to our analysis is a current-voltage relation that accounts for the low-density plasma along the magnetic flux tubes that connect the Io torus and Jupiter. This low density region, ~ 2 RJ from Jupiter's center, can significantly limit the field-aligned current, essentially acting as a "high-latitude current choke". Once parallel electric fields are introduced, the governing equations that couple Jupiter's ionosphere to the Io torus become nonlinear and, while the large-scale behavior is similar to that expected with no parallel electric field, there are substantial deviations on smaller scales. The solutions, bound by properties of the Io torus and Jupiter's ionosphere, indicate that the parallel potentials are on the order of 1 kV when constrained by peak energy fluxes of ~1 miliWatt per meter squared. The parallel potentials that we predict are significantly lower than earlier reports.

Ergun, R. E.; Ray, L.; Delamere, P. A.; Bagenal, F.; Dols, V.; Su, Y.

2008-12-01

65

Simulations of field-aligned currents: Application of theory of thin filament motion to Io's plasma wake  

NASA Astrophysics Data System (ADS)

Io's plasma wake was treated as a tail of magnetic flux tubes perturbed by Io successively. The evolution of an Io-perturbed flux tube was studied numerically via magnetohydrodynamics (MHD) approach of a thin filament. Our simulations suggest that the mechanism for producing wake aurora could not be explained by either Alfvén wave or electric circuit alone, rather, the underlying physics possesses the characteristics typical for both Alfvén wave and corotational lag models. An upstream-coming flux tube must be in contact with Io for approximately 500 s, until a tilt angle of about 4° has been developed, before it is released downstream. A magnetic field depression forms downstream as a result of the continual departure of the flux tubes from Io, which in turn has significant influence on the motion of a flux tube. A perturbed flux tube would undergo a subcorotational motion in Io's plasma wake. This motion is inevitably modulated by Alfvén wave bouncing back and forth between the equatorial plane and the boundary of Io plasma torus. The scale of the subcorotation region is in the order of 1 Jovian radius (RJ). The distribution of the simulated field-aligned currents downstream is consistent with the observed wake aurora brightness profile; in particular, the periodic structure in the current distribution is in agreement with recent infrared and FUV observations showing the presence of secondary spots in the auroral emissions.

Chen, C. X.

2007-03-01

66

Analysis of the wake field effects in the PEP-II storage rings with extremely high currents  

NASA Astrophysics Data System (ADS)

We present the history and analysis of different wake field effects throughout the operational life of the PEP-II SLAC B-factory. Although the impedance of the high and low energy rings is small, the intense high-current beams generated a lot of power. The effects from these wake fields are: heating and damage of vacuum beam chamber elements like RF seals, vacuum valves, shielded bellows, BPM buttons and ceramic tiles; vacuum spikes, vacuum instabilities and high detector background; and beam longitudinal and transverse instabilities. We also discuss the methods used to eliminate these effects. Results of this analysis and the PEP-II experience may be very useful in the design of new storage rings and light sources.

Novokhatski, A.; Seeman, J.; Sullivan, M.

2014-01-01

67

Aircraft wake vortices: a comparison of wind-tunnel data with field trial measurements by laser radar  

Microsoft Academic Search

A method is presented for comparison of measurements on aircraft wake vortices obtained using two very different techniques: 1) five-hole probe measurements on a 1\\/13.6 (7.35%) scale half-model of an Airbus A321 were made in a wind tunnel, and 2) coherent laser radar (lidar) measurements were made in full-scale field trials at Toulouse Blagnac Airport. The lidar measurements provide line-of-sight

Michael Harris; J. Michael Vaughan; Klaus Huenecke; Caren Huenecke

2000-01-01

68

Speckle Suppression with the Project 1640 Integral Field Spectrograph  

E-print Network

Project 1640 is a high-contrast imaging instrument recently commissioned at Palomar observatory. A combination of a coronagraph with an integral field spectrograph (IFS), Project 1640 is designed to detect and characterize extrasolar planets, brown dwarfs, and circumstellar material orbiting nearby stars. In this paper, we present our data processing techniques for improving upon instrument raw sensitivity via the removal of quasi-static speckles. Our approach utilizes the chromatic image diversity provided by the IFS in combination with the locally-optimized combination of images (LOCI) algorithm to suppress the intensity of residual contaminating light in close angular proximity to target stars. We describe the Project 1640 speckle suppression pipeline (PSSP) and demonstrate the ability to detect companions with brightness comparable to and below that of initial speckle intensities using on-sky commissioning data. Our preliminary results indicate that suppression factors of at least one order of magnitude a...

Crepp, Justin R; Brenner, Douglas; Oppenheimer, Ben R; Zimmerman, Neil; Hinkley, Sasha; Parry, Ian; King, David; Vasisht, Gautam; Beichman, Charles; Hillenbrand, Lynne; Dekany, Richard; Shao, Mike; Burruss, Rick; Roberts, Lewis C; Bouchez, Antonin; Roberts, Jenny; Soummer, Remi

2010-01-01

69

Suppression and control of leakage field in electromagnetic helical microwiggler  

NASA Astrophysics Data System (ADS)

An electromagnetic helical microwiggler with three poles in one period installing permanent magnets to suppress the leakage field has been designed and manufactured. The gap field of the wiggler is adjustable by controlling the position of an iron screw set in each retainer fixing the permanent magnet blocks. In a test wiggler with a period of 7.9 mm and magnetic pole gap length of 4.6 mm, the field has been measured 2.3 kG which was more than two times larger than that of the wiggler without permanent magnets.

Ohigashi, Nobuhisa; Ishida, Takayuki; Tsunawaki, Yoshiaki; Imasaki, Kazuo; Fujita, Masayuki; Asakawa, Makoto; Kuruma, Shin-ichiro; Yamanaka, Chiyoe; Nakai, Sadao; Mima, Kunioki

1996-02-01

70

A closed-form solution of wake-fields in an elliptical pill-box by using an elliptical coordinate system  

SciTech Connect

It was known from a complete model analysis that the wake potential in the pill-box cavity is predominantly determined by a few longitudinal modes counting from the fundamental longitudinal mode. An approach to find the longitudinal modes of an elliptical cavity is developed by means of the coordinate transformation method. It is found that the field configuration and eigenfrequencies of the elliptical cavity can be expressed in a closed form in terms of Mathieu functions. Inserting the closed form solution of modes into the previous analytical formula for the wake field, the wake field is expressed too in a closed form solution, which is convenient for numerical calculation. Thus, a numerical method to calculate expediently the wake field is developed, and a model calculation is presented. {copyright} 1989 American Institute of Physics

Yang, J.S.; Chen, K.W. (Center for Accelerator Science and Technology, The University of Texas at Arlington, Arlington, TX 76019 (US))

1989-10-15

71

The scaling of far-field wake angle of non-axisymmetric pressure disturbance  

E-print Network

It has been recently emphasized that the angle of maximum wave amplitude $\\alpha$ in the wake of a disturbance of finite size can be significantly narrower than the maximum value $\\alpha_K = \\sin^{-1}(1/3) \\simeq 19.47^\\mathrm{o}$ predicted by the classical analysis of Kelvin. For axisymmetric disturbance, simple argument based on the Cauchy-Poisson initial-value problem suggests that the wake angle decreases following a Mach-like law at large velocity, $\\alpha \\simeq Fr_L^{-1}$, where $Fr_L=U/\\sqrt{gL}$ is the Froude number based on the disturbance velocity $U$, its size $L$, and gravity $g$. In this paper we extend this analysis to the case of non-axisymmetric disturbances, relevant to real ships. We find that, for intermediate Froude numbers, the wake angle follows an intermediate scaling law $\\alpha \\simeq Fr_L^{-2}$, in agreement with the recent prediction of Noblesse \\textit{et al.} [``Why can ship wakes appear narrower than Kelvin's angle?'' to appear in Eur. J. Mech. B/Fluids (2014)]. We show that bey...

Moisy, Frederic

2014-01-01

72

Characterization of electron self-injection in laser wake field acceleration due to the parametric resonance  

SciTech Connect

The wave-breaking processes originating from a parametric resonance in the wake of a laser pulse in the absence of pulse overfocusing are thoroughly analyzed via multidimensional particle-in-cell simulations. The processes play a key role in the electron self-injection in the laser-driven acceleration of high energy, monoenergetic electrons in plasma channels. The resonance character of the charge loading in the first, second, and third injections is shown; its effect on the electron acceleration is demonstrated.

Zhidkov, A. [Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196 (Japan); Japan Science and Technology Agency (JST), CREST, 2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan); Koga, J. [APRC, Japan Atomic Energy Agency, 8-1-7 Umemidai, Kizugawa-shi, Souraku, Kyoto 619-0215 (Japan); Hosokai, T.; Kodama, R. [PPC, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan); Japan Science and Technology Agency (JST), CREST, 2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan); Fujii, T.; Oishi, Y.; Nemoto, K. [Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196 (Japan)

2010-08-15

73

Speckle Suppression with the Project 1640 Integral Field Spectrograph  

NASA Astrophysics Data System (ADS)

Project 1640 is a high-contrast imaging instrument recently commissioned at the Palomar observatory. A combination of a coronagraph with an integral-field spectrograph (IFS), Project 1640 is designed to detect and characterize extrasolar planets, brown dwarfs, and circumstellar material orbiting nearby stars. In this paper, we present our data processing techniques for improving upon instrument raw sensitivity via the removal of quasi-static speckles. Our approach utilizes the chromatic image diversity provided by the IFS in combination with the locally optimized combination of images algorithm to suppress the intensity of residual contaminating light in close angular proximity to target stars. We describe the Project 1640 speckle suppression pipeline and demonstrate its ability to detect companions with brightness comparable to and below that of initial speckle intensities using on-sky commissioning data. Our preliminary results indicate that suppression factors of at least one order of magnitude are consistently possible, reaching 5? contrast levels of 2.1 × 10-5 at 1'' in the H band in 20 minutes of on-source integration time when non-common-path errors are reasonably well calibrated. These results suggest that near-infrared contrast levels of order ?10-7 at subarcsecond separations will soon be possible for Project 1640 and similarly designed instruments that receive a diffraction-limited beam corrected by adaptive optics systems employing deformable mirrors with high actuator density.

Crepp, Justin R.; Pueyo, Laurent; Brenner, Douglas; Oppenheimer, Ben R.; Zimmerman, Neil; Hinkley, Sasha; Parry, Ian; King, David; Vasisht, Gautam; Beichman, Charles; Hillenbrand, Lynne; Dekany, Richard; Shao, Mike; Burruss, Rick; Roberts, Lewis C.; Bouchez, Antonin; Roberts, Jenny; Soummer, Rémi

2011-03-01

74

HIGH-GRADIENT, HIGH-TRANSFORMER-RATIO, DIELECTRIC WAKE FIELD ACCELERATOR  

SciTech Connect

The Phase I work reported here responds to DoE'ss stated need "...to develop improved accelerator designs that can provide very high gradient (>200 MV/m for electrons...) acceleration of intense bunches of particles." Omega-P'�s approach to this goal is through use of a ramped train of annular electron bunches to drive a coaxial dielectric wakefield accelerator (CDWA) structure. This approach is a direct extension of the CDWA concept from acceleration in wake fields caused by a single drive bunch, to the more efficient acceleration that we predict can be realized from a tailored (or ramped) train of several drive bunches. This is possible because of a much higher transformer ratio for the latter. The CDWA structure itself has a number of unique features, including: a high accelerating gradient G, potentially with G > 1 GeV/m; continuous energy coupling from drive to test bunches without transfer structures; inherent transverse focusing forces for particles in the accelerated bunch; highly stable motion of high charge annular drive bunches; acceptable alignment tolerances for a multi-section system. What is new in the present approach is that the coaxial dielectric structure is now to be energized by-not one-�but by a short train of ramped annular-shaped drive bunches moving in the outer coaxial channel of the structure. We have shown that this allows acceleration of an electron bunch traveling along the axis in the inner channel with a markedly higher transformer ratio T than for a single drive bunch. As described in this report, the structure will be a GHz-scale prototype with cm-scale transverse dimensions that is expected to confirm principles that can be applied to the design of a future THz-scale high gradient (> 500 MV/m) accelerator with mm-scale transverse dimensions. We show here a new means to significantly increase the transformer ratio T of the device, and thereby to significantly improve its suitability as a flexible and effective component in a future high energy, high gradient accelerator facility. We predict that the T of a high gradient CDWA can be increased by a substantial factor; this enhancement is dramatically greater than what has been demonstrated heretofore. This large enhancement in T that we predict arises from using a train of three or four drive bunches in which the spacing of the bunches and their respective charges are selected according to a simple principle that requires each bunch lose energy to the wakefields at the same rate, so as not to sacrifice drive beam efficiency�¢����as would be the case if one bunch exhausted its available energy while others had not. It is anticipated that results from the study proposed here can have a direct impact on design of the dielectric accelerator in a TeV-scale collider concept, and in the accelerator for an x-ray FEL.

Jay L. Hirshfield

2012-04-12

75

Compression and Suppression of Shifting Receptive Field Activity in Frontal Eye Field Neurons  

PubMed Central

Before each saccade, neurons in frontal eye field anticipate the impending eye movement by showing sensitivity to stimuli appearing where the neuron's receptive field will be at the end of the saccade, referred to as the future field (FF) of the neuron. We explored the time course of this anticipatory activity in monkeys by briefly flashing stimuli in the FF at different times before saccades. Different neurons showed substantial variation in FF time course, but two salient observations emerged. First, when we compared the time span of stimulus probes before the saccade to the time span of FF activity, we found a striking temporal compression of FF activity, similar to compression seen for perisaccadic stimuli in human psychophysics. Second, neurons with distinct FF activity also showed suppression at the time of the saccade. The increase in FF activity and the decrease with suppression were temporally independent, making the patterns of activity difficult to separate. We resolved this by constructing a simple model with values for the start, peak, and duration of FF activity and suppression for each neuron. The model revealed the different time courses of FF sensitivity and suppression, suggesting that information about the impending saccade triggering suppression reaches the frontal eye field through a different pathway, or a different mechanism, than that triggering FF activity. Recognition of the variations in the time course of anticipatory FF activity provides critical information on its function and its relation to human visual perception at the time of the saccade. PMID:24227735

Cavanaugh, James; Wurtz, Robert H.

2013-01-01

76

Suppression of probe background signals via B1 field inhomogeneity  

SciTech Connect

A new approach combining a long pulse with the DEPTH sequence (Cory and Ritchey, Journal of Magnetic Resonance, 1988) greatly improves the efficiency for suppressing probe background signals arising from spinning modules. By applying a long initial excitation pulse in the DEPTH sequence, instead of a {pi}/2 pulse, the inhomogeneous B{sub 1} fields outside the coil can dephase the background coherence in the nutation frame. The initial long pulse and the following two consecutive EXORCYCLE {pi} pulses function complementarily and prove most effective in removing background signals from both strong and weak B{sub 1} fields. Experimentally, the length of the long pulse can be optimized around odd multiples of the {pi}/2 pulse, depending on the individual probe design, to preserve signals inside the coil while minimizing those from probe hardware. This method extends the applicability of the DEPTH sequence to probes with small differences in B{sub 1} field strength between the inside and outside of the coil, and can readily combine with well-developed double resonance experiments for quantitative measurement. In general, spin systems with weak internal interactions are required to attain efficient and uniform excitation for powder samples, and the principles to determine the applicability are discussed qualitatively in terms of the relative strength of spin interactions, r.f. power and spinning rate.

Feng, Jian; Reimer, Jeffrey

2011-01-27

77

Flow field behind a fixed bluff body in a vertical pipe simulating a wake of a Taylor bubble  

NASA Astrophysics Data System (ADS)

The hydrodynamic structure of the wake of an axisymmetric bluff body that simulates the shape of a moving elongated (Taylor) bubble in a vertical pipe was studied using Particle Image Velocimetry in laminar and turbulent background flows. The distribution of the mean axial and radial velocity components in the wake, as well as the spatial variation of the normal and shear stresses are presented and compared with the corresponding quantities in the wake of the gas bubble. The accumulated results enable estimates of the spatial variation of turbulent energy production term in the undeveloped separated flow in the wake of the bluff body.

Babin, V.; Barnea, D.; Shemer, L.

2013-10-01

78

Analysis of unstable vortical structure in a propeller wake affected by a simulated hull wake  

Microsoft Academic Search

A two-frame PIV (particle image velocimetry) technique was used to investigate the flow characteristics of a complicated propeller\\u000a wake influenced by a hull wake. As the propeller is significantly affected by the hull wake of a marine vessel, measurements\\u000a of the propeller wake under the hull wake are certainly needed for more reliable validation of numerical predictions. Velocity\\u000a field measurements

Bu-Geun Paik; Kyung-Youl Kim; Jung-Yeop Lee; Sang-Joon Lee

2010-01-01

79

Effects of plasma density on relativistic self-injection for electron laser wake-field acceleration  

SciTech Connect

Density effects on the dynamics of a cavity produced in the wake of an ultraintense (a{sub 0}=eE/mc{omega}>>1) and short ({omega}{sub pl}{tau}/{pi}<1) laser pulse and on the duration of accelerated electrons are studied via two-dimensional particle-in-cell simulation. Formation of a nonbreaking cavity is a crucial part of relativistic self-injection of plasma electrons from the front of a laser pulse and their further acceleration leading to a beam-quality femtosecond bunch. This self-injection appears in a uniform plasma when the group velocity of the pulse becomes smaller than the maximal electron velocity accelerated in the ponderomotive bias, {phi}=mc{sup 2}a{sub 0}{sup 2}/2. However with increasing density, this mechanism starts to contend with relativistic wave breaking. Though additional injection due to the relativistic wave breaking increases the total charge of energetic electrons, the duration of the bunch increases to the picosecond range and its energy distribution becomes a Maxwellian.

Zhidkov, A. [Department of Accelerator Physics and Engineering, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage, Chiba 263-8555 (Japan); General Physics Institute, Russian Academy of Sciences, Vavilov 38, Moscow (Russian Federation); Koga, J. [Advanced Photon Research Center, Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizu, Souraku, Kyoto 619-0215 (Japan); Hosokai, T.; Uesaka, M. [Nuclear Engineering Research Laboratory, University of Tokyo, 22-2 Shirane-shirakata, Tokai, Naka, Ibaraki 319-1188 (Japan); Kinoshita, K. [Department of Accelerator Physics and Engineering, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage, Chiba 263-8555 (Japan)

2004-12-01

80

E-157: A 1.4-m-long plasma wake field acceleration experiment using a 30 GeV electron beam from the Stanford Linear Accelerator  

E-print Network

E-157: A 1.4-m-long plasma wake field acceleration experiment using a 30 GeV electron beam from the Stanford Linear Accelerator Center Linac* M. J. Hogan, R. Assmann,a) F.-J. Decker, R. Iverson, P. Raimondi, S. Rokni, R. H. Siemann, D. Walz, and D. Whittum Stanford Linear Accelerator Center, Stanford

81

L'OASIS Ti:sapphire laser facility: a multi-beam, multi-terawatt system for wake-field acceleration studies  

Microsoft Academic Search

A highly automated and remote controlled Ti:sapphire CPA laser system provides synchronized beams of 2 times 1.0-TW, 12-TW, and 100-TW peak-power, in a unique, radiation shielded facility specially designed to study laser wake-field acceleration in plasmas

Wim P. Leemans; Cameron G. R. Geddes; Jeroen van Tilborg; Michael Dickinson; Scott DiMaggio; Don Syversrud; Joe Wallig; Nathan Ybarrolaza; C. Toth

2004-01-01

82

Measurements of fish's wake by PIV  

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

83

Simulations of field-aligned currents: Application of theory of thin filament motion to Io's plasma wake  

Microsoft Academic Search

Io's plasma wake was treated as a tail of magnetic flux tubes perturbed by Io successively. The evolution of an Io-perturbed flux tube was studied numerically via magnetohydrodynamics (MHD) approach of a thin filament. Our simulations suggest that the mechanism for producing wake aurora could not be explained by either Alfvén wave or electric circuit alone, rather, the underlying physics

C. X. Chen

2007-01-01

84

Influence of Plasma Wake Around Satellite Body on Characteristics of Electric Field Sensor  

Microsoft Academic Search

Knowledge of the characteristics of wire antennas in magnetized plasma used as sensors for electric field observations by scientific satellites in geospace is necessary to determine the absolute intensity and the phase of the electric field wave because the observation data about electric field are available as voltage signal. Two important characteristics are the effective length and the antenna impedance.

R. Higashi; T. Imachi; S. Yagitani

2007-01-01

85

A multi-beam, multi-terawatt Ti:sapphire laser system for laser wake-field acceleration studies  

SciTech Connect

The Lasers, Optical Accelerator Systems Integrated Studies (L'OASIS) Lab of LBNL operates a highly automated and remotely controlled Ti:sapphire chirped pulse amplification (CPA) laser system that provides synchronized beams of 2x1.0 TW, 12 TW, and 100 TW peak-power, in a unique, radiation shielded facility. The system has been specially designed for studying high field laser-plasma interactions and particularly aimed for the investigations of laser wake-field particle acceleration. It generates and recombines multiple beams having different pulse durations, wavelengths, and pulse energies for various stages of plasma preparation, excitation, and diagnostics. The amplifier system is characterized and continuously monitored via local area network (LAN) from a radiation shielded control room by an array of diagnostics, including beam profile monitoring cameras, remote controlled alignment options, self-correcting beam-pointing stabilization loops, pulse measurement tools, such as single-shot autocorrelator for pulse duration and third-order correlator for contrast measurements, FROG for pulse shape studies.

Toth, Cs.; Geddes, C.G.R.; Tilborg, J. van; Leemans, W.P. [L'OASIS Group, Accelerator and Fusion Research Division, Lawrence Berkeley National Laboratory, BLDG 71R0259, 1 Cyclotron Rd., Berkeley, CA 94720 (United States)

2004-12-07

86

Electron Acceleration by a Wake Field Forced by an Intense Ultrashort Laser Pulse  

Microsoft Academic Search

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these

V. Malka; S. Fritzler; E. Lefebvre; M.-M. Aleonard; F. Burgy; J.-P. Chambaret; J.-F. Chemin; K. Krushelnick; G. Malka; S. P. D. Mangles; Z. Najmudin; M. Pittman; J.-P. Rousseau; J.-N. Scheurer; B. Walton; A. E. Dangor

2002-01-01

87

Overview of helicopter wake and airloads technology  

NASA Technical Reports Server (NTRS)

An overview of helicopter aerodynamics technology is presented with emphasis on rotor wake and airloads methodology developed at the United Technologies Research Center (UTRC). The evolution over the past twenty years of various levels of computerized wake geometry models at UTRC, such as undistorted wake, prescribed empirical wake, predicted distorted wake, and generalized wake models for the hover and forward flight regimes, is reviewed. The requirement for accurate wake modeling for flow field and airload prediction is demonstrated by comparisons of theoretical and experimental results. These results include blade pressure distributions predicted from a recently developed procedure for including the rotor wake influence in a full potential flow analysis. Predictions of the interactional aerodynamics of various helicopter components (rotor, fuselage, and tail) are also presented. It is concluded that, with advanced computers and the rapidly progressing computational aerodynamics technology, significant progress toward reliable prediction of helicopter airloads is forseeable in the near future.

Landgrebe, A. J.

1985-01-01

88

Evidence of photon acceleration by laser wake fields C. D. Murphya  

E-print Network

using both a novel photon-kinetic code and a three-dimensional particle-in-cell code. In addition to the wide-ranging applications in the field of compact particle accelerators, the concept of wave kinetics of the plasma perturbations are much larger than the photon wave- length and period. In this case, geometric

Strathclyde, University of

89

Ionization suppression of diatomic molecules in different wavelength laser fields  

NASA Astrophysics Data System (ADS)

We theoretically study ionization of the N2 and O2 molecules and focus on the ionization suppression at various wavelengths. We find that the ionization rate of the N2 molecules is not significantly suppressed over a wide range of laser wavelengths. The ratio of the ion yields of N2+ to that of Ar+, however, slightly decreases as the laser wavelength increases, and this ratio for the low laser intensity is slightly larger than that for the high laser intensity. The interference effect greatly modulates the photoelectron energy spectra: an interference valley appearing in the photoelectron energy spectra. For the O2 molecules, the ionization suppression is always significant, and clearly depends on the laser wavelength and the laser intensity. The ratio of the ion yields of O2+ to that of Xe+ becomes larger as the laser wavelength (intensity) increases under given laser intensity (wavelength). Overall photoelectron energy spectra of O2 are suppressed, especially for the low-energy range.

Ren, Xianghe; Yang, Jing; Niu, Yufeng; Zhang, Jingtao

2014-10-01

90

Evaluation of rhizobacterial indicators of tobacco black root rot suppressiveness in farmers' fields.  

PubMed

Very few soil quality indicators include disease-suppressiveness criteria. We assessed whether 64 16S rRNA microarray probes whose signals correlated with tobacco black root rot suppressiveness in greenhouse analysis could also discriminate suppressive from conducive soils under field conditions. Rhizobacterial communities of tobacco and wheat sampled in 2 years from four farmers' fields of contrasted suppressiveness status were compared. The 64 previously identified indicator probes correctly classified 72% of 29 field samples, with nine probes for Azospirillum, Gluconacetobacter, Sphingomonadaceae, Planctomycetes, Mycoplasma, Lactobacillus crispatus and Thermodesulforhabdus providing the best prediction. The whole probe set (1033 probes) revealed strong effects of plant, field location and year on rhizobacterial community composition, and a smaller (7% variance) but significant effect of soil suppressiveness status. Seventeen additional probes correlating with suppressiveness status in the field (noticeably for Agrobacterium, Methylobacterium, Ochrobactrum) were selected, and combined with the nine others, they improved correct sample classification from 72% to 79% (100% tobacco and 63% wheat samples). Pseudomonas probes were not informative in the field, even those targeting biocontrol pseudomonads producing 2,4-diacetylphloroglucinol, nor was quantitative polymerase chain reaction for 2,4-diacetylphloroglucinol-synthesis gene phlD. This study shows that a subset of 16S rRNA probes targeting diverse rhizobacteria can be useful as suppressiveness indicators under field conditions. PMID:24992533

Kyselková, Martina; Almario, Juliana; Kopecký, Jan; Ságová-Mare?ková, Markéta; Haurat, Jacqueline; Muller, Daniel; Grundmann, Geneviève L; Moënne-Loccoz, Yvan

2014-08-01

91

Generation of femtosecond electron bunches and hard-X-rays by ultra-intense laser wake field acceleration in a gas jet  

Microsoft Academic Search

Summary form only given. Femtosecond electron beams and hard X-rays with ??0.1 nm may find various applications in biology, chemistry, and molecular electronics giving a new time-scale probe analysis. Such short electron beams can be produced in the wake field acceleration by short relativistically intense laser pulses and then Thomson scattering of a second laser pulse can serve for efficient

A. Zhidkov; M. Uesaka; T. Hosokai; K. Kinoshita

2004-01-01

92

Results from SLAC Experiment on Plasma Wake field Acceleration over One Meter  

NASA Astrophysics Data System (ADS)

In the E-157 experiment, a 30 GeV electron beam of 2e10 electrons in a 0.65mm long bunch is propagated through a 1.4m long lithium plasma (created by UV ionization) of density ~ 2e14/cm^3. The beam density is greater than the plasma density and the head of the bunch expels the plasma electrons leaving behind an ion channel with transverse focusing fields of up to several thousand Tesla/m. There are three types of ongoing studies: (1) The zeroth order effect is the ``thick plasma lens" where the induced focusing-channel causes the beam to undergo so-called betatron oscillations where the envelope of the beam oscillates radially. (2) Transverse head-tail effects are of higher order: non-axially-symmetric longitudinal charge distributions cause different slices of the beam to oscillate radially with different periods, or may even be unstable, distorting the beam further. (3) For electron bunch lengths on the order of half the plasma wavelength, the ion column immediately behind the head of the electron bunch will cause the main bunch to lose energy. The plasma electrons expelled from the beam will rush back to the axis and produce a strong accelerating force (order GeV/m) for the particles in the tail of the same bunch. The betatron oscillations are studied by scanning the plasma density and observing time-integrated images of optical transition radiation and Cherenkov radiation from foils downstream of the plasma. Energy changes in the beam are observed from time-resolved images of the Cherenkov radiator located in a dispersive section of the downstream beamline. Head-tail effects are seen in these images and can be isolated from energy gain by acquiring time-resolved images from the Cherenkov radiator in the non-dispersive direction. Beam-position monitors and beamline feedback signals also reveal information regarding beam centroid motion induced by the plasma. Progress on the experiment will be reported.

Clayton, Christopher E.

2000-04-01

93

Transverse evolution of a long relativistic electron beam governed by the Vlasov-Poisson-type pair of equations within the plasma wake field dynamics in the local regime  

NASA Astrophysics Data System (ADS)

An analysis of the transverse plasma wake field dynamics excited by a long relativistic electron beam is carried out in a purely local regime, in which the transverse beam spot-size is much bigger than the plasma wavelength. This is done by using the Vlasov equation, that governs the kinetic spatiotemporal evolution of the transverse paraxial beam transport. A self-consistent description of the beam dynamics is then provided by coupling the Vlasov equation with a Poisson-type equation. Furthermore, the latter relates the beam density with the plasma wake potential energy. Remarkably, the approach here proposed seems to be suitable for describing the beam self-modulation and the prediction of the beam collapse.

Fedele, Renato; Akhter, Tahmina; Jovanovi?, Dušan; De Nicola, Sergio; Mannan, Abdul

2014-07-01

94

Neurometabolic coupling differs for suppression within and beyond the classical receptive field in visual cortex.  

PubMed

Neurons in visual cortex exhibit two major types of stimulus elicited suppression. One, cross-orientation suppression, occurs within the classical receptive field (CRF) when an orthogonal grating is superposed on one at optimal orientation. The second, surround suppression, occurs when the size of an optimally oriented grating extends beyond the CRF. Previous proposals suggest that intracortical inhibition is responsible for surround suppression whereas feedforward processes may underlie cross-orientation suppression. To gain more insight concerning these types of suppression, we have included measurements of metabolic function in addition to neural responses. We made co-localized measurements of multiple unit neural activity and tissue oxygen concentrations in the striate cortex of anaesthetized cats while using visual stimuli to activate the two kinds of suppression. Results show that the amplitude of the initial negative oxygen response increases with stimulus size but neural responses decrease as size extends beyond the CRF. This shows that oxygen consumption increases with stimulus size regardless of reduced neural response. On the other hand, amplitudes of both the initial negative oxygen component and the neural responses are simultaneously attenuated by the orthogonal mask in cross-orientation suppression. These different neurometabolic response patterns are consistent with suggestions that the two types of suppressive processes arise from different neural mechanisms. PMID:21558164

Li, Baowang; Freeman, Ralph D

2011-07-01

95

A Limited Role for Suppression in the Central Field of Individuals with Strabismic Amblyopia  

PubMed Central

Background Although their eyes are pointing in different directions, people with long-standing strabismic amblyopia typically do not experience double-vision or indeed any visual symptoms arising from their condition. It is generally believed that the phenomenon of suppression plays a major role in dealing with the consequences of amblyopia and strabismus, by preventing images from the weaker/deviating eye from reaching conscious awareness. Suppression is thus a highly sophisticated coping mechanism. Although suppression has been studied for over 100 years the literature is equivocal in relation to the extent of the retina that is suppressed, though the method used to investigate suppression is crucial to the outcome. There is growing evidence that some measurement methods lead to artefactual claims that suppression exists when it does not. Methodology/Results Here we present the results of an experiment conducted with a new method to examine the prevalence, depth and extent of suppression in ten individuals with strabismic amblyopia. Seven subjects (70%) showed no evidence whatsoever for suppression and in the three individuals who did (30%), the depth and extent of suppression was small. Conclusions Suppression may play a much smaller role in dealing with the negative consequences of strabismic amblyopia than previously thought. Whereas recent claims of this nature have been made only in those with micro-strabismus our results show extremely limited evidence for suppression across the central visual field in strabismic amblyopes more generally. Instead of suppressing the image from the weaker/deviating eye, we suggest the visual system of individuals with strabismic amblyopia may act to maximise the possibilities for binocular co-operation. This is consistent with recent evidence from strabismic and amblyopic individuals that their binocular mechanisms are intact, and that, just as in visual normals, performance with two eyes is better than with the better eye alone in these individuals. PMID:22649494

Barrett, Brendan T.; Panesar, Gurvinder K.; Scally, Andrew J.; Pacey, Ian E.

2012-01-01

96

Power suppression from disparate mass scales in effective scalar field theories of inflation and quintessence  

SciTech Connect

A scalar potential coupled to other fields of large disparate masses will exhibit power suppression of the quantum loop corrections from these massive fields. Quintessence fields in the dark energy regime and inflaton fields during inflation often have a very large background field value. Thus any other field with its mass dependent on the quintessence/inflaton background field value through a moderate coupling will become very massive during the dark energy/inflation phase and its quantum corrections to the scalar effective potential will be suppressed. This concept is developed in this paper using the decoupling theorem. The problem then reduces to a quantitative question of the size of suppression effects within the parameter space of coupling constants, scalar field background value and renormalization scale. Some numerical examples are presented both for inflation and quintessence, but the approach is general and can be applied to any scalar field effective potential. The consequences to dark energy of the decoupling effect developed here is that the quintessence field need not just be an incredibly weakly interacting field, often included as an add-on to generate dark energy and having no other purpose. Instead, this quintessence field could play a central role in the particle physics dynamics at early times and then the other fields simply decouple from it at late times before the onset of the dark energy phase. For inflation a consequence is coupling of the inflaton to other heavy fields can be much larger.

Bastero-Gil, Mar [Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada-18071 (Spain); Berera, Arjun [School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ (United Kingdom); Jackson, Brendan M., E-mail: mbg@ugr.es, E-mail: ab@ph.ed.ac.uk, E-mail: bmj@roe.ac.uk [Institute for Astronomy, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3HJ (United Kingdom)

2011-07-01

97

Characterization of cavity wakes  

NASA Astrophysics Data System (ADS)

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

Kidd, James A.

98

Direct Numerical Simulations of a Stratified Self-Propelled Wake  

NASA Astrophysics Data System (ADS)

Direct Numerical Simulations (DNS) of a self-propelled wake subject to stratification are utilized to study the characteristics of the mean flow and turbulence fluctuations in the wake. Results include wake height, wake width, peak defect velocity, r.m.s. velocity fluctuations and turbulent fluxes, < uiuj> and . Buoyancy is found to decrease the vertical growth of the wake, suppress vertical fluctuations, and lead to internal wave emission. The lack of existing numerical and experimental studies of a truly momentumless wake precludes comparing the present results with prior studies, as was done by us for the towed wake. Differences between the case of the towed wake and the self-propelled case will be highlighted and discussed.

Brucker, Kyle; Sarkar, Sutanu

2007-11-01

99

Suppression of the Richtmyer-Meshkov Instability in the Presence of a Magnetic Field  

SciTech Connect

We present numerical evidence from two dimensional simulations that the growth of the Richtmyer-Meshkov instability is suppressed in the presence of a magnetic field. A bifurcation occurs during the refraction of the incident shock on the density interface which transports baroclinically generated vorticity away from the interface to a pair of slow or intermediate magnetosonic shocks. Consequently, the density interface is devoid of vorticity and its growth and associated mixing is completely suppressed.

Ravi Samtaney

2003-03-21

100

Speckle Suppression with the Project 1640 Integral Field Spectrograph  

Microsoft Academic Search

Project 1640 is a high-contrast imaging instrument recently commissioned at the Palomar observatory. A combination of a coronagraph with an integral-field spectrograph (IFS), Project 1640 is designed to detect and characterize extrasolar planets, brown dwarfs, and circumstellar material orbiting nearby stars. In this paper, we present our data processing techniques for improving upon instrument raw sensitivity via the removal of

Justin R. Crepp; Laurent Pueyo; Douglas Brenner; Ben R. Oppenheimer; Neil Zimmerman; Sasha Hinkley; Ian Parry; David King; Gautam Vasisht; Charles Beichman; Lynne Hillenbrand; Richard Dekany; Mike Shao; Rick Burruss; Lewis C. Roberts; Antonin Bouchez; Jenny Roberts; Rémi Soummer

2011-01-01

101

Speckle Suppression with the Project 1640 Integral Field Spectrograph  

Microsoft Academic Search

Project 1640 is a high-contrast imaging instrument recently commissioned at\\u000aPalomar observatory. A combination of a coronagraph with an integral field\\u000aspectrograph (IFS), Project 1640 is designed to detect and characterize\\u000aextrasolar planets, brown dwarfs, and circumstellar material orbiting nearby\\u000astars. In this paper, we present our data processing techniques for improving\\u000aupon instrument raw sensitivity via the removal of

Justin R. Crepp; Laurent Pueyo; Douglas Brenner; Ben R. Oppenheimer; Neil Zimmerman; Sasha Hinkley; Ian Parry; David King; Gautam Vasisht; Charles Beichman; Lynne Hillenbrand; Richard Dekany; Mike Shao; Rick Burruss; Antonin Bouchez; Jenny Roberts; Remi Soummer

2010-01-01

102

Magnetic Field Suppression of Flow in Semiconductor Melt  

NASA Technical Reports Server (NTRS)

One of the most promising approaches for the reduction of convection during the crystal growth of conductive melts (semiconductor crystals) is the application of magnetic fields. Current technology allows the experimentation with very intense static fields (up to 80 KGauss) for which nearly convection free results are expected from simple scaling analysis in stabilized systems (vertical Bridgman method with axial magnetic field). However, controversial experimental results were obtained. The computational methods are, therefore, a fundamental tool in the understanding of the phenomena accounting during the solidification of semiconductor materials. Moreover, effects like the bending of the isomagnetic lines, different aspect ratios and misalignments between the direction of the gravity and magnetic field vectors can not be analyzed with analytical methods. The earliest numerical results showed controversial conclusions and are not able to explain the experimental results. Although the generated flows are extremely low, the computational task is a complicated because of the thin boundary layers. That is one of the reasons for the discrepancy in the results that numerical studies reported. Modeling of these magnetically damped crystal growth experiments requires advanced numerical methods. We used, for comparison, three different approaches to obtain the solution of the problem of thermal convection flows: (1) Spectral method in spectral superelement implementation, (2) Finite element method with regularization for boundary layers, (3) Multiquadric method, a novel method with global radial basis functions, that is proven to have exponential convergence. The results obtained by these three methods are presented for a wide region of Rayleigh and Hartman numbers. Comparison and discussion of accuracy, efficiency, reliability and agreement with experimental results will be presented as well.

Fedoseyev, A. I.; Kansa, E. J.; Marin, C.; Volz, M. P.; Ostrogorsky, A. G.

2000-01-01

103

Airloads, wakes, and aeroelasticity  

NASA Technical Reports Server (NTRS)

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

Johnson, Wayne

1990-01-01

104

Efficient and stable transgene suppression via RNAi in field-grown poplars.  

PubMed

The efficiency and stability of RNA interference (RNAi) in perennial species, particularly in natural environments, is poorly understood. We studied 56 independent poplar RNAi transgenic events in the field over 2 years. A resident BAR transgene was targeted with two different types of RNAi constructs: a 475-bp IR of the promoter sequence and a 275-bp IR of the coding sequence, each with and without the presence of flanking matrix attachment regions (MARs). RNAi directed at the coding sequence was a strong inducer of gene silencing; 80% of the transgenic events showed more than 90% suppression. In contrast, RNAi targeting the promoter resulted in only 6% of transgenic events showing more than 90% suppression. The degree of suppression varied widely but was highly stable in each event over 2 years in the field, and had no association with insert copy number or the presence of MARs. RNAi remained stable during a winter to summer seasonal cycle, a time when expression of the targeted transgene driven by an rbcS promoter varied widely. When strong gene suppression was induced by an IR directed at the promoter sequence, it was accompanied by methylation of the homologous promoter region. DNA methylation was also observed in the coding region of highly suppressed events containing an IR directed at the coding sequence; however, the methylation degree and pattern varied widely among those suppressed events. Our results suggest that RNAi can be highly effective for functional genomics and biotechnology of perennial plants. PMID:17929189

Li, Jingyi; Brunner, Amy M; Shevchenko, Olga; Meilan, Richard; Ma, Cathleen; Skinner, Jeffrey S; Strauss, Steven H

2008-08-01

105

Background gradient suppression in stimulated echo NMR diffusion studies using magic pulsed field gradient ratios.  

PubMed

By evaluating the spin echo attenuation for a generalized 13-interval PFG NMR sequence consisting of pulsed field gradients with four different effective intensities (F(p/r) and G(p/r)), magic pulsed field gradient (MPFG) ratios for the prepare (G(p)/F(p)) and the read (G(r)/F(r)) interval are derived, which suppress the cross term between background field gradients and the pulsed field gradients even in the cases where the background field gradients may change during the z-store interval of the pulse sequence. These MPFG ratios depend only on the timing of the pulsed gradients in the pulse sequence and allow a convenient experimental approach to background gradient suppression in NMR diffusion studies with heterogeneous systems, where the local properties of the (internal) background gradients are often unknown. If the pulsed field gradients are centered in the tau-intervals between the pi and pi/2 rf pulses, these two MPFG ratios coincide to eta=G(p/r)/F(p/r)=1-8/[1+(1/3)(delta/tau)(2)]. Since the width of the pulsed field gradients (delta) is bounded by 0< or =delta< or =tau, eta can only be in the range of 5< or =-eta< or =7. The predicted suppression of the unwanted cross terms is demonstrated experimentally using time-dependent external gradients which are controlled in the NMR experiment as well as spatially dependent internal background gradients generated by the magnetic properties of the sample itself. The theoretical and experimental results confirm and extend the approach of Sun et al. (J. Magn. Reson. 161 (2003) 168), who recently introduced a 13-interval type PFG NMR sequence with two asymmetric pulsed magnetic field gradients suitable to suppress unwanted cross terms with spatially dependent background field gradients. PMID:14729028

Galvosas, Petrik; Stallmach, Frank; Kärger, Jörg

2004-02-01

106

The Spontaneous Magnetic Field Generation and Suppression of Heat Conduction in Clusters of Galaxies  

Microsoft Academic Search

We show that magnetic fields are spontaneously generated in the plasmas which have the temperature inhomogeneity and the heat conduction is spontaneously suppressed. This is based on the microscopic plasma instability that the anisotropic velocity distribution induced by the temperature gradient derives the low frequency growing transverse magnetic waves. We have shown that the physical mechanism for this growth is

Nobuhiro Okabe; Makoto Hattori

2004-01-01

107

Suppression of electron spin decoherence of the diamond NV center by a transverse magnetic field  

NASA Astrophysics Data System (ADS)

We demonstrate that the spin decoherence of nitrogen vacancy (NV) centers in diamond can be suppressed by a transverse magnetic field if the electron spin bath is the primary decoherence source. The NV spin coherence, created in “a decoherence-free subspace,” is protected by the transverse component of the zero-field splitting, increasing the spin-coherence time about twofold. The decoherence due to the electron spin bath is also suppressed at magnetic fields stronger than ˜25 G when applied parallel to the NV symmetry axis. Our method can be used to extend the spin-coherence time of similar spin systems for applications in quantum computing, field sensing, and other metrologies.

Shin, Chang S.; Avalos, Claudia E.; Butler, Mark C.; Wang, Hai-Jing; Seltzer, Scott J.; Liu, Ren-Bao; Pines, Alexander; Bajaj, Vikram S.

2013-10-01

108

Wind turbine wake aerodynamics  

Microsoft Academic Search

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

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

2003-01-01

109

Spectral element discontinuous Galerkin simulations for wake potential calculations : NEKCEM.  

SciTech Connect

In this paper we present high-order spectral element discontinuous Galerkin simulations for wake field and wake potential calculations. Numerical discretizations are based on body-conforming hexagonal meshes on Gauss-Lobatto-Legendre grids. We demonstrate wake potential profiles for cylindrically symmetric cavity structures in 3D, including the cases for linear and quadratic transitions between two cross sections. Wake potential calculations are carried out on 2D surfaces for various bunch sizes.

Min, M.; Fischer, P. F.; Chae, Y.-C.

2008-01-01

110

Suppression of cooling by strong magnetic fields in white dwarf stars.  

PubMed

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young. PMID:25327247

Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

2014-11-01

111

Suppression of cooling by strong magnetic fields in white dwarf stars  

NASA Astrophysics Data System (ADS)

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

Valyavin, G.; Shulyak, D.; Wade, G. A.; Antonyuk, K.; Zharikov, S. V.; Galazutdinov, G. A.; Plachinda, S.; Bagnulo, S.; Fox Machado, L.; Alvarez, M.; Clark, D. M.; Lopez, J. M.; Hiriart, D.; Han, Inwoo; Jeon, Young-Beom; Zurita, C.; Mujica, R.; Burlakova, T.; Szeifert, T.; Burenkov, A.

2014-11-01

112

Experimental evaluation of a flat wake theory for predicting rotor inflow-wake velocities  

NASA Technical Reports Server (NTRS)

The theory for predicting helicopter inflow-wake velocities called flat wake theory was correlated with several sets of experimental data. The theory was developed by V. E. Baskin of the USSR, and a computer code known as DOWN was developed at Princeton University to implement the theory. The theory treats the wake geometry as rigid without interaction between induced velocities and wake structure. The wake structure is assumed to be a flat sheet of vorticity composed of trailing elements whose strength depends on the azimuthal and radial distributions of circulation on a rotor blade. The code predicts the three orthogonal components of flow velocity in the field surrounding the rotor. The predictions can be utilized in rotor performance and helicopter real-time flight-path simulation. The predictive capability of the coded version of flat wake theory provides vertical inflow patterns similar to experimental patterns.

Wilson, John C.

1992-01-01

113

Fringing field suppression for liquid crystal gratings using equivalent capacitance configuration  

NASA Astrophysics Data System (ADS)

A liquid crystal grating with high spatial frequency and equivalent capacitance configuration is proposed, where two layers of periodical ground electrodes are interlaced and aligned with the addressing electrodes. The equivalent capacitance configuration can reduce the fringing field effect efficiently owing to the generated electric field resisting the fringing field and redistributing the equivalent voltage exerting on the liquid crystal layer. The phase modulation depth and far-field diffraction patterns both for conventional and novel configurations were simulated. The results show that phase modulation is greatly enhanced and the maximum diffraction efficiency for a sinusoidal phase grating is 33.86%, which indicates that the equivalent capacitance configuration provides a good solution for suppressing the fringing field effect.

Yang, Lei; Xia, Jun; Zhang, Xiaobing; Xie, Yi; Kang, Mingwu; Zhang, Qiuzhi

2014-10-01

114

Critical Magnetic Field Strength for Suppression of the Richtmyer-Meshkov Instability in Plasmas  

NASA Astrophysics Data System (ADS)

The critical strength of a magnetic field required for the suppression of the Richtmyer-Meshkov instability (RMI) is investigated numerically by using a two-dimensional single-mode analysis. For the cases of magnetohydrodynamic parallel shocks, the RMI can be stabilized as a result of the extraction of vorticity from the interface. A useful formula describing a critical condition for magnetohydrodynamic RMI is introduced and is successfully confirmed by direct numerical simulations. The critical field strength is found to be largely dependent on the Mach number of the incident shock. If the shock is strong enough, even low-? plasmas can be subject to the growth of the RMI.

Sano, Takayoshi; Inoue, Tsuyoshi; Nishihara, Katsunobu

2013-11-01

115

Growth of randomness in a spatially developing wake  

NASA Technical Reports Server (NTRS)

The spatially evolving vortical structures and randomness statistics at various downstream locations of a plane wake are examined. The growth of the continuous components of the power spectra in such a wake are systematically investigated. Selective amplification of the fundamental mode from the broadband noise is found in the linear region. The fundamental mode of large-magnitude locking in the wake suppresses the noise growth. When the wake is forced by large-amplitude random phase noise, nonlinear interactions of a few beating modes whose frequencies are close to the fundamental one play a crucial role in randomizing the wake. In this case, the vorticity contours show randomizing vortical structure of the wake. Pairing motion of the same-sign vortices is seen in the randomizing region.

Maekawa, Hiroshi; Mansour, Nagi N.; Buell, Jeffrey C.

1989-01-01

116

Differential study on molecular suppressed ionization in intense linearly and circularly polarized laser fields  

SciTech Connect

We present a differential study on above-threshold ionization of the O{sub 2} (N{sub 2}) molecule as well as the companion atom Xe (Ar) (with close ionization potential) produced by linearly and circularly polarized laser fields (25 fs, 795 nm). The photoelectron angular distributions of the companion target are similar at the same laser condition. In both linearly and circularly polarized fields, we observe that the photoelectron yields of O{sub 2} are suppressed in the entire energy spectral range as compared with Xe with fully differential measurements, but not for the N{sub 2}-Ar pair. This is different from the prediction of photoelectron energy spectra by the model including the interference terms [Phys. Rev. Lett. 85, 2280 (2000)], from which the low-energy photoelectrons of O{sub 2} were expected to be strongly suppressed in both linearly and circularly polarized laser fields. Resorting to the basic strong-field ionization picture, we believe that the lower orbital-dependent multiphoton excitation or tunneling possibility of O{sub 2} as compared with Xe is responsible for this effect. High-resolution fully differential data pose a stringent test on the current strong-field calculations on molecules.

Deng Yongkai; Liu Yunquan; Liu Xianrong; Liu Hong; Yang Yudong; Wu Chengyin; Gong Qihuang [Department of Physics and State Key Laboratory for Mesoscopic Physics, Peking University, Beijing 100871 (China)

2011-12-15

117

Anisotropic Stark Effect and Electric-Field Noise Suppression for Phosphorus Donor Qubits in Silicon  

E-print Network

We report the use of novel, capacitively terminated coplanar waveguide (CPW) resonators to measure the Stark shift of phosphorus donor qubits in Si. We confirm that valley repopulation leads to an anisotropic spin-orbit Stark shift dependent on electric and magnetic field orientations relative to the Si crystal. Using the measured values for the Stark shift, we predict magnetic fields for which the spin-orbit Stark effect cancels the hyperfine Stark effect, suppressing decoherence from electric-field noise. By measuring the linear Stark effect, we show that such sources of decoherence can be non-negligible due to strain. From our data, we estimate the effective electric field due to strain in our samples. Values for the spin-orbit and hyperfine Stark parameters are reported.

A. J. Sigillito; A. M. Tyryshkin; S. A. Lyon

2014-09-11

118

Anisotropic Stark Effect and Electric-Field Noise Suppression for Phosphorus Donor Qubits in Silicon  

E-print Network

We report the use of novel, capacitively terminated coplanar waveguide (CPW) resonators to measure the Stark shift of phosphorus donor qubits in Si. We confirm that valley repopulation leads to an anisotropic spin-orbit Stark shift dependent on electric and magnetic field orientations relative to the Si crystal. Using the measured values for the Stark shift, we predict magnetic fields for which the spin-orbit Stark effect cancels the hyperfine Stark effect, suppressing decoherence from electric-field noise. By measuring the linear Stark effect, we show that such sources of decoherence can be non-negligible due to strain. From our data, we estimate the effective electric field due to strain in our samples. Values for the spin-orbit and hyperfine Stark parameters are reported.

Sigillito, A J; Lyon, S A

2014-01-01

119

High lift wake investigation  

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

120

Application of laser velocimetry to aircraft wake-vortex measurements  

NASA Technical Reports Server (NTRS)

The theory and use of a laser velocimeter that makes simultaneous measurements of vertical and longitudinal velocities while rapidly scanning a flow field laterally are described, and its direct application to trailing wake-vortex research is discussed. Pertinent measurements of aircraft wake-vortex velocity distributions obtained in a wind tunnel and water towing tank are presented. The utility of the velocimeter to quantitatively assess differences in wake velocity distributions due to wake dissipating devices and span loading changes on the wake-generating model is also demonstrated.

Ciffone, D. L.; Orloff, K. L.

1977-01-01

121

Visualization on fish's wake  

NASA Astrophysics Data System (ADS)

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

Li, Xuemin; Lu, Xiyun; Yin, Xiezhen

2002-05-01

122

Contrail formation in aircraft wakes  

Microsoft Academic Search

The process of the formation and early evolution of a condensation trail (`contrail') in the near field of an aircraft wake was numerically studied by means of a mixed Eulerian\\/Lagrangian two-phase flow approach. Large-eddy simulations were used for the carrier phase, while, for the dispersed phase, a Lagrangian particle tracking method was used, coupled with a microphysics model to account

Roberto Paoli; Jerome Hélie; Thierry Poinsot

2004-01-01

123

Wake Vortex Influence on Ambient Potential Temperature  

NASA Technical Reports Server (NTRS)

The two-dimensional version of the Terminal Area Simulation System (TASS) was used to numerically simulate the interaction of wake vortices from closely separated aircraft. The aircraft parameters and separations are taken from observed data at an actual airport. The wake vortices are generated near the runway threshold for four successive aircraft. The ambient conditions are characterized by light crosswinds and stable stratification. This movie shows the effect that the vortices have upon the ambient potential temperature field.

1997-01-01

124

Noradrenergic Modulation of Wakefulness/Arousal  

PubMed Central

The locus coeruleus-noradrenergic systems supplies norepinephrine throughout the central nervous system. State-dependent neuronal discharge activity of locus coeruleus noradrenergic neurons has long-suggested a role of this system in the induction of an alert waking state. Work over the past two decades provides unambiguous evidence that the locus coeruleus, and likely other noradrenergic nuclei, exert potent wake-promoting actions via an activation of noradrenergic ?- and ?1-receptors located within multiple subcortical structures, including the general regions of the medial septal area, the medial preoptic area and, most recently, the lateral hypothalamus. Conversely, global blockade of ?- and ?1-receptors or suppression of norepinephrine release results in profound sedation. The wake-promoting action of central noradrenergic neurotransmission has clinical implications for treatment of sleep/arousal disorders, such as insomnia and narcolepsy, and clinical conditions associated with excessive arousal, such as post-traumatic stress disorder. PMID:22296742

Berridge, Craig W.; Schmeichel, Brooke E.; Espana, Rodrigo A.

2011-01-01

125

Suppression of multipactor discharge on a dielectric surface by an external magnetic field  

SciTech Connect

The multipactor discharge on a dielectric surface in an external magnetic field is simulated by using the particle-in-cell method, and the electron number, energy, the velocity of the yield of secondary electrons, and the power deposited on dielectric surface in the process of multipactor discharge are investigated. The effects of the strength of the external magnetic field on multipactor are studied. The results show that when the external magnetic field reaches a certain value, the multipactor is weaker than that in the case of no external magnetic field and becomes much lighter versus the strength of the external magnetic field in the half microwave period in which the ExB drift pulls the electrons back to dielectric surface. And in the other half microwave period in which the ExB drift pushes the electrons away from the dielectric surface, the multipactor is cut off. So the power capability can be increased to the fourfold by the suppression of multipactor by applying an external magnetic field.

Cai Libing; Zhu Xiangqin; Wang Yue; Xuan Chun; Xia Hongfu [Northwest Institute of Nuclear Technology, P. O. Box 69-12, Xi'an 710024 (China); Wang Jianguo [Northwest Institute of Nuclear Technology, P. O. Box 69-12, Xi'an 710024 (China); School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an 710049 (China)

2011-07-15

126

Wind tunnel measurements for dispersion modelling of vehicle wakes  

NASA Astrophysics Data System (ADS)

Wind tunnel measurements downwind of reduced scale car models have been made to study the wake regions in detail, test the usefulness of existing vehicle wake models, and draw key information needed for dispersion modelling in vehicle wakes. The experiments simulated a car moving in still air. This is achieved by (i) the experimental characterisation of the flow, turbulence and concentration fields in both the near and far wake regions, (ii) the preliminary assessment of existing wake models using the experimental database, and (iii) the comparison of previous field measurements in the wake of a real diesel car with the wind tunnel measurements. The experiments highlighted very large gradients of velocities and concentrations existing, in particular, in the near-wake. Of course, the measured fields are strongly dependent on the geometry of the modelled vehicle and a generalisation for other vehicles may prove to be difficult. The methodology applied in the present study, although improvable, could constitute a first step towards the development of mathematical parameterisations. Experimental results were also compared with the estimates from two wake models. It was found that they can adequately describe the far-wake of a vehicle in terms of velocities, but a better characterisation in terms of turbulence and pollutant dispersion is needed. Parameterised models able to predict velocity and concentrations with fine enough details at the near-wake scale do not exist.

Carpentieri, Matteo; Kumar, Prashant; Robins, Alan

2012-12-01

127

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

128

Secondary energy growth and turbulence suppression in conducting channel flow with streamwise magnetic field  

NASA Astrophysics Data System (ADS)

The effects of a streamwise magnetic field on conducting channel flow are studied by analyzing secondary linear perturbations evolving on streamwise streaks and by direct numerical simulations of relaminarization. By means of an optimal perturbation approach, magnetic damping is found to increase the streamwise wavelength of the most amplified secondary perturbations and to reduce their amplification level. Complete suppression of secondary instability is observed at a critical magnetic interaction parameter that depends on the streak amplitude and on the Reynolds number when the transient evolution of the streaky basic flow is taken into account. Relaminarization in the direct numerical simulation occurs at lower values of the interaction parameter than the critical values from the stability computations for the streak amplitudes considered. The dependence of these threshold values of the interaction parameters on the Reynolds number is fairly similar between simulations and stability analysis. Relaminarization thresholds from the simulations are also in good agreement with experiments on pipe flow with streamwise magnetic field.

Dong, Shuai; Krasnov, Dmitry; Boeck, Thomas

2012-07-01

129

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

NASA Technical Reports Server (NTRS)

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

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

1974-01-01

130

On the control and suppression of the Rayleigh-Taylor instability using electric fields  

NASA Astrophysics Data System (ADS)

It is shown theoretically that an electric field can be used to control and suppress the classical Rayleigh-Taylor instability found in stratified flows when a heavy fluid lies above lighter fluid. Dielectric fluids of arbitrary viscosities and densities are considered and a theory is presented to show that a horizontal electric field (acting in the plane of the undisturbed liquid-liquid surface), causes growth rates and critical stability wavenumbers to be reduced thus shifting the instability to longer wavelengths. This facilitates complete stabilization in a given finite domain above a critical value of the electric field strength. Direct numerical simulations based on the Navier-Stokes equations coupled to the electrostatic fields are carried out and the linear theory is used to critically evaluate the codes before computing into the fully nonlinear stage. Excellent agreement is found between theory and simulations, both in unstable cases that compare growth rates and in stable cases that compare frequencies of oscillation and damping rates. Computations in the fully nonlinear regime supporting finger formation and roll-up show that a weak electric field slows down finger growth and that there exists a critical value of the field strength, for a given system, above which complete stabilization can take place. The effectiveness of the stabilization is lost if the initial amplitude is large enough or if the field is switched on too late. We also present a numerical experiment that utilizes a simple on-off protocol for the electric field to produce sustained time periodic interfacial oscillations. It is suggested that such phenomena can be useful in inducing mixing. A physical centimeter-sized model consisting of stratified water and olive oil layers is shown to be within the realm of the stabilization mechanism for field strengths that are approximately 2 × 104 V/m.

Cimpeanu, Radu; Papageorgiou, Demetrios T.; Petropoulos, Peter G.

2014-02-01

131

Cavities of Weak Magnetic Field Strength in the Wake of FTEs: Results from Global Magnetospheric MHD Simulations  

NASA Technical Reports Server (NTRS)

We use the global magnetohydrodynamic (MHD) code BATS-R-US to model multipoint observations of Flux Transfer Event (FTE) signatures. Simulations with high spatial and temporal resolution predict that cavities of weak magnetic field strength protruding into the magnetosphere trail FTEs. These predictions are consistent with recently reported multi-point Cluster observations of traveling magnetopause erosion regions (TMERs).

Kuznetsova, M. M.; Sibeck, D. G.; Hesse, M.; Wang, Y.; Rastaetter, L.; Toth, G.; Ridley, A.

2009-01-01

132

Nonlinear collisionless plasma wakes of small particles  

SciTech Connect

The wake behind a spherical particle smaller than the Debye length ({lambda}{sub De}) in flowing plasma is calculated using a particle-in-cell code. The results with different magnitudes of charge reveal substantial nonlinear effects down to values that for a floating particle would correspond to a particle radius {approx}10{sup -2{lambda}}{sub De}. The peak potential in the oscillatory wake structure is strongly suppressed by nonlinearity, never exceeding {approx}0.4 times the unperturbed ion energy. By contrast, the density peak arising from ion focusing can be many times the ambient. Strong heating of the ions occurs in the nonlinear regime. Direct ion absorption by the particle is not important for the far wake unless the radius exceeds 10{sup -1{lambda}}{sub De}, and is therefore never significant (for the far wake) in the linear regime. Reasonable agreement with full-scale linear response calculations are obtained in the linear regime. The wake wavelength is confirmed and an explanation, in terms of the conical potential structure, is proposed for experimentally-observed oblique alignment of different-sized grains.

Hutchinson, I. H. [Plasma Science and Fusion Center and Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)

2011-03-15

133

Jovian plasma torus interaction with Europa. Plasma wake structure and effect of inductive magnetic field: 3D Hybrid kinetic simulation  

E-print Network

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect a to variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo Orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa (Cassidy et al., 2007; Shematovich et al., 2005). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream backgr...

Lipatov, A S; Paterson, W R; Sittler, E C; Hartle, R E; Simpson, D G

2012-01-01

134

Sustained suppression of type-I edge-localized modes with dominantly n = 2 magnetic fields in DIII-D  

NASA Astrophysics Data System (ADS)

Type-I edge-localized modes (ELMs) have been suppressed in DIII-D (Luxon et al 2003 Nucl. Fusion 43 1813) H-mode discharges with a H98Y2 confinement factor near 1.0 using magnetic perturbations (MPs) with dominant toroidal mode number n = 2. This expands access to the ELM-suppressed regime, which was previously attainable in DIII-D only with n = 3 fields. ELM suppression is obtained with two rows of internal coils for 1.8 s with normalized beta of 1.9 and average triangularity of 0.53, corresponding to a scaled version of ITER scenario 2 at an ITER relevant electron collisionality of 0.2. The applied field reduces the pedestal pressure and edge current via the density without degrading the edge thermal transport barrier. ELITE calculations find that the resulting profiles are stable to intermediate-n peeling-ballooning modes. ELM suppression is found within different ranges of q95 depending on the coil configuration used to generate the MP. The edge safety factors associated with suppression do not correspond to those that maximize the pitch-resonant components of the applied vacuum field. Instead, ELM suppression is correlated with an increase in the amplification of kink-resonant components of the calculated ideal MHD plasma response field.

Lanctot, M. J.; Buttery, R. J.; de Grassie, J. S.; Evans, T. E.; Ferraro, N. M.; Hanson, J. M.; Haskey, S. R.; Moyer, R. A.; Nazikian, R.; Osborne, T. H.; Orlov, D. M.; Snyder, P. B.; Wade, M. R.; the DIII-D Team

2013-08-01

135

Wake Vortex Minimization  

NASA Technical Reports Server (NTRS)

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

1977-01-01

136

LOW-MASS PLANETS IN PROTOPLANETARY DISKS WITH NET VERTICAL MAGNETIC FIELDS: THE PLANETARY WAKE AND GAP OPENING  

SciTech Connect

Some regions in protoplanetary disks are turbulent, while some regions are quiescent (e.g. the dead zone). In order to study how planets open gaps in both inviscid hydrodynamic disk (e.g. the dead zone) and the disk subject to magnetorotational instability (MRI), we carried out both shearing box two-dimensional inviscid hydrodynamical simulations and three-dimensional unstratified magnetohydrodynamical (MHD) simulations (having net vertical magnetic fields) with a planet at the box center. We found that, due to the nonlinear wave steepening, even a low mass planet can open gaps in both cases, in contradiction to the ''thermal criterion'' for gap opening. In order to understand if we can represent the MRI turbulent stress with the viscous {alpha} prescription for studying gap opening, we compare gap properties in MRI-turbulent disks to those in viscous HD disks having the same stress, and found that the same mass planet opens a significantly deeper and wider gap in net vertical flux MHD disks than in viscous HD disks. This difference arises due to the efficient magnetic field transport into the gap region in MRI disks, leading to a larger effective {alpha} within the gap. Thus, across the gap, the Maxwell stress profile is smoother than the gap density profile, and a deeper gap is needed for the Maxwell stress gradient to balance the planetary torque density. Comparison with previous results from net toroidal flux/zero flux MHD simulations indicates that the magnetic field geometry plays an important role in the gap opening process. We also found that long-lived density features (termed zonal flows) produced by the MRI can affect planet migration. Overall, our results suggest that gaps can be commonly produced by low mass planets in realistic protoplanetary disks, and caution the use of a constant {alpha}-viscosity to model gaps in protoplanetary disks.

Zhu Zhaohuan; Stone, James M.; Rafikov, Roman R., E-mail: zhzhu@astro.princeton.edu, E-mail: jstone@astro.princeton.edu, E-mail: rrr@astro.princeton.edu [Department of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544 (United States)

2013-05-10

137

Synergistic Effects of Turbine Wakes and Atmospheric Stability on Power Production at an Onshore Wind Farm  

SciTech Connect

This report examines the complex interactions between atmospheric stability and turbine-induced wakes on downwind turbine wind speed and power production at a West Coast North American multi-MW wind farm. Wakes are generated when the upwind flow field is distorted by the mechanical movement of the wind turbine blades. This has two consequences for downwind turbines: (1) the downwind turbine encounters wind flows with reduced velocity and (2) the downwind turbine encounters increased turbulence across multiple length scales via mechanical turbulence production by the upwind turbine. This increase in turbulence on top of ambient levels may increase aerodynamic fatigue loads on the blades and reduce the lifetime of turbine component parts. Furthermore, ambient atmospheric conditions, including atmospheric stability, i.e., thermal stratification in the lower boundary layer, play an important role in wake dissipation. Higher levels of ambient turbulence (i.e., a convective or unstable boundary layer) lead to higher turbulent mixing in the wake and a faster recovery in the velocity flow field downwind of a turbine. Lower levels of ambient turbulence, as in a stable boundary layer, will lead to more persistent wakes. The wake of a wind turbine can be divided into two regions: the near wake and far wake, as illustrated in Figure 1. The near wake is formed when the turbine structure alters the shape of the flow field and usually persists one rotor diameter (D) downstream. The difference between the air inside and outside of the near wake results in a shear layer. This shear layer thickens as it moves downstream and forms turbulent eddies of multiple length scales. As the wake travels downstream, it expands depending on the level of ambient turbulence and meanders (i.e., travels in non-uniform path). Schepers estimates that the wake is fully expanded at a distance of 2.25 D and the far wake region begins at 2-5 D downstream. The actual distance traveled before the wake recovers to its inflow velocity is dependent on the amount ambient turbulence, the amount of wind shear, and topographical and structural effects. The maximum velocity deficit is estimated to occur at 1-2 D but can be longer under low levels of ambient turbulence. Our understanding of turbine wakes comes from wind tunnel experiments, field experiments, numerical simulations, and from studies utilizing both experimental and modeling methods. It is well documented that downwind turbines in multi-Megawatt wind farms often produce less power than upwind turbine rows. These wake-induced power losses have been estimated from 5% to up to 40% depending on the turbine operating settings (e.g., thrust coefficient), number of turbine rows, turbine size (e.g., rotor diameter and hub-height), wind farm terrain, and atmospheric flow conditions (e.g., ambient wind speed, turbulence, and atmospheric stability). Early work by Elliott and Cadogan suggested that power data for different turbulent conditions be segregated to distinguish the effects of turbulence on wind farm power production. This may be especially important for downwind turbines within wind farms, as chaotic and turbulent wake flows increase stress on downstream turbines. Impacts of stability on turbine wakes and power production have been examined for a flat terrain, moderate size (43 turbines) wind farm in Minnesota and for an offshore, 80 turbine wind farm off the coast of Denmark. Conzemius found it difficult to distinguish wakes (i.e., downwind velocity deficits) when the atmosphere was convective as large amounts of scatter were present in the turbine nacelle wind speed data. This suggested that high levels of turbulence broke-up the wake via large buoyancy effects, which are generally on the order of 1 km in size. On the other hand, they found pronounced wake effects when the atmosphere was very stable and turbulence was either suppressed or the length scale was reduced as turbulence in this case was mechanically produced (i.e., friction forces). This led to larger reductions at downwind turbines and maximum ve

Wharton, S; Lundquist, J K; Marjanovic, N

2012-01-25

138

Wind and Wake Sensing with UAV Formation Flight: System Development and Flight Testing  

NASA Astrophysics Data System (ADS)

Wind turbulence including atmospheric turbulence and wake turbulence have been widely investigated; however, only recently it become possible to use Unmanned Aerial Vehicles (UAVs) as a validation tool for research in this area. Wind can be a major contributing factor of adverse weather for aircraft. More importantly, it is an even greater risk towards UAVs because of their small size and weight. Being able to estimate wind fields and gusts can potentially provide substantial benefits for both unmanned and manned aviation. Possible applications include gust suppression for improving handling qualities, a better warning system for high wind encounters, and enhanced control for small UAVs during flight. On the other hand, the existence of wind can be advantageous since it can lead to fuel savings and longer duration flights through dynamic soaring or thermal soaring. Wakes are an effect of the lift distribution across an aircraft's wing or tail. Wakes can cause substantial disturbances when multiple aircraft are moving through the same airspace. In fact, the perils from an aircraft flying through the wake of another aircraft is a leading cause of the delay between takeoff times at airports. Similar to wind, though, wakes can be useful for energy harvesting and increasing an aircraft's endurance when flying in formation which can be a great advantage to UAVs because they are often limited in flight time due to small payload capacity. Formation flight can most often be seen in manned aircraft but can be adopted for use with unmanned systems. Autonomous flight is needed for flying in the "sweet spot" of the generated wakes for energy harvesting as well as for thermal soaring during long duration flights. For the research presented here formation flight was implemented for the study of wake sensing and gust alleviation. The major contributions of this research are in the areas of a novel technique to estimate wind using an Unscented Kalman filter and experimental wake sensing data using UAVs in formation flight. This has been achieved and well documented before in manned aircraft but very little work has been done on UAV wake sensing especially during flight testing. This document describes the development and flight testing of small unmanned aerial system (UAS) for wind and wake sensing purpose including a Ground Control Station (GCS) and UAVs. This research can be stated in four major components. Firstly, formation flight was obtained by integrating a formation flight controller on the WVU Phastball Research UAV aircraft platform from the Flight Control Systems Laboratory (FCSL) at West Virginia University (WVU). Second, a new approach to wind estimation using an Unscented Kalman filter (UKF) is discussed along with results from flight data. Third, wake modeling within a simulator and wake sensing during formation flight is shown. Finally, experimental results are used to discuss the "sweet spot" for energy harvesting in formation flight, a novel approach to cooperative wind estimation, and gust suppression control for a follower aircraft in formation flight.

Larrabee, Trenton Jameson

139

Fast fat-suppressed reduced field-of-view temperature mapping using 2DRF excitation pulses  

NASA Astrophysics Data System (ADS)

The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11 × 1140 ?s echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3 T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies.

Yuan, Jing; Mei, Chang-Sheng; Madore, Bruno; McDannold, Nathan J.; Panych, Lawrence P.

2011-05-01

140

Crosswind Shear Gradient Affect on Wake Vortices  

NASA Technical Reports Server (NTRS)

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

Proctor, Fred H.; Ahmad, Nashat N.

2011-01-01

141

Wake distribution prediction on the propeller plane in ship design using artificial intelligence  

Microsoft Academic Search

In this study, a new neuro-fuzzy technique is applied to estimate the wake field distribution on propeller plane of ship. The wake distribution data of stern flow fields have been collected systematically by model tests of ship. When a correlation between geometrical hull information and wake distribution of ship is grasped through the collected data, the obtained correlation can be

S.-Y. Kim; B. Y. Moon

2006-01-01

142

Acceleration of nonmonoenergetic electron bunches injected into a wake wave  

SciTech Connect

The trapping and acceleration of nonmonoenergetic electron bunches in a wake field wave excited by a laser pulse in a plasma channel is studied. Electrons are injected into the region of the wake wave potential maximum at a velocity lower than the phase velocity of the wave. The paper analyzes the grouping of bunch electrons in the energy space emerging in the course of acceleration under certain conditions of their injection into the wake wave and minimizing the energy spread for such electrons. The factors determining the minimal energy spread between bunch electrons are analyzed. The possibility of monoenergetic acceleration of electron bunches generated by modern injectors in a wake wave is analyzed.

Kuznetsov, S. V., E-mail: shenau@rambler.ru [Russian Academy of Sciences, Joint Institute for High Temperatures (Russian Federation)

2012-07-15

143

Bachelor thesis: "Validation of an engineering model of the near wake wind field of wind turbines based on nacelle based lidar measurements"  

E-print Network

this is possible with lidar systems and at ForWind ­ Uni Oldenburg we are performing analysis of unique. Wind turbine, meteorological and lidar data have to be synchronized, checked and selected properly thesis in validation of an engineering near wake model Sketch lidar measurement and deterministic wind

Peinke, Joachim

144

Neurophysiological support of consciousness during waking and sleep  

Microsoft Academic Search

The aim of this review was to try to establish the current neurophysiological knowledge capable of explaining the differences of mental functioning during the different stages of sleep and waking. The analysis focused on the cortical state. Waking is characterized by electrophysiological activities (low voltage and gamma range EEG field patterns, unitary activities) and cerebral blood flow reflecting an activated

C Gottesmann

1999-01-01

145

RF interference suppression in a cardiac synchronization system operating in a high magnetic field NMR imaging system  

SciTech Connect

An electrocardiographic (ECG) unit suitable for cardiac-synchronized nuclear magnetic resonance imaging in high magnetic fields is presented. The unit includes lossy transmission lines as ECG leads in order to suppress radio frequency (RF) interference in the electrocardiogram. The unit's immunity to RF interference is demonstrated.

Damji, A.A.; Snyder, R.E.; Ellinger, D.C.; Witkowski, F.X.; Allen, P.S.

1988-11-01

146

Mach-like capillary-gravity wakes  

E-print Network

We determine experimentally the angle $\\alpha$ of maximum wave amplitude in the far-field wake behind a vertical surface-piercing cylinder translated at constant velocity $U$ for Bond numbers $Bo_D = D / \\lambda_c$ ranging between 0.1 and 4.2, where $D$ is the cylinder diameter and $\\lambda_c$ the capillary length. In all cases the wake angle is found to follow a Mach-like law at large velocity, $\\alpha \\sim U^{-1}$, but with different scalings depending on the value of $Bo_D$. For small $Bo_D$ (large capillary effects), the wake angle approximately follows the law $\\alpha \\simeq c_{\\rm g,min} / U$, where $c_{\\rm g,min}$ is the minimum group velocity of capillary-gravity waves. For larger $Bo_D$ (weak capillary effects), we recover at large velocity the law $\\alpha \\sim \\sqrt{gD}/U$ recently found for ship wakes [Rabaud and Moisy, "Ship Wakes: Kelvin or Mach Angle?", Phys. Rev. Lett. {\\bf 110}, 214503 (2013)]. Using the general property of dispersive waves that the characteristic wavelength of the wavepacket ...

Moisy, F

2014-01-01

147

Dissipation of turbulence in the wake of a wind turbine  

NASA Astrophysics Data System (ADS)

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

Lundquist, J. K.; Bariteau, L.

2013-12-01

148

Wake Vortex Advisory System (WakeVAS) Concept of Operations  

NASA Technical Reports Server (NTRS)

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

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

2003-01-01

149

Role of the basal ganglia in the control of sleep and wakefulness  

PubMed Central

The basal ganglia (BG) act as a cohesive functional unit that regulates motor function, habit formation, and reward/addictive behaviors; but the debate has only recently started on how the BG maintain wakefulness and suppress sleep to achieve all these fundamental functions of the BG. Neurotoxic lesioning, pharmacological approaches, and the behavioral analyses of genetically modified animals revealed that the striatum and globus pallidus are important for the control of sleep and wakefulness. Here, we discuss anatomical and molecular mechanisms for sleep-wake regulation in the BG and propose a plausible model in which the nucleus accumbens integrates behavioral processes with wakefulness through adenosine and dopamine receptors. PMID:23465424

Lazarus, Michael; Chen, Jiang-Fan; Urade, Yoshihiro; Huang, Zhi-Li

2013-01-01

150

Numerical Simulation of Wakes in a Weakly Stratified Fluid  

E-print Network

This paper describes some preliminary numerical studies using large eddy simulation of full-scale submarine wakes. Submarine wakes are a combination of the wake generated by a smooth slender body and a number of superimposed vortex pairs generated by various control surfaces and other body appendages. For this preliminary study, we attempt to gain some insight into the behavior of full-scale submarine wakes by computing separately the evolution the self-propelled wake of a slender body and the motion of a single vortex pair in both a non-stratified and a stratified environment. An important aspect of the simulations is the use of an iterative procedure to relax the initial turbulence field so that turbulent production and dissipation are in balance.

Rottman, James W; Innis, George E; O'Shea, Thomas T; Novikov, Evgeny

2014-01-01

151

Electromagnetic signature of human cortical dynamics during wakefulness and sleep  

E-print Network

Electromagnetic signature of human cortical dynamics during wakefulness and sleep Signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.5 Spatial reach of LFP & Electromagnetic Lead field . . . . . . . . . . . . . . . 35 2 Studies 45 4 Overview 47 4.1 Electromagnetic properties of the extracellular medium

Destexhe, Alain

152

Brain Wake-Ups  

NSDL National Science Digital Library

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

1997-01-01

153

Direct Numerical Simulation of a Weakly Stratified Turbulent Wake  

NASA Technical Reports Server (NTRS)

Direct numerical simulation (DNS) is used to investigate a time-dependent turbulent wake evolving in a stably stratified background. A large initial Froude number is chosen to allow the wake to become fully turbulent and axisymmetric before stratification affects the spreading rate of the mean defect. The uncertainty introduced by the finite sample size associated with gathering statistics from a simulation of a time-dependent flow is reduced, compared to earlier simulations of this flow. The DNS reveals the buoyancy-induced changes to the turbulence structure, as well as to the mean-defect history and the terms in the mean-momentum and turbulence-kinetic-energy budgets, that characterize the various states of this flow - namely the three-dimensional (essentially unstratified), non-equilibrium (or 'wake-collapse') and quasi-two-dimensional (or 'two-component') regimes observed elsewhere for wakes embedded in both weakly and strongly stratified backgrounds. The wake-collapse regime is not accompanied by transfer (or 'reconversion') of the potential energy of the turbulence to the kinetic energy of the turbulence, implying that this is not an essential feature of stratified-wake dynamics. The dependence upon Reynolds number of the duration of the wake-collapse period is demonstrated, and the effect of the details of the initial/near-field conditions of the wake on its subsequent development is examined.

Redford, J. A.; Lund, T. S.; Coleman, Gary N.

2014-01-01

154

Particle Access and Charging Environments in the Lunar Wake  

NASA Technical Reports Server (NTRS)

A plasma wake a region of low density, high temperature plasma forms on the far side of the Moon when solar wind, magnetosheath, and magnetotail plasma flows past the Moon [Manka, 1973; Ogilvie et al., 1996; Farrell et al., 1998; Halekas et al., 2005]. Ion populations in these flows typically have much smaller thermal velocity than bulk speed and are therefore excluded from the plasma wake while the large thermal electron velocity allows the lighter negatively charged particles to stream ahead of the ions into the wake. Charge separation due to electrons streaming ahead of the ions into the wake from the wake boundary establishes an ambipolar electric field which impedes the motion of electron flow and accelerates ions into the wake [Ogilvie et al., 1996; Farrell et al., 1997]. We have conducted a theoretical study of acceleration (and deceleration) of charged particles in lunar plasma environments, which investigated the mechanisms responsible for allowing solar wind entry into the lunar wake, and for producing energetic particle distributions observed within the lunar wake. To this end, the investigation utilized a macroscale 3D hybrid particle-in-cell numerical model of the interaction of the Moon with external plasma environments to compute electric fields in the lunar environment for a variety of external plasma conditions and interplanetary magnetic field orientations. Ion dynamics were attained from the hybrid code while electron dynamics were determined by considering electron test particle trajectories through the fields established in the hybrid code. Results from the code will be presented to evaluate charging environments within the lunar wake.

Parker, Linda; Minow, Joseph; Singh, Nagendra; Araveti, Venkata S.; Venkiteswaran, Karthik

2010-01-01

155

Statistical Study of the Lunar Plasma Wake Outer Boundary  

NASA Astrophysics Data System (ADS)

The Moon does not have an intrinsic magnetic field and lacks the conductivity necessary to develop an induced magnetosphere. Therefore, the interaction of the Moon with the solar wind is dominated by impact absorption of solar wind particles on the day side and the generation of a plasma wake on the night side. A plasma density gradient forms between the flowing solar wind and the plasma wake, causing solar wind plasma to gradually refill the wake region. Electrons fill the wake first, pulling ions in after them via ambi-polar diffusion. Despite the existence of comprehensive new plasma measurements of the lunar wake region, relatively little attention has been devoted to the shape and variability in location of its outer boundary. Improved knowledge of this boundary condition for the physical processes associated with wake refilling would provide useful tests for simulations and theoretical models of the lunar plasma interaction. The ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) spacecraft mission is a two-probe lunar mission derived from the THEMIS (Time History of Events and Macroscale Interactions During Substorms) mission, repurposed to study the lunar space and planetary environment. Over the course of the mission there have been numerous passes of the ARTEMIS spacecraft through the lunar wake, at distances of up to seven lunar radii from the Moon. They have occurred for a variety of external conditions. We present a statistical study of tens of selected wake-crossing events of the ARTEMIS probes in 2011, using data primarily from the ARTEMIS fluxgate magnetometers (FGMs) and electrostatic analyzers (ESAs) to identify when the spacecraft entered and exited the wake. We study the shape of the outer wake boundary and its response to external conditions using two different techniques: one defines the wake boundary by a sharp decrease in ion density, the other by a decrease in magnetic field magnitude. We investigate how the wake boundary changes in response to solar wind parameters such as plasma beta, ion velocity, ion temperature, and magnetic field cone and clock angles. These results are compared with earlier wake crossing studies and computational modeling.

Ames, W. F.; Brain, D. A.; Poppe, A.; Halekas, J. S.; McFadden, J. P.; Glassmeier, K.; Angelopoulos, V.

2012-12-01

156

Short bunch wake potentials for a chain of TESLA cavities  

NASA Astrophysics Data System (ADS)

The modification of wake fields from a single cavity to a quasi-periodic structure of cavities is of great concern, especially for applications using very short bunches. We extend our former study (Novokhatski, 1997 [1]). A strong modification of wake fields along a train of cavities was clearly found for bunch lengths lower than 1 mm. In particular, the wakes induced by the bunch, as it proceeds down the successive cavities, decrease in amplitude and become more linear around the bunch center, with a profile very close to the integral of the charge density. The loss factor, decreasing also with the number of cells, becomes independent of bunch length for very short bunches and tends asymptotically to a finite value. This nice behavior of wake fields for short bunches presents good opportunity for application of very short bunches in Linear Colliders and X-ray Free Electron Lasers.

Novokhatski, Alexander; Mosnier, Alban

2014-11-01

157

RNA interference suppression of lignin biosynthesis increases fermentable sugar yields for biofuel production from field-grown sugarcane.  

PubMed

The agronomic performance, cell wall characteristics and enzymatic saccharification efficiency of transgenic sugarcane plants with modified lignin were evaluated under replicated field conditions. Caffeic acid O-methyltransferase (COMT) was stably suppressed by RNAi in the field, resulting in transcript reduction of 80%-91%. Along with COMT suppression, total lignin content was reduced by 6%-12% in different transgenic lines. Suppression of COMT also altered lignin composition by reducing syringyl units and p-coumarate incorporation into lignin. Reduction in total lignin by 6% improved saccharification efficiency by 19%-23% with no significant difference in biomass yield, plant height, stalk diameter, tiller number, total structural carbohydrates or brix value when compared with nontransgenic tissue culture-derived or transgenic control plants. Lignin reduction of 8%-12% compromised biomass yield, but increased saccharification efficiency by 28%-32% compared with control plants. Biomass from transgenic sugarcane lines that have 6%-12% less lignin requires approximately one-third of the hydrolysis time or 3- to 4-fold less enzyme to release an equal or greater amount of fermentable sugar than nontransgenic plants. Reducing the recalcitrance of lignocellulosic biomass to saccharification by modifying lignin biosynthesis is expected to greatly benefit the economic competitiveness of sugarcane as a biofuel feedstock. PMID:23551338

Jung, Je Hyeong; Vermerris, Wilfred; Gallo, Maria; Fedenko, Jeffrey R; Erickson, John E; Altpeter, Fredy

2013-08-01

158

Asymptotic behavior of a flat plate wake  

NASA Technical Reports Server (NTRS)

An experimental study has been conducted to investigate the far-field, self-similar properties of a flat plate wake. A plane turbulent wake was generated at the trailing edge of a smooth splitter plate separating two legs of a Mixing Layer Wind Tunnel, with both initial boundary layers tripped. For the present study, both legs were operated at a free-steam velocity in the test section of 15 m/s, giving a Reynolds number based on wake momentum thickness of about 1750. Single profile measurements were obtained at five streamwise locations using a Pitot probe for the mean velocity measurements and a single cross-wire probe for the turbulence data, which included statistics up to third order. The mean flow data indicated a self-similar behavior beyond a streamwise distance equivalent to about 350 wake momentum thicknesses. However, the turbulence data show better collapse beyond a distance equivalent to about 500 momentum thicknesses, with all the measured peak Reynolds stresses achieving constant, asymptotic levels. The asymptotic mean flow behavior and peak primary stress levels agree well with theoretical predictions based on a constant eddy viscosity model. The present data also agree reasonably well with previous measurements, of which only one set extends into the self-similar region. Detailed comparisons with previous data are presented and discussed in this report.

Weygandt, James H.; Mehta, Rabindra D.

1989-01-01

159

Wake in faint television meteors  

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

160

Noise generated by a propeller in a wake  

NASA Technical Reports Server (NTRS)

Propeller performance and noise were measured on two model scale propellers operating in an anechoic flow environment with and without a wake. Wake thickness of one and three propeller chords were generated by an airfoil which spanned the full diameter of the propeller. Noise measurements were made in the relative near field of the propeller at three streamwise and three azimuthal positions. The data show that as much as 10 dB increase in the OASPL results when a wake is introduced into an operating propeller. Performance data are also presented for completeness.

Block, P. J. W.

1984-01-01

161

Wake Sensor Evaluation Program and Results of JFK-1 Wake Vortex Sensor Intercomparisons  

NASA Technical Reports Server (NTRS)

The overall approach should be to: (1) Seek simplest, sufficiently robust, integrated ground based sensor systems (wakes and weather) for AVOSS; (2) Expand all sensor performance cross-comparisons and data mergings in on-going field deployments; and (3) Achieve maximal cost effectiveness through hardware/info sharing. An effective team is in place to accomplish the above tasks.

Barker, Ben C., Jr.; Burnham, David C.; Rudis, Robert P.

1997-01-01

162

Wake-Up Call  

NSDL National Science Digital Library

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

Rubin, Lisa M.; Herreid, Clyde F.

2002-01-01

163

Phantom for assessment of fat suppression in large field-of-view diffusion-weighted magnetic resonance imaging  

NASA Astrophysics Data System (ADS)

We present the development and application of a phantom for assessment and optimization of fat suppression over a large field-of-view in diffusion-weighted magnetic resonance imaging at 1.5 T and 3 T. A Perspex cylinder (inner diameter 185 mm, height 300 mm) which contains a second cylinder (inner diameter 140 mm) was constructed. The inner cylinder was filled with water doped with copper sulphate and sodium chloride and the annulus was filled with corn oil, which closely matches the spectrum and longitudinal relaxation times of subcutaneous abdominal fat. Placement of the phantom on the couch at 45° to the z-axis presented an elliptical cross-section, which was of a similar size and shape to axial abdominal images. The use of a phantom for optimization of fat suppression allowed quantitative comparison between studies without the differences introduced by variability between human subjects. We have demonstrated that the phantom is suitable for selection of inversion delay times, spectral adiabatic inversion recovery delays and assessment of combinatorial methods of fat suppression. The phantom is valuable in protocol development and the assessment of new techniques, particularly in multi-centre trials.

Winfield, J. M.; Douglas, N. H. M.; deSouza, N. M.; Collins, D. J.

2014-05-01

164

Chitin Amendment Increases Soil Suppressiveness toward Plant Pathogens and Modulates the Actinobacterial and Oxalobacteraceal Communities in an Experimental Agricultural Field  

PubMed Central

A long-term experiment on the effect of chitin addition to soil on the suppression of soilborne pathogens was set up and monitored for 8 years in an experimental field, Vredepeel, The Netherlands. Chitinous matter obtained from shrimps was added to soil top layers on two different occasions, and the suppressiveness of soil toward Verticillium dahliae, as well as plant-pathogenic nematodes, was assessed, in addition to analyses of the abundances and community structures of members of the soil microbiota. The data revealed that chitin amendment had raised the suppressiveness of soil, in particular toward Verticillium dahliae, 9 months after the (second) treatment, extending to 2 years following treatment. Moreover, major effects of the added chitin on the soil microbial communities were detected. First, shifts in both the abundances and structures of the chitin-treated soil microbial communities, both of total soil bacteria and fungi, were found. In addition, the abundances and structures of soil actinobacteria and the Oxalobacteraceae were affected by chitin. At the functional gene level, the abundance of specific (family-18 glycoside hydrolase) chitinase genes carried by the soil bacteria also revealed upshifts as a result of the added chitin. The effects of chitin noted for the Oxalobacteraceae were specifically related to significant upshifts in the abundances of the species Duganella violaceinigra and Massilia plicata. These effects of chitin persisted over the time of the experiment. PMID:23811512

Cretoiu, Mariana Silvia; Korthals, Gerard W.; Visser, Johnny H. M.

2013-01-01

165

Observation of multipactor suppression in a dielectric-loaded accelerating structure using an applied axial magnetic field  

SciTech Connect

Efforts by a number of institutions to develop a Dielectric-Loaded Accelerating (DLA) structure capable of supporting high gradient acceleration when driven by an external radio frequency source have been ongoing over the past decade. Single surface resonant multipactor has been previously identified as one of the major limitations on the practical application of DLA structures in electron accelerators. In this paper, we report the results of an experiment that demonstrated suppression of multipactor growth in an X-band DLA structure through the use of an applied axial magnetic field. This represents an advance toward the practical use of DLA structures in many accelerator applications.

Jing, C.; Konecny, R.; Antipov, S. [Euclid Techlabs, LLC, 5900 Harper Rd., Solon, Ohio 44139 (United States) [Euclid Techlabs, LLC, 5900 Harper Rd., Solon, Ohio 44139 (United States); High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Chang, C. [Science and Technology on High Power Microwave Laboratory, Xi'an City 710024 (China) [Science and Technology on High Power Microwave Laboratory, Xi'an City 710024 (China); Institute of Energy, Tsinghua University, Beijing 100084 (China); Gold, S. H. [Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375 (United States)] [Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375 (United States); Schoessow, P.; Kanareykin, A. [Euclid Techlabs, LLC, 5900 Harper Rd., Solon, Ohio 44139 (United States)] [Euclid Techlabs, LLC, 5900 Harper Rd., Solon, Ohio 44139 (United States); Gai, W. [High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)] [High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)

2013-11-18

166

A new methodology for free wake analysis using curved vortex elements  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

167

Binocular Neurons in Parastriate Cortex: Interocular 'Matching' of Receptive Field Properties, Eye Dominance and Strength of Silent Suppression  

PubMed Central

Spike-responses of single binocular neurons were recorded from a distinct part of primary visual cortex, the parastriate cortex (cytoarchitectonic area 18) of anaesthetized and immobilized domestic cats. Functional identification of neurons was based on the ratios of phase-variant (F1) component to the mean firing rate (F0) of their spike-responses to optimized (orientation, direction, spatial and temporal frequencies and size) sine-wave-luminance-modulated drifting grating patches presented separately via each eye. In over 95% of neurons, the interocular differences in the phase-sensitivities (differences in F1/F0 spike-response ratios) were small (?0.3) and in over 80% of neurons, the interocular differences in preferred orientations were ?10°. The interocular correlations of the direction selectivity indices and optimal spatial frequencies, like those of the phase sensitivies and optimal orientations, were also strong (coefficients of correlation r ?0.7005). By contrast, the interocular correlations of the optimal temporal frequencies, the diameters of summation areas of the excitatory responses and suppression indices were weak (coefficients of correlation r ?0.4585). In cells with high eye dominance indices (HEDI cells), the mean magnitudes of suppressions evoked by stimulation of silent, extra-classical receptive fields via the non-dominant eyes, were significantly greater than those when the stimuli were presented via the dominant eyes. We argue that the well documented ‘eye-origin specific’ segregation of the lateral geniculate inputs underpinning distinct eye dominance columns in primary visual cortices of mammals with frontally positioned eyes (distinct eye dominance columns), combined with significant interocular differences in the strength of silent suppressive fields, putatively contribute to binocular stereoscopic vision. PMID:24927276

Wang, Chun; Dreher, Bogdan

2014-01-01

168

Wakes in inhomogeneous plasmas  

E-print Network

The Debye shielding of a charge immersed in a flowing plasma is an old classic problem in plasma physics. It has been given renewed attention in the last two decades in view of experiments with complex plasmas, where charged dust particles are often levitated in a region with strong ion flow. Efforts to describe the shielding of the dust particles in such conditions have been focused on the homogeneous plasma approximation, which ignores the substantial inhomogeneity of the levitation region. We address the role of the plasma inhomogeneity by rigorously calculating the point charge potential in the collisionless Bohm sheath. We demonstrate that the inhomogeneity can dramatically modify the wake, making it non-oscillatory and weaker.

Kompaneets, Roman; Nosenko, Vladimir; Morfill, Gregor E

2014-01-01

169

WEST SIDE OF WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

WEST SIDE OF WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHEAST AT SOUTHWEST CORNER SHOWING OVERHANGS (01/02/2008) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

170

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING SOUTHEAST AT NORTHWEST ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING SOUTHEAST AT NORTHWEST CORNER FROM ACROSS TARMAC (12/25/2007) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

171

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHWEST AT SOUTHEAST ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHWEST AT SOUTHEAST CORNER OF LOBBY OF BUILDING (12/29/2007) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

172

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHEAST AT WEST ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHEAST AT WEST SIDE SHOWING FLAG, GUN, ENGINES (12/29/2007) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

173

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHEAST AT SOUTHWEST ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING NORTHEAST AT SOUTHWEST CORNER FROM BEHIND CONTROL TOWER (12/28/2007) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

174

SOUTHWEST CORNER OF WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

SOUTHWEST CORNER OF WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING EAST AT WEST FAÇADE WITH SCALE POLE (01/02/2008) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

175

An optically modulated zero-field atomic magnetometer with suppressed spin-exchange broadening  

E-print Network

We demonstrate an optically pumped $^{87}$Rb magnetometer in a microfabricated vapor cell based on a zero-field dispersive resonance generated by optical modulation of the $^{87}$Rb ground state energy levels. The magnetometer is operated in the spin-exchange relaxation-free regime where high magnetic field sensitivities can be achieved. This device can be useful in applications requiring array-based magnetometers where radio frequency magnetic fields can induce cross-talk among adjacent sensors or affect the source of the magnetic field being measured.

Jimenez-Martinez, Ricardo; Kitching, John

2014-01-01

176

An optically modulated zero-field atomic magnetometer with suppressed spin-exchange broadening.  

PubMed

We demonstrate an optically pumped (87)Rb magnetometer in a microfabricated vapor cell based on a zero-field dispersive resonance generated by optical modulation of the (87)Rb ground state energy levels. The magnetometer is operated in the spin-exchange relaxation-free regime where high magnetic field sensitivities can be achieved. This device can be useful in applications requiring array-based magnetometers where radio frequency magnetic fields can induce cross-talk among adjacent sensors or affect the source of the magnetic field being measured. PMID:24784676

Jiménez-Martínez, R; Knappe, S; Kitching, J

2014-04-01

177

An optically modulated zero-field atomic magnetometer with suppressed spin-exchange broadening  

NASA Astrophysics Data System (ADS)

We demonstrate an optically pumped 87Rb magnetometer in a microfabricated vapor cell based on a zero-field dispersive resonance generated by optical modulation of the 87Rb ground state energy levels. The magnetometer is operated in the spin-exchange relaxation-free regime where high magnetic field sensitivities can be achieved. This device can be useful in applications requiring array-based magnetometers where radio frequency magnetic fields can induce cross-talk among adjacent sensors or affect the source of the magnetic field being measured.

Jiménez-Martínez, R.; Knappe, S.; Kitching, J.

2014-04-01

178

Imaging analysis of clock neurons reveals light buffers the wake-promoting effect of dopamine  

Microsoft Academic Search

How animals maintain proper amounts of sleep yet remain flexible to changes in environmental conditions remains unknown. We found that environmental light suppressed the wake-promoting effects of dopamine in fly brains. The ten large lateral-ventral neurons (l-LNvs), a subset of clock neurons, are wake-promoting and respond to dopamine, octopamine and light. Behavioral and imaging analyses suggested that dopamine is a

Yuhua Shang; Paula Haynes; Nicolás Pírez; Kyle I Harrington; Fang Guo; Jordan Pollack; Pengyu Hong; Leslie C Griffith; Michael Rosbash

2011-01-01

179

Comparison of spatial integration and surround suppression characteristics in spiking activity and the local field potential  

E-print Network

potential (LFP), containing low-frequency fluctuations in the electrical potential generated by local on the interpretation of human fMRI BOLD data and on our understanding of the mechanisms of local field potential excitatory cells and inhibitory interneurons. The functional significance of the LFP has been demonstrated

Thiele, Alexander

180

Suppressing Multi-Channel Ultra-Low-Field MRI Measurement Noise Using Data Consistency and Image Sparsity  

PubMed Central

Ultra-low-field (ULF) MRI (B0?=?10–100 µT) typically suffers from a low signal-to-noise ratio (SNR). While SNR can be improved by pre-polarization and signal detection using highly sensitive superconducting quantum interference device (SQUID) sensors, we propose to use the inter-dependency of the k-space data from highly parallel detection with up to tens of sensors readily available in the ULF MRI in order to suppress the noise. Furthermore, the prior information that an image can be sparsely represented can be integrated with this data consistency constraint to further improve the SNR. Simulations and experimental data using 47 SQUID sensors demonstrate the effectiveness of this data consistency constraint and sparsity prior in ULF-MRI reconstruction. PMID:23626710

Lin, Fa-Hsuan; Vesanen, Panu T.; Hsu, Yi-Cheng; Nieminen, Jaakko O.; Zevenhoven, Koos C. J.; Dabek, Juhani; Parkkonen, Lauri T.; Simola, Juha; Ahonen, Antti I.; Ilmoniemi, Risto J.

2013-01-01

181

Wake potentials of the ILC Interaction Region  

SciTech Connect

The vacuum chamber of the ILC Interaction Region (IR) is optimized for best detector performance. It has special shaping to minimize additional backgrounds due to the metal part of the chamber. Also, for the same reason this thin vacuum chamber does not have water cooling. Therefore, small amounts of power, which may be deposited in the chamber, can be enough to raise the chamber to a high temperature. One of the sources of 'heating' power is the electromagnetic field of the beam. This field diffracts by non-regularities of the beam pipe and excites free-propagating fields, which are then absorbed by the pipe wall. In addition we have a heating power of the image currents due to finite conductivity of the metallic wall. We will discuss these effects as updating the previous results. The conclusions of this report are: (1) The amount of the beam energy loss in IR is almost equal to the energy loss in one ILC (TESLA) accelerating cryo-module; (2) Addition energy spread at IR is very small; (3) Spectrum of the wake fields is limited 300 GHz; (4) Average power of the wake fields excited in IR is 30 W for nominal ILC parameters; and (5) Pulse power in this case is 6 kilowatts.

Novokhatski, A.; /SLAC

2011-08-16

182

Suppression of hidden order in URu2Si2 under pressure and restoration in magnetic field  

NASA Astrophysics Data System (ADS)

We describe here recent inelastic neutron scattering experiments on the heavy fermion compound URu2Si2 realized in order to clarify the nature of the hidden order (HO) phase which occurs below T0 = 17.5 K at ambient pressure. The choice was to measure at a given pressure P where the system will go, by lowering the temperature, successively from paramagnetic (PM) to HO and then to antiferromagnetic phase (AF). Furthermore, in order to verify the selection of the pressure, a macroscopic detection of the phase transitions was also achieved in situ via its thermal expansion response detected by a strain gauge glued on the crystal. Just above Px = 0.5 GPa, where the ground state switches from HO to AF, the Q0 = (1,0,0) excitation disappears while the excitation at the incommensurate wavevector Q1 = (1.4, 0, 0) remains. Thus, the Q0 = (1, 0, 0) excitation is intrinsic only in the HO phase. This result is reinforced by studies where now pressure and magnetic field H can be used as tuning variable. Above Px, the AF phase at low temperature is destroyed by a magnetic field larger than HAF (collapse of the AF Q0 = (1, 0,0) Bragg reflection). The field reentrance of the HO phase is demonstrated by the reappearance of its characteristic Q0 = (1, 0, 0) excitation. The recovery of a PM phase will only be achieved far above HAF at HM approx 35 T. To determine the P-H-T phase diagram of URu2Si2, macroscopic measurements of the thermal expansion were realized with a strain gauge. The reentrant magnetic field increases strongly with pressure. Finally, to investigate the interplay between superconductivity (SC) and spin dynamics, new inelastic neutron scattering experiments are reported down to 0.4 K, far below the superconducting critical temperature TSC approx 1.3 K as measured on our crystal by diamagnetic shielding.

Hassinger, E.; Aoki, D.; Bourdarot, F.; Knebel, G.; Taufour, V.; Raymond, S.; Villaume, A.; Flouquet, J.

2010-11-01

183

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

SciTech Connect

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

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

2010-07-15

184

Distributed control in a mean-field cortical network model: Implications for seizure suppression  

NASA Astrophysics Data System (ADS)

Brain electrical stimulation (BES) has long been suggested as a means of controlling pathological brain activity. In epilepsy, control of a spatially localized source, the seizure focus, may normalize neuronal dynamics. Consequently, most BES research has been directed at controlling small, local, neuronal populations. At a higher level, pathological seizure activity can be viewed as a network event that may begin without a clear spatial focus or in multiple sites and spread rapidly through a distributed cortical network. In this paper, we begin to address the implications of local control in a network scenario. To do so, we explore the efficacy of local BES when deployed over a larger-scale neuronal network, for instance, using a grid of stimulating electrodes on the cortex. By introducing a mean-field model of neuronal interactions we are able to identify limitations in network controllability based on physiological constraints that suggest the need for more nuanced network control strategies.

Ching, ShiNung; Brown, Emery N.; Kramer, Mark A.

2012-08-01

185

A Study of Wake Development and Structure in Constant Pressure Gradients  

NASA Technical Reports Server (NTRS)

Motivated by the application to high-lift aerodynamics for commercial transport aircraft, a systematic investigation into the response of symmetric/asymmetric planar turbulent wake development to constant adverse, zero, and favorable pressure gradients has been conducted. The experiments are performed at a Reynolds number of 2.4 million based on the chord of the wake generator. A unique feature of this wake study is that the pressure gradients imposed on the wake flow field are held constant. The experimental measurements involve both conventional LDV and hot wire flow field surveys of mean and turbulent quantities including the turbulent kinetic energy budget. In addition, similarity analysis and numerical simulation have also been conducted for this wake study. A focus of the research has been to isolate the effects of both pressure gradient and initial wake asymmetry on the wake development. Experimental results reveal that the pressure gradient has a tremendous influence on the wake development, despite the relatively modest pressure gradients imposed. For a given pressure gradient, the development of an initially asymmetric wake is different from the initially symmetric wake. An explicit similarity solution for the shape parameters of the symmetric wake is obtained and agrees with the experimental results. The turbulent kinetic energy budget measurements of the symmetric wake demonstrate that except for the convection term, the imposed pressure gradient does not change the fundamental flow physics of turbulent kinetic energy transport. Based on the turbulent kinetic energy budget measurements, an approach to correct the bias error associated with the notoriously difficult dissipation estimate is proposed and validated through the comparison of the experimental estimate with a direct numerical simulation result.

Thomas, Flint O.; Nelson, R. C.; Liu, Xiaofeng

2000-01-01

186

The computation of induced drag with nonplanar and deformed wakes  

NASA Technical Reports Server (NTRS)

The classical calculation of inviscid drag, based on far field flow properties, is reexamined with particular attention to the nonlinear effects of wake roll-up. Based on a detailed look at nonlinear, inviscid flow theory, it is concluded that many of the classical, linear results are more general than might have been expected. Departures from the linear theory are identified and design implications are discussed. Results include the following: Wake deformation has little effect on the induced drag of a single element wing, but introduces first order corrections to the induced drag of a multi-element lifting system. Far field Trefftz-plane analysis may be used to estimate the induced drag of lifting systems, even when wake roll-up is considered, but numerical difficulties arise. The implications of several other approximations made in lifting line theory are evaluated by comparison with more refined analyses.

Kroo, Ilan; Smith, Stephen

1991-01-01

187

Improving actuator disk wake model  

NASA Astrophysics Data System (ADS)

The wind energy industry has traditionally relied on simple wake models for estimating Wind Turbine (WT) wake losses. Despite limitations, low requirements in terms of detailed rotor information makes their use feasible, unlike more complex models, such as Blade Element Method (BEM) or Actuator Line. Froude's Actuator Disk (AD) does not suffer the simpler model's limitation of prescribing the wake through a closed set of equations, while sharing with them the low rotor data requirements. On the other hand they require some form of parametrization to close the model and calculate total thrust acting on the flow. An Actuator Disk model was developed, using an iterative algorithm based on Froude's one-dimensional momentum theory to determine the WT's performance, proving to be successful in estimating the performance of both machines in undisturbed flow and in the wake of an upstream machines. Before Froude's AD limitations compared to more complex rotor models, load distributions emulating those of a BEM model were tested. The results show that little impact is obtained at 3 rotor diameters downstream and beyond, agreeing with common definition of a far-wake that starts at 1-2 diameters downstream, where rotor characteristics become negligible and atmospheric flow effects dominate.

Costa Gomes, V. M. M. G.; Palma, J. M. L. M.; Silva Lopes, A.

2014-06-01

188

Aerodynamic interaction between vortical wakes and the viscous flow about a circular cylinder  

NASA Technical Reports Server (NTRS)

In the design analysis of conventional aircraft configurations, the prediction of the strong interaction between vortical wakes and the viscous flow field about bodies is of considerable importance. Interactions between vortical wakes and aircraft components are even more common on rotorcraft and configurations with lifting surfaces forward of the wing. An accurate analysis of the vortex-wake interaction with aircraft components is needed for the optimization of the payload and the reduction of vibratory loads. However, the three-dimensional flow field beneath the rotor disk and the interaction of the rotor wake with solid bodies in the flow field are highly complex. The present paper has the objective to provide a basis for the considered interactions by studying a simpler problem. This problem involves the two-dimensional interaction of external wakes with the viscous flow about a circular cylinder.

Stremel, P. M.

1985-01-01

189

Suppression of ringing in the tuned input circuit of a SQUID detector used in low-field NMR measurements  

NASA Astrophysics Data System (ADS)

In low-field NMR measurements, we employ a high temperature superconducting quantum interference device (SQUID) as a detector with an inductively coupled liquid-nitrogen-cooled LC tuned input circuit. However, ringing across the LC circuit appears after the sudden switch-off of the prepolarizing magnetic field. This ringing leads to instability of the SQUID readout and prevents the acquisition of short-relaxation-time signals. We developed and tested two simple and effective FET-based Q switch circuits with adjustable parameters which suppress the ringing. Each of these Q switches makes it possible to record free induction decay signals with a Larmor frequency of 1.2 kHz and an effective relaxation time constant of 30 ms. A gradually changing current caused by the release of charges stored in the p-n junction of the FET, which delays the Q value recovery of the LC circuit, can only be observed by the SQUID because of its frequency-independent sensitivity.

Dong, Hui; Zhang, Yi; Krause, Hans-Joachim; Xie, Xiaoming; Braginski, Alex I.; Offenhäusser, Andreas

2009-12-01

190

Suppression of Amblyomma americanum (Ixodida: Ixodidae) for short-term field operations utilizing cypermethrin and lambda-cyhalothrin.  

PubMed

Tick-borne diseases pose significant risks to U.S. military personnel who conduct operations, both domestic and abroad. To determine the feasibility of protecting personnel from tick vectors during short-term field deployments, acaricides cypermethrin (Demon WP, Syngenta, Greensboro, NC) and lambda-cyhalothrin (Surrender Pestabs, CSI, Pasadena, TX) were applied to plots within two separate field sites on Camp Blanding Joint Training Center in Starke, FL, from May to June 2011. We analyzed their effectiveness in reducing tick counts for 6 wk after application. In total, 8,193 ticks were identified and counted, of which > 99% were a mix of nymphs and adult-stage Amblyomma americanum (L.). Our results indicate that both cypermethrin and lambda-cyhalothrin were effective in significantly reducing tick numbers and preventing entry into treated plots for 6 wk after application. Thus, these two acaracides can be used to effectively suppress tick populations and provide residual protection in small geographic areas of recreation or public health significance. PMID:24897866

Hughes, Tony H; Richardson, Alec G; Hoel, David F; Mejeoumov, Tracy; Farooq, Mohammad; Stoops, Craig A

2014-05-01

191

TESLA Report 2003-19 THE SHORT-RANGE TRANSVERSE WAKE  

E-print Network

TESLA Report 2003-19 THE SHORT-RANGE TRANSVERSE WAKE FUNCTION FOR TESLA ACCELERATING STRUCTURE T of a Free Electron Laser in TESLA project requires very short bunches. It results in a very long interaction calculate the short-range transverse wakefields of the TESLA linac accelerating structure. Wake fields

192

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

NASA Technical Reports Server (NTRS)

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

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

1999-01-01

193

Canopy wake measurements using multiple scanning wind LiDARs  

NASA Astrophysics Data System (ADS)

Canopy wakes have been shown, in controlled wind tunnel experiments, to significantly affect the fluxes of momentum, heat and other scalars at the land and water surface over distances of ~O(1 km), see Markfort et al. (EFM, 2013). However, there are currently no measurements of the velocity field downwind of a full-scale forest canopy. Point-based anemometer measurements of wake turbulence provide limited insight into the extent and details of the wake structure, whereas scanning Doppler wind LiDARs can provide information on how the wake evolves in space and varies over time. For the first time, we present measurements of the velocity field in the wake of a tall patch of forest canopy. The patch consists of two uniform rows of 35-meter tall deciduous, plane trees, which border either side of the Allée de Dorigny, near the EPFL campus. The canopy is approximately 250 m long, and it is 35 m wide, along the direction of the wind. A challenge faced while making field measurements is that the wind rarely intersects a canopy normal to the edge. The resulting wake flow may be deflected relative to the mean inflow. Using multiple LiDARs, we measure the evolution of the wake due to an oblique wind blowing over the canopy. One LiDAR is positioned directly downwind of the canopy to measure the flow along the mean wind direction and the other is positioned near the canopy to evaluate the transversal component of the wind and how it varies with downwind distance from the canopy. Preliminary results show that the open trunk space near the base of the canopy results in a surface jet that can be detected just downwind of the canopy and farther downwind dissipates as it mixes with the wake flow above. A time-varying recirculation zone can be detected by the periodic reversal of the velocity vector near the surface, downwind of the canopy. The implications of canopy wakes for measurement and modeling of surface fluxes will be discussed.

Markfort, Corey D.; Carbajo Fuertes, Fernando; Valerio Iungo, Giacomo; Stefan, Heinz; Porté-Agel, Fernando

2014-05-01

194

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

2012-12-01

195

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

PubMed Central

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

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

2012-01-01

196

Wakes in Inertial Fusion Plasmas  

NASA Astrophysics Data System (ADS)

Plasma wave wakes, which are the collective oscillatory response near the plasma frequency to the propagation of particles or electromagnetic waves through a plasma, play a critical role in many plasma processes. New results from backwards stimulated Raman scattering (BSRS), in which wakes with phase velocities much less than the speed of light are induced by the beating of counter-propagating light waves, and from electron beam stopping, in which the wakes are produced by the motion of relativistically propagating electrons through the dense plasma, are discussed. Both processes play important roles in Inertial Confinement Fusion (ICF). In BSRS, laser light is scattered backwards out of the plasma, decreasing the energy available to compress the ICF capsule and affecting the symmetry of where the laser energy hits the hohlraum wall in indirect drive ICF. The plasma wave wake can also generate superthermal electrons that can preheat the core and/or the ablator. Electron beam stopping plays a critical role in the Fast Ignition (FI) ICF concept, in which a beam of relativistic electrons is used to heat the target core to ignition temperatures after the compression stage. The beam stopping power determines the effectiveness of the heating process. This dissertation covers new discoveries on the importance of plasma wave wakes in both BSRS and electron beam stopping. In the SRS studies, 1D particle-in-cell (PIC) simulations using OSIRIS are performed, which model a short-duration (˜500/?0 --1FWHM) counter-propagating scattered light seed pulse in the presence of a constant pump laser with an intensity far below the absolute instability threshold for plasma waves undergoing Landau damping. The seed undergoes linear convective Raman amplification and dominates over the amplification of fluctuations due to particle discreteness. The simulation results are in good agreement with results from a coupled-mode solver when special relativity and the effects of finite size PIC simulation particles are accounted for. Linear gain spectra including both effects are discussed. Extending the PIC simulations past when the seed exits the simulation domain reveals bursts of large-amplitude scattering in many cases, which do not occur in simulations without the seed pulse. These bursts can have amplitudes several times greater than the amplified seed pulse, and an examination of the orbits of particles trapped in the wake illustrates that the bursts are caused by a reduction of Landau damping due to particle trapping. This large-amplitude scattering is caused by the seed inducing a wake earlier in the simulation, thus modifying the distribution function. Performing simulations with longer duration seeds leads to parts of the seeds reaching amplitudes several times more than the steady-state linear theory results, similarly caused by a reduction of Landau damping. Simulations with continuous seeds demonstrate that the onset of inflation depends on the seed wavelength and incident intensity, and oscillations in the reflectivity are observed at a frequency equal to the difference between the seed frequency and the frequency at which the inflationary SRS grows. In the electron beam stopping studies, 3D PIC simulations are performed of relativistic electrons with a momentum of 10mec propagating in a cold FI core plasma. Some of the simulations use one simulation particle per real particle, and particle sizes much smaller than the interparitcle spacing. The wake made by a single electron is compared against that calculated using cold fluid theory assuming the phase velocity of the wake is near the speed of light. The results agree for the first wavelength of the wake. However, the shape of the wake changes for succeeding wavelengths and depends on the background plasma temperature, with the concavity pointing in the direction the electron is moving in cold plasmas and in the opposite direction as the plasma temperature increases. In the warm plasma the curvature is described by electrostatic Vlasov theory (for vparticle >> vth) and is due

Ellis, Ian Norman

197

Reexamining X-mode suppression and fine structure in artificial E region field-aligned plasma density irregularities  

NASA Astrophysics Data System (ADS)

Artificial field-aligned plasma density irregularities (FAIs) were generated in the E region of the ionosphere above the High Frequency Active Auroral Research Program facility during campaigns in May and August of 2012 and observed using a 30 MHz coherent scatter radar imager in Homer, Alaska. The purpose of this ionospheric modification experiment was to measure the threshold pump power required to excite thermal parametric instabilities by O-mode heating and to investigate the suppression of the FAIs by simultaneous X-mode heating. We find that the threshold pump power for irregularity excitation was consistent with theoretical predictions and increased by approximately a factor of 2 when X-mode heating was present. A modified version of the Another Model of the Ionosphere (SAMI2) ionospheric model was used to simulate the threshold experiments and suggested that the increase was entirely due to enhanced D region absorption associated with X-mode heating. Additionally, a remarkable degree of fine structure possibly caused by natural gradient drift instability in the heater-modified volume was observed in experiments performed during geomagnetically active conditions.

Miceli, R. J.; Hysell, D. L.; Munk, J.; McCarrick, M.; Huba, J. D.

2013-09-01

198

Magnetic Fluctuations and Turbulence in the Venus Magnetosheath and Wake  

E-print Network

Recent research has shown that distinct physical regions in the Venusian induced magnetosphere are recognizable from the variations of strength and of wave/fluctuation activity of the magnetic field. In this paper the statistical properties of magnetic fluctuations are investigated in the Venusian magnetosheath, terminator, and wake regions. The latter two regions were not visited by previous missions. We found 1/f fluctuations in the magnetosheath, large-scale structures near the terminator and more developed turbulence further downstream in the wake. Location independent short-tailed non-Gaussian statistics was observed.

Z. Vörös; T. L. Zhang; M. P. Leubner; M. Volwerk; M. Delva; W. Baumjohann; K. Kudela

2008-06-11

199

Analytical model of rotor wake aerodynamics in ground effect  

NASA Technical Reports Server (NTRS)

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

Saberi, H. A.

1983-01-01

200

Analytical and experimental studies of wakes behind circularly capped bubbles  

Microsoft Academic Search

The wakes behind circularly capped bubbles are examined by means of an experimental and analytical study. A single two-inch diameter bubble is injected into a six by three foot fluid column, one half inch thick, producing an essentially two-dimensional flow. Aspirin powder placed in the fluid column just prior to bubble release highlights the structure of the flow field before

W. F. Bessler

1984-01-01

201

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

202

Passive propulsion in vortex wakes  

Microsoft Academic Search

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

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

2006-01-01

203

Cooling Signs in Wake Debate  

ERIC Educational Resources Information Center

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

Samuels, Christina A.

2011-01-01

204

CONTROL OF SLEEP AND WAKEFULNESS  

PubMed Central

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

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

2013-01-01

205

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

NASA Technical Reports Server (NTRS)

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

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

1998-01-01

206

Effect of wakes on land-atmosphere fluxes  

NASA Astrophysics Data System (ADS)

Wakes affect land-atmosphere fluxes of momentum and scalars, including water vapor and trace gases. Canopies and bluff bodies, including forests, buildings and topography, cause boundary layer flow separation, significantly extend flow recovery, and lead to a break down of standard Monin-Obukhov similarity relationships in the atmospheric boundary layer (ABL). Wakes generated by these land surface features persist for significant distances affecting a large fraction of the Earth's terrestrial surface. This effect is currently not accounted for in land-atmosphere modeling, and little is known about how heterogeneity of wake-generating features effect land surface fluxes. Additionally flux measurements, made in wake-affected regions, do not satisfy the homogeneous requirements for the standard eddy correlation (EC) method. This phenomenon often referred to as sheltering has been shown to affect momentum and kinetic energy fluxes into lakes from the atmosphere (Markfort et al. 2010). This presentation will highlight results from controlled wind tunnel experiments of neutral and thermally stratified boundary layers, using PIV and custom x-wire/cold-wire anemometry, designed to understand how the physical structure of upstream bluff bodies or porous canopies and thermal stability affect the separation zone, boundary layer recovery and surface fluxes. We also compare these results to field measurements taken with a Doppler LiDAR in the wake of a canopy and a building. We have found that there is a nonlinear relationship between porosity and flow separation behind a canopy to clearing transition. Results will provide the basis for new parameterizations to account for wake effects on land-atmosphere fluxes and corrections for EC measurements over open fields, lakes, and wetlands.

Markfort, C. D.; Zhang, W.; Porte-Agel, F.; Stefan, H. G.

2011-12-01

207

Absolute instability of the Gaussian wake profile  

NASA Technical Reports Server (NTRS)

Linear parallel-flow stability theory has been used to investigate the effect of viscosity on the local absolute instability of a family of wake profiles with a Gaussian velocity distribution. The type of local instability, i.e., convective or absolute, is determined by the location of a branch-point singularity with zero group velocity of the complex dispersion relation for the instability waves. The effects of viscosity were found to be weak for values of the wake Reynolds number, based on the center-line velocity defect and the wake half-width, larger than about 400. Absolute instability occurs only for sufficiently large values of the center-line wake defect. The critical value of this parameter increases with decreasing wake Reynolds number, thereby indicating a shrinking region of absolute instability with decreasing wake Reynolds number. If backflow is not allowed, absolute instability does not occur for wake Reynolds numbers smaller than about 38.

Hultgren, Lennart S.; Aggarwal, Arun K.

1987-01-01

208

Wake shed by an accelerating carangiform fish  

NASA Astrophysics Data System (ADS)

We reveal an important fact that momentum change observed in the wake of an accelerating carangiform fish does not necessarily elucidate orientations of propulsive forces produced. An accelerating Crucian Carp (Carassius auratus) was found to shed a wake with net forward fluid momentum, which seemed drag-producing. Based on Newton's law, however, an accelerating fish is expected to shed a thrust wake with net rearward fluid momentum, rather than a drag wake. The unusual wake pattern observed is considered to be resulted primarily from the effect of pressure gradient created by accelerating movements of the fish. Ambient fluids tend to be sucked into low pressure zones behind an accelerating fish, resulting in forward orientations of jets recognizable in the wake. Accordingly, as to an accelerating fish, identifying force orientations from the wake requires considering also the effect of pressure gradient.

Ting, Shang-Chieh; Yang, Jing-Tang

2008-11-01

209

Planetary Ion fluxes in the Venus Wake  

NASA Astrophysics Data System (ADS)

Measurements conducted with the ASPERA-4 instrument and the magnetometer of the Venus Express spacecraft show that the kinetic pressure of planetary O+ ions measured in the Venus wake can be significantly larger than the local magnetic pressure and, as a result, those ions are not being driven by magnetic forces but by the kinetic energy of the solar wind. Beams of planetary O+ ions with those properties have been detected in several orbits of the Venus Express through the wake as the spacecraft traverses by the noon-midnight plane along its near polar trajectory. Peak values of the kinetic pressure of the O+ ions are sufficient to produce superalfvenic flow conditions. It is suggested that such O+ ion beams are eroded from the magnetic polar regions of the Venus ionosphere where the solar wind carves out plasma channels that extend downstream from those regions. Issues related to the acceleration of planetary ions as the solar wind interacts with the Venus ionosphere are related to the energetics of the plasma. When the kinetic pressure of the particle populations involved in the interaction is smaller than the local magnetic pressure the latter will be dominant and hence the particles will follow trajectories dictated by the magnetic field. Such conditions should occur by the magnetic barrier that is formed over the dayside Venus ionosphere where the interplanetary magnetic fluxes pile up thus leading to enhanced values of the magnetic field intensity. Different conditions are expected when the kinetic pressure of the plasma is larger than the local magnetic pressure. In this case the latter will be convected by the particle fluxes as it occurs in the superalfvenic solar wind. Plasma conditions applicable to the planetary ions that stream in the Venus wake and that have been removed from the Venus ionosphere can be examined using the plasma and magnetic field data obtained from the Venus Express (VEX) measurements. A suitable example is provided by the plasma and the magnetic pressure profiles that were obtained from the data in orbit 123 on August 22-2006 and that are reproduced in Figure 1. The profiles in the lower panel show that the peak kinetic pressure of the O+ ions becomes substantially larger than the local magnetic pressure (between 01:48 UT and 02:00 UT) and also that within a wide region of the wake (between ~02:00 UT and ~02:25 UT) the kinetic pressure becomes smaller than the magnetic pressure. Values of the ratio of the kinetic to the magnetic pressure that are obtained from both profiles are given in the upper panel to show that in the region where the peak kinetic pressure of the O+ ions are measured that ratio is substantially larger than one thus indicating that the local ions move under superalfvenic conditions. The opposite is true in other regions of the wake where values of that ratio are smaller than one and thus the plasma is subalfvenic.

Pérez-de-Tejada, H.; Lundin, R.; Durand-Manterola, H.; Barabash, S.; Zhang, T. L.; Sauvaud, J. A.; Reyes-Ruiz, M.

2012-09-01

210

Brain mechanisms that control sleep and waking  

NASA Astrophysics Data System (ADS)

This review paper presents a brief historical survey of the technological and early research that laid the groundwork for recent advances in sleep-waking research. A major advance in this field occurred shortly after the end of World War II with the discovery of the ascending reticular activating system (ARAS) as the neural source in the brain stem of the waking state. Subsequent research showed that the brain stem activating system produced cortical arousal via two pathways: a dorsal route through the thalamus and a ventral route through the hypothalamus and basal forebrain. The nuclei, pathways, and neurotransmitters that comprise the multiple components of these arousal systems are described. Sleep is now recognized as being composed of two very different states: rapid eye movements (REMs) sleep and non-REM sleep. The major findings on the neural mechanisms that control these two sleep states are presented. This review ends with a discussion of two current views on the function of sleep: to maintain the integrity of the immune system and to enhance memory consolidation.

Siegel, Jerome

211

Negative wake behind a sphere rising in viscoelastic fluids: a lattice Boltzmann investigation.  

PubMed

We investigate the complex flow field around a sphere rising in a Maxwell fluid by means of the lattice Boltzmann simulation to provide insights into the strange negative wake experimentally observed behind a bubble or particle in non Newtonian fluids. The influence of the rise velocity, sphere diameter, and fluid's rheology is considered through two dimensionless numbers: the Deborah number De and the Reynolds number Re. Our simulation shows that the negative wake appears behind the sphere when De>2 . On the other hand, the shape of the negative wake described by the opening angle theta of the upward flow cone surrounding the negative wake is mainly determined by the Reynolds number Re. These results reveal that the physical origin of the negative wake stems mainly from the competition between the elastic and viscous stresses in the fluid. PMID:17279993

Frank, Xavier; Li, Huai Z

2006-11-01

212

Wind tunnel measurements in the wake of a simple structure in a simulated atmospheric flow  

NASA Technical Reports Server (NTRS)

Measurements of longitudinal mean velocity and turbulence intensity were made in the wake of a rectangular model building in a simulated atmospheric boundary-layer wind. The model building was a 1:50 scale model of a structure used in a wake measurement program at the George C. Marshall Space Flight Center 8-tower boundary-layer facility. The approach wind profile and measurement locations were chosen to match the field site conditions. The wakes of the building in winds from azimuths of 0 and 47 degrees referenced to the normal to the building long axis were examined. The effect of two lines of trees upwind of the building on the wake and the importance of the ratio of the building height to boundary-layer thickness on the extent of the wake were determined.

Hansen, A. C.; Peterka, J. A.; Cermak, J. E.

1975-01-01

213

Experimental investigation about the effect of non-axisymmetric wake impact on a low speed axial compressor  

Microsoft Academic Search

Non-axisymmetric wake impact experiments were carried out after the best exciting frequency for a low speed axial compressor\\u000a had been found by axisymmetric wake impact experiments. When the number and circumferential distribution of inlet guide vanes\\u000a (IGV) are logical the wakes of non-axisymmetric IGVs can exert beneficial unsteady exciting effect on their downstream rotor\\u000a flow fields and improve the compressor’s

Jianyong Liu; Yajun Lu; Zhiping Li

2010-01-01

214

Wake effects in a Fayette 95-IIS wind turbine array  

SciTech Connect

A group of 35 wind turbines on the Castello Ranch in Altamont Pass, California, was investigated to quantify array wake effects (losses in energy production due to operation of upwind turbines) and the factors influencing them. Approximately 65 hours of field measurements were made in summer 1986, with turbine energy production and wind velocity data recorded for various scenarios of array operation. Customized software and hardware were developed and installed by Fayette to facilitate these measurements. The existence of wake effects was fairly well established. Relative energy-production losses averaged 6% at the second row, when the first row was operating, and 7 to 8% at the third row, when the first two were operating. Apparently, then, the impact of the first row on the third (at a 21-rotor-diameter distance) was minimal. Ambient wind speed did not appear to affect the relative wind speed pattern within the array due to wakes, but because of the shape of the performance curve, it did affect relative energy production losses (particularly for the low-RPM mode of machine operation). The influences of ambient atmospheric conditions, such as stability, turbulence, and shear on the array wakes, were also investigated by testing over a range of the conditions available during a typical 24-hour day at the test site. None of these variables showed any significant effect on the degree of wake-induced energy losses. While the results of this study apply only to this specific array and type of wind turbine, the methodology could be applied to study wake effects at other wind farms. 6 refs., 7 figs., 20 tabs.

Simon, R.L.; Matson, D.F.; Fuchs, J.M.

1987-09-01

215

Flow and turbulence conditions in the wake of a H-section in cross flow  

NASA Astrophysics Data System (ADS)

Both steady and unsteady wakes of a torsional aeroelastic wind energy converter are examined. The converter consists of a H-section prism with attached pendulums for adjusting the natural frequency of the system. To provide an understanding of the wake characteristics of the device, a particle image velocimetry system was used to measure the mean and fluctuating velocity fields from which various single-point statistics were calculated. For the steady case, wake profiles were obtained at angles of attack from -30° to +30° (at 5° intervals) with the H-section fixed at each angle. In the unsteady case, the H-section was free to vibrate due to flow-structure interaction and a torsional vibration was produced with angles of attack from -30° to +30°. Turbulent wake profiles obtained for the H-section at fixed angles of attack are compared with the oscillating cases and the differences between the various single-point turbulence velocity statistics are discussed. The presented results indicate that the overall dynamic wake deficit for the oscillating case is less than the static wake deficit. Furthermore, the dynamic wake deficit, which is about 20% of U? is dispersed across the entire section; whereas, the static wake is concentrated around the trailing edge side of the converter.

Schmit, R. F.; Glauser, M. N.; Ahmadi, G.

2004-02-01

216

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.

217

TWO-CHANNEL DIELECTRIC WAKE FIELD ACCELERATOR  

SciTech Connect

Experimental results are reported for test beam acceleration and deflection in a two-channel, cm-scale, rectangular dielectric-lined wakefield accelerator structure energized by a 14-MeV drive beam. The dominant waveguide mode of the structure is at {approx}30 GHz, and the structure is configured to exhibit a high transformer ratio ({approx}12:1). Accelerated bunches in the narrow secondary channel of the structure are continuously energized via Cherenkov radiation that is emitted by a drive bunch moving in the wider primary channel. Observed energy gains and losses, transverse deflections, and changes in the test bunch charge distribution compare favorably with predictions of theory.

Jay L. Hirshfield

2012-05-30

218

Numerical prediction of wakes in cascades and compressor rotors including the effects of mixing. II - Rotor passage flow and wakes including the effects of spanwise mixing  

NASA Technical Reports Server (NTRS)

The results of a numerical investigation to predict the flow field including wakes and mixing in axial-flow compressor rotors are presented. The wake behavior in a moderately loaded compressor rotor is studied numerically using a 3D incompressible Navier-Stokes solver with a high Reynolds number form of a turbulence model. The equations are solved using a time dependent implicit technique. The agreement between the measured data and the predictions is good; including the blade boundary-layer profiles, wake mean-velocity profiles, and decay. The ability of the pseudocompressibility scheme to predict the entire flow field including the near and far wake profiles and its decay characteristics, effect of loading, and the viscous losses of a 3D rotor flow field are demonstrated. The mixing in the downstream regions away from the hub and annulus walls is dominated by wake diffusion. In regions away from the walls the radial mixing is predominantly caused by the transport of mass, momentum, and energy by the radial component of velocity in the wake.

Lakshminarayana, B.; Suryavamshi, N.

1991-01-01

219

Evolution of Rotor Wake in Swirling Flow  

NASA Technical Reports Server (NTRS)

A theory is presented for modeling the evolution of rotor wakes as a function of axial distance in swirling mean flows. The theory, which extends an earlier work to include arbitrary radial distributions of mean swirl, indicates that swirl can significantly alter the wake structure of the rotor especially at large downstream distances (i.e., for moderate to large rotor-stator spacings). Using measured wakes of a representative scale model fan stage to define the mean swirl and initial wake perturbations, the theory is used to predict the subsequent evolution of the wakes. The results indicate the sensitivity of the wake evolution to the initial profile and the need to have complete and consistent initial definition of both velocity and pressure perturbations.

El-Haldidi, Basman; Atassi, Hafiz; Envia, Edmane; Podboy, Gary

2000-01-01

220

RWF rotor-wake-fuselage code software reference guide  

NASA Technical Reports Server (NTRS)

The RWF (Rotor-Wake-Fuselage) code was developed from first principles to compute the aerodynamics associated with the complex flow field of helicopter configurations. The code is sized for a single, multi-bladed main rotor and any configuration of non-lifting fuselage. The mathematical model for the RWF code is based on the integration of the momentum equations and Green's theorem. The unknowns in the problem are the strengths of prescribed singularity distributions on the boundaries of the flow. For the body (fuselage) a surface of constant strength source panels is used. For the rotor blades and rotor wake a surface of constant strength doublet panels is used. The mean camber line of the rotor airfoil is partitioned into surface panels. The no-flow boundary condition at the panel centroids is modified at each azimuthal step to account for rotor blade cyclic pitch variation. The geometry of the rotor wake is computers at each time step of the solution. The code produces rotor and fuselage surface pressures, as well as the complex geometry of the evolving rotor wake.

Berry, John D.

1991-01-01

221

Spatiotemporal spectral analysis of a forced cylinder wake.  

PubMed

The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at Re=100. Two-dimensional particle image velocimetry on the midplane perpendicular to the axis of a cylinder is used to characterize the spatial development of the flow and its stability properties. The lock-in phenomenon that determines the boundaries between regions of the forcing parameter space where the wake is globally unstable or convectively unstable [see Thiria and Wesfreid, J. Fluids Struct. 25, 654 (2009) for a review] is scrutinized using the experimental data. A method based on the analysis of power density spectra of the flow allows us to give a detailed description of the forced wake, shedding light on the energy distribution in the different frequency components and in particular on a cascade-like mechanism evidenced for a high amplitude of the forcing oscillation. In addition, a calculation of the drag from the velocity field is performed, allowing us to relate the resulting force on the body to the wake properties. PMID:22181499

D'Adamo, Juan; Godoy-Diana, Ramiro; Wesfreid, José Eduardo

2011-11-01

222

Histaminergic Control of Sleep-Wake Cycles: Recent Therapeutic Advances for Sleep and Wake Disorders  

Microsoft Academic Search

The role of histaminergic neurotransmission in the promotion of waking has been extensively studied in pre- clinical species. Appreciation for the role of histamine continues to expand with increasing understanding of the interac- tion of histamine within the broad network of neuromodulators that regulate sleep and wake. The effects of histamine on waking are transduced through the H1 and the

A. J. Barbier; M. J. Bradbury

2007-01-01

223

VORTEX ASSYMETRY IN ISLAND WAKES Ayah Lazar (1); Alexandre Stegner (2); Rui Caldeira (3); Romain Pennel (4); Changming Dong (5);  

E-print Network

Pennel (4); Changming Dong (5); H.Didelle (6) and S.Viboud (7) (1) Geophysics and Planetary Sciences, Tel-field wake one or two diameter behind the island. The stabilizing impact of the stratification may lead islands wakes in the biological enrichment and the retention of surface pollutants is an area of growing

Stegner Alexandre

224

On the generation of a reverse Von Krmn street for the controlled cylinder wake in the laminar regime  

E-print Network

On the generation of a reverse Von Kármán street for the controlled cylinder wake in the laminar that can be achieved under rotary sinusoidal control for the circular cylinder wake in the laminar regime correction field with negative drag is observable for this controlled flow configuration. The significant

Bergmann, Michel

225

Plasma wakefield acceleration studies using the quasi-static code WAKE  

E-print Network

The quasi-static code WAKE [P. Mora and T. Antonsen, Phys. Plasmas {\\bf 4}, 217(1997)] is upgraded to model the propagation of an ultra-relativistic charged particle beam through a warm background plasma in plasma wakefield acceleration. The upgraded code is benchmarked against the full particle-in-cell code OSIRIS [Hemker et al., Phys. Rev. ST Accel. Beams {\\bf 3}, 061301(2000)] and the quasi-static code QuickPIC [Huang et al., J. Comp. Phys. {\\bf 217}, 658 (2006)]. The effect of non-zero plasma temperature on the peak accelerating electric field is studied for a two bunch electron beam driver with parameters corresponding to the plasma wakefield acceleration experiments at FACET. It is shown that plasma temperature does not affect the energy gain and spread of the accelerated particles despite suppressing the peak accelerating electric field. The role of plasma temperature in improving the numerical convergence of the electric field with the grid resolution is discussed.

Jain, Neeraj; Antonsen, T M; Mori, Warren B; An, Weiming

2014-01-01

226

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

227

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

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

228

Feasibility of wake vortex monitoring systems for air terminals  

NASA Technical Reports Server (NTRS)

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

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

1972-01-01

229

GEOTAIL observation of upstream ULF waves associated with lunar wake  

NASA Astrophysics Data System (ADS)

Left-handed, circular polarized ULF waves with frequency of 0.3-1.1 Hz were detected by GEOTAIL at 27 lunar radii upstream of the moon when the spacecraft was magnetically connected with the lunar wake. The wave was detected twice at 16:45-17:00 and 18:55-19:02 on October 25, 1994, when the spacecraft and the moon were on the dawn side of the Earth's magnetosphere. The ULF wave was propagating in a direction nearly parallel to the background magnetic field. The observed frequency and polarization are explained by reversal of polarization of right-handed, sunward-propagating electron whistler waves with frequencies above 1.4 Hz in the solar wind frame of reference, which were excited through the interaction with electron beams flowing in anti-sunward direction downstream of the lunar wake. The downstream flow of electron beam is explained by filtering effect of the potential drop at the boundary of the lunar wake. Low-energy components of electrons are reflected back by the potential drop, and the rest components, with energies higher than that of the electric potential penetrate through the wake. The velocity distribution of downstream electrons would be modified to have some bump or shoulder in energy range to form a beam, which is likely to excite whistler mode wave through cyclotron resonance. The lowest energy of the resonant electrons was calculated to be 0.96-2.5 (keV) from the lower boundary of the detected frequency. The variation in the lowest frequency suggests that there are some regions of the lunar wake where potential drop is reduced.

Nakagawa, T.; Takahashi, Y.; Iizima, M.

2003-09-01

230

Physiologically-based modeling of sleep-wake regulatory networks.  

PubMed

Mathematical modeling has played a significant role in building our understanding of sleep-wake and circadian behavior. Over the past 40 years, phenomenological models, including the two-process model and oscillator models, helped frame experimental results and guide progress in understanding the interaction of homeostatic and circadian influences on sleep and understanding the generation of rapid eye movement sleep cycling. Recent advances in the clarification of the neural anatomy and physiology involved in the regulation of sleep and circadian rhythms have motivated the development of more detailed and physiologically-based mathematical models that extend the approach introduced by the classical reciprocal-interaction model. Using mathematical formalisms developed in the field of computational neuroscience to model neuronal population activity, these models investigate the dynamics of proposed conceptual models of sleep-wake regulatory networks with a focus on generating appropriate sleep and wake state transition patterns as well as simulating disease states and experimental protocols. In this review, we discuss several recent physiologically-based mathematical models of sleep-wake regulatory networks. We identify common features among these models in their network structures, model dynamics and approaches for model validation. We describe how the model analysis technique of fast-slow decomposition, which exploits the naturally occurring multiple timescales of sleep-wake behavior, can be applied to understand model dynamics in these networks. Our purpose in identifying commonalities among these models is to propel understanding of both the mathematical models and their underlying conceptual models, and focus directions for future experimental and theoretical work. PMID:24530893

Booth, Victoria; Diniz Behn, Cecilia G

2014-04-01

231

The Effect of Wake Passing on Turbine Blade Film Cooling  

NASA Technical Reports Server (NTRS)

The effect of upstream blade row wake passing on the showerhead film cooling performance of a downstream turbine blade has been investigated through a combination of experimental and computational studies. The experiments were performed in a steady-flow annular turbine cascade facility equipped with an upstream rotating row of cylindrical rods to produce a periodic wake field similar to that found in an actual turbine. Spanwise, chordwise, and temporal resolution of the blade surface temperature were achieved through the use of an array of nickel thin-film surface gauges covering one unit cell of showerhead film hole pattern. Film effectiveness and Nusselt number values were determined for a test matrix of various injectants, injectant blowing ratios, and wake Strouhal numbers. Results indicated a demonstratable reduction in film effectiveness with increasing Strouhal number, as well as the expected increase in film effectiveness with blowing ratio. An equation was developed to correlate the span-average film effectiveness data. The primary effect of wake unsteadiness was found to be correlated well by a chordwise-constant decrement of 0.094-St. Measurable spanwise film effectiveness variations were found near the showerhead region, but meaningful unsteady variations and downstream spanwise variations were not found. Nusselt numbers were less sensitive to wake and injection changes. Computations were performed using a three-dimensional turbulent Navier-Stokes code which was modified to model wake passing and film cooling. Unsteady computations were found to agree well with steady computations provided the proper time-average blowing ratio and pressure/suction surface flow split are matched. The remaining differences were isolated to be due to the enhanced mixing in the unsteady solution caused by the wake sweeping normally on the pressure surface. Steady computations were found to be in excellent agreement with experimental Nusselt numbers, but to overpredict experimental film effectiveness values. This is likely due to the inability to match actual hole exit velocity profiles and the absence of a credible turbulence model for film cooling.

Heidmann, James David

1996-01-01

232

Pressure Transients within MCS Mesohighs and Wake Lows  

Microsoft Academic Search

By animating enhanced coarse surface pressure observations of 12 1985 Preliminary Regional Experiment for Storm-Scale Operational Research Meteorology (PRE-STORM) mesoscale convective systems (MCSs) the authors exposed 92 transitory highs and lows living within virtually all of the systems' mesohighs and wake lows. A quasi-Lagrangian (feature following, not material following) analysis of the pressure fields produced five primary results. First, these

Jason C. Knievel; Richard H. Johnson

1998-01-01

233

Influence of coherent structures in the gas-particle circular cylinder wake flow  

Microsoft Academic Search

To investigate the influence of coherent structures in the gas-particle wake flow, direct numerical simulation (DNS) method\\u000a was adopted to compute a two-dimensional particle laden wake flow. A high accuracy spectral element method (SEM) was employed\\u000a to simulate the gas flow field and a Lagrangian approach was used to compute the particles movement. Numerical results showed\\u000a that at the same

Ji Feng; LIU Lan; Fan Jian-ren; Cen Ke-fa

2005-01-01

234

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

SciTech Connect

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

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

1983-03-01

235

Fear in the Wake of Terror  

E-print Network

Fear in the Wake of Terror PSYCHOLOGY Fear in the Wake of Terror Only the Big Bang Was More facing our society, as reflected in their titles: the Max Planck Institute for the Study of Religious will concentrate on the change in religious attitudes, social structures and identities in immigrant communities

Falge, Eva

236

Dream bizarreness and waking thought in schizophrenia.  

PubMed

Dream diaries and reports of daytime waking thought were collected from five schizophrenia patients and matched controls. It was more difficult for blind judges to differentiate the patients' than the controls' dream reports from reports of waking thought, and patients reported shorter but more bizarre dreams than did the controls. PMID:20471693

Noreika, Valdas; Valli, Katja; Markkula, Juha; Seppälä, Katriina; Revonsuo, Antti

2010-08-15

237

The suppressed negative bias illumination-induced instability in In-Ga-Zn-O thin film transistors with fringe field structure  

NASA Astrophysics Data System (ADS)

This study investigates the suppressed negative gate bias illumination stress (NBIS) -induced instability of via-type amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) with fringe field (FF) structures. The less negative threshold voltage shifts of devices after NBIS are showed when device has larger FF structures. This finding is attributed to more dispersive distribution of photo-generated holes in the width direction of a-IGZO during NBIS, which reduce the hole trapping phenomenon in the front channel interface. The a-IGZO TFT with FF structure is expected to be an effective method to increase the electrical reliability of devices after NBIS.

Chen, Yu-Chun; Chang, Ting-Chang; Li, Hung-Wei; Hsieh, Tien-Yu; Chen, Te-Chih; Wu, Chang-Pei; Chou, Cheng-Hsu; Chung, Wang-Cheng; Chang, Jung-Fang; Tai, Ya-Hsiang

2012-11-01

238

Determination of Wind Turbine Near-Wake Length Based on Stability Analysis  

NASA Astrophysics Data System (ADS)

A numerical study on the wake behind a wind turbine is carried out focusing on determining the length of the near-wake based on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method. The wake is perturbed by applying stochastic or harmonic excitations in the neighborhood of the tips of the blades. The flow field is then analyzed to obtain the stability properties of the tip vortices in the wake of the wind turbine. As a main outcome of the study it is found that the amplification of specific waves (traveling structures) along the tip vortex spirals is responsible for triggering the instability leading to wake breakdown. The presence of unstable modes in the wake is related to the mutual inductance (vortex pairing) instability where there is an out-of-phase displacement of successive helix turns. Furthermore, using the non-dimensional growth rate, it is found that the pairing instability has a universal growth rate equal to ?/2. Using this relationship, and the assumption that breakdown to turbulence occurs once a vortex has experienced sufficient growth, we provide an analytical relationship between the turbulence intensity and the stable wake length. The analysis leads to a simple expression for determining the length of the near wake. This expression shows that the near wake length is inversely proportional to thrust, tip speed ratio and the logarithmic of the turbulence intensity.

Sørensen, Jens N.; Mikkelsen, Robert; Sarmast, Sasan; Ivanell, Stefan; Henningson, Dan

2014-06-01

239

Schlieren investigation of the square cylinder wake: Joint influence of buoyancy and orientation  

NASA Astrophysics Data System (ADS)

The present work examines the combined influence of orientation and buoyancy on vortex shedding from a heated square cylinder with the main flow in the vertically upward direction in the aiding buoyancy configuration. The wake of the cylinder is imaged by a schlieren technique. The study investigates the effect of buoyancy, Reynolds number, and angle of incidence of square cylinder with respect to the incoming flow. The Reynolds numbers based on the cylinder edge have been set equal to 56, 87, and 100. Eight different orientations (? =0°, 5°, 10°, 15°, 20°, 30°, 40°, and 45°) and a Richardson number range of 0.031-0.291 have been considered. Instantaneous as well as time-averaged schlieren images, velocity profiles, vortex formation length, Strouhal number, and power spectra are reported. Results show that there is no vortex shedding at Re=56 for the zero angle of incidence. Vortex shedding is initiated at this Reynolds number for a higher angle of incidence, indicating that cylinder orientation plays a favoring role in destabilizing the wake. For orientations other than 0° and 45°, the time-averaged wake is asymmetric. With an increase in the angle of incidence, the shear layers roll up over a shorter distance. With heating, the fluid particles in the shear layer are further accelerated and a marginal increase in Strouhal number with Richardson number is observed. As the heating level increases to a higher value, buoyancy delivers sufficient momentum into the wake, diminishes the velocity deficit, and completely eliminates vortex shedding. Suppression of vortex shedding is observed at a certain critical Richardson number that depends on Reynolds number and the angle of incidence. The approach toward complete suppression and the corresponding wake structures are of interest. The present study demonstrates that the properties of the cylinder wake are intricately related to both the heating level and its orientation relative to the incoming flow.

Kakade, A. A.; Singh, S. K.; Panigrahi, P. K.; Muralidhar, K.

2010-05-01

240

SAR observation and numerical modeling of tidal current wakes at the East China Sea offshore wind farm  

NASA Astrophysics Data System (ADS)

TerraSAR-X (TS-X) Synthetic Aperture Radar (SAR) image acquired at the East China Sea offshore wind farm presents distinct wakes at a kilometer scale on the lee of the wind turbines. The presumption was that these wakes were caused by wind movement around turbine blades. However, wind analysis using spaceborne radiometer data, numerical weather prediction, and in situ measurements suggest that the prevailing wind direction did not align with the wakes. By analyzing measurement at the tidal gauge station and modeling of the tidal current field, these trailing wakes are interpreted to have formed when a strong tidal current impinged on the cylindrical monopiles of the wind turbines. A numerical simulation was further conducted to reproduce the tidal current wake under such conditions. Comparison of the simulated surface velocity in the wake region with the TS-X sea surface backscatter intensity shows a similar trend. Consequently, turbulence intensity (T.I.) of the tidal current wakes over multiple piles is studied using the TS-X observation. It is found that the T.I. has a logarithmic relation with distance. Furthermore, another case study showing wakes due to wind movement around turbine blades is presented to discuss the differences in the tidal current wakes and wind turbine wakes. The conclusion is drawn that small-scale wakes formed by interaction of the tidal current and the turbine piles could be also imaged by SAR when certain conditions are satisfied. The study is anticipated to draw more attentions to the impacts of offshore wind foundations on local hydrodynamic field.

Li, XiaoMing; Chi, Lequan; Chen, Xueen; Ren, YongZheng; Lehner, Susanne

2014-08-01

241

POD Analysis of a Wind Turbine Wake in a Turbulent Atmospheric Boundary Layer  

NASA Astrophysics Data System (ADS)

The wake of a single wind turbine is modeled using an actuator disk model and large eddy simulations. As inflow condition a numerically generated turbulent atmospheric boundary layer is used. The proper orthogonal decomposition (POD) is applied to a plane perpendicular to the main flow in the far wake of the turbine. Reconstructions of the field are investigated depending on the numbers of POD modes used. Even though a great number of modes is needed to recover a great part of the turbulent kinetic energy, our results indicate that relevant aspects of a wake flow can be recovered using only a few modes. Particularly, the dynamics of the average velocity over a potential disk in the wake can partially be captured using only three modes.

Bastine, D.; Witha, B.; Wächter, M.; Peinke, J.

2014-06-01

242

Nonlinear resonant stimulation and no-feedback control of the wake behind a circular cylinder  

NASA Astrophysics Data System (ADS)

Recently the concept of a nonfeedback control has been developed, based on having an accurate model of the unperturbed system from which an appropriate driving force can be calculated. In this work the application of the nonlinear resonant stimulation concept of Reiser, Hübler, and Lüscher [Z. Naturforsch. Teil A 42, 803 (1987)] and the nonfeedback control method of Hübler and Lüscher [Naturwissenschaften 76, 67 (1989)] is experimentally investigated. The experimental system that is considered is the periodic wake behind a circular cylinder at low Reynolds numbers. It will be shown that successful control, i.e., an increase or decrease of the width of the wake region, can be achieved by using an external sound excitation. Both experimental results and numerical simulations based on a low-dimensional model reconstructed from experimental data are in a good quantitative agreement. In addition we discuss the physical properties of a stimulated wake flow and the general problems of the experimental application of the control to such flows. Furthermore, an experimental setup is proposed from which a higher efficiency for the wake control and the possibility to achieve all kinds of preselected dynamics, such as a total suppression of the wake flow, can be expected. The advantages of this type of model-based control, its physical interpretation, and its applicability to technical flows will be discussed in detail.

Ohle, Frank; Lange, Marc

1996-01-01

243

Transitions in the vortex wake behind the plunging profile  

NASA Astrophysics Data System (ADS)

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

Koz?owski, Tomasz; Kudela, Henryk

2014-12-01

244

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

NASA Technical Reports Server (NTRS)

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

Sarpkaya, T.

2004-01-01

245

Large HAWT wake measurement and analysis  

NASA Technical Reports Server (NTRS)

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.

1995-01-01

246

Formulations of the endophytic bacterium Bacillus subtilis Tu-100 suppress Sclerotinia sclerotiorum on oilseed rape and improve plant vigor in field trials conducted at separate locations.  

PubMed

Sclerotinia sclerotiorum causes serious yield losses in crops in the People's Republic of China. Two formulations of oilseed rape seed containing the bacterium Bacillus subtilis Tu-100 were evaluated for suppression of this pathogen in field trials conducted at two independent locations. The pellet formulation significantly reduced disease (incidence and disease index) and increased plant dry mass, while the wrap formulation significantly reduced disease incidence and significantly increased plant dry mass at both field locations. Mean seed yield per 120 plants with both formulations of isolate Tu-100 was significantly greater than the appropriate controls, but at only one of the locations. Both formulations provided stable B. subtilis Tu-100 biomass (?10(5) CFU·g(-1)) and seed germination (?85%) over a 6 month period at room temperature. Polymerase chain reaction and DNA sequence analysis identified ituC and ituD, and bacAB and bacD in the genome of isolate Tu-100. These genes are involved in the biosynthesis of iturin and bacilysin. Iturin was detected in culture filtrates from isolate Tu-100, with thin layer chromatography. Detection of bacilysin was not attempted. Experiments reported here indicate the commercial viability of B. subtilis Tu-100 for suppression of S. sclerotiorum on oilseed rape. PMID:21767217

Hu, Xiaojia; Roberts, Daniel P; Maul, Jude E; Emche, Sarah E; Liao, Xing; Guo, Xuelan; Liu, Yeying; McKenna, Laurie F; Buyer, Jeffrey S; Liu, Shengyi

2011-07-01

247

Nitrogen-Implanted Silicon Oxynitride: A Coating for Suppressing Field Emission From Stainless Steel Used in High-Voltage Applications  

Microsoft Academic Search

In this paper, the authors examine the field emission performance of stainless steel polished to varying degrees, both before and after being coated with a nitrogen-implanted silicon oxynitride layer. The deposition procedure utilizes the simultaneous sputtering of silicon dioxide from a dielectric quartz window and ion implantation of nitrogen from an RF inductively coupled plasma. Here, the scanning field emission

Nimel D. Theodore; Brian C. Holloway; Dennis M. Manos; Richard Moore; Carlos Hernandez; Tong Wang; H. Frederick Dylla

2006-01-01

248

Dynamic wake meandering Gunner C. Larsen, Helge Aa. Madsen, Ferhat  

E-print Network

Dynamic wake meandering modeling Gunner C. Larsen, Helge Aa. Madsen, Ferhat Bingöl, Jakob Mann. Larsen, and Robert Mikkelsen, Title: Dynamic wake meandering modeling Department: Wind Energy Department. The basic conjecture behind the dynamic wake meandering model is that wake transportation in the atmospheric

249

Review of Idealized Aircraft Wake Vortex Models  

NASA Technical Reports Server (NTRS)

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

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

2014-01-01

250

NASA Wake Vortex Research for Aircraft Spacing  

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

251

Vortex interactions and decay in aircraft wakes  

NASA Technical Reports Server (NTRS)

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

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

1977-01-01

252

Wake-Vortex Hazards During Cruise  

NASA Technical Reports Server (NTRS)

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

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

1998-01-01

253

Simulations of accelerating currents in Io's plasma wake  

NASA Astrophysics Data System (ADS)

The evolution of an Io-perturbed flux tube was studied numerically via magnetohydrodynamics MHD approach of a thin filament Our simulations suggest that the mechanism for producing wake aurora could not be explained by either Alfvén wave or electric circuit alone An upstream-coming flux tube must be in contact with Io for approximately 500 seconds until a tilt angle of about 4 degrees has been developed before it is released downstream A magnetic field depression forms downstream as a result of the continual departure of the flux tubes from Io which in turn has significant influence on the motion of a flux tube A perturbed flux tube would undergo a subcorotational motion in Io s plasma wake This motion is inevitably modulated by Alfvén wave bouncing back and forth between the equatorial plane and the boundary of Io plasma torus The scale of the subcorotation region is in the order of 1 Jovian radius The distribution of the simulated accelerating currents downstream is consistent with the observed wake aurora brightness profile

Chen, C. X.

254

Analysis of long distance wakes behind a row of turbines - a parameter study  

NASA Astrophysics Data System (ADS)

Large Eddy Simulations (LES) of the long distance wake behind a row of 10 turbines are conducted to predict wake recovery. The Navier-Stokes solver EllipSys3D is used in combination with the actuator disc concept. Neutral atmospheric conditions are assumed in combination with synthetic turbulence using the Mann method. Both the wind shear profile and turbulence are introduced into the flow field using body forces. Previous simulations using the same simulation method to model the Horns Rev wind farm showed a higher wake recovery at long distances compared to measurements. The current study investigates further the sensitivity to parameters such as the grid resolution, Reynolds number, the turbulence characteristics as well as the impact of using different internal turbine spacings. The clearest impact on the recovery behind the farm could be seen from the turbulence intensity of the incoming flow. The impact of the wind shear on the turbulence intensity in the domain needs further studies. A lower turbulence level gives slower wake recovery as expected. A slower wake recovery can also be seen for a higher grid resolution. The Reynolds number, apart from when using a very low value, has a small impact on the result. The variation of the internal spacing is seen to have a relatively minor impact on the farm wake recovery.

Eriksson, O.; Nilsson, K.; Breton, S.-P.; Ivanell, S.

2014-06-01

255

Detailed investigation of thermal convection in a liquid metal under a horizontal magnetic field: suppression of oscillatory flow observed by velocity profiles.  

PubMed

Thermal convection experiments in a liquid gallium layer were carried out with various intensities of uniform horizontal magnetic fields. The gallium layer was in a rectangular vessel with a 4:1:1 length ratio (1 is the height), where the magnetic field is applied in the direction normal to the longest vertical wall. An ultrasonic velocity profiling method was used to visualize the spatiotemporal variations in the flow pattern, and the temperature fluctuations in the gallium layer were also monitored. The observed flow pattern without a magnetic field shows oscillating rolls with axes normal to the longest vertical wall of the vessel. The oscillatory motion of the flow pattern was suppressed when increasing the applied magnetic field. The flow behavior was characterized by the fluctuation amplitude of the oscillation and the frequency in the range of Rayleigh numbers from 9.3 x 10³ to 3.5 x 10? and Chandrasekhar numbers 0-1900. The effect of the horizontal magnetic field on the flow pattern may be summarized into three regimes with increases in the magnetic intensity: (1) no effect of the magnetic field, (2) a decrease in the oscillation of the roll structure, and (3) a steady two-dimensional roll structure with no oscillation. These regimes may be explained as a result of an increase in the dominance of Lorentz forces over inertial forces. The power spectrum from the temperature time series showed the presence of a convective-inertial subrange above Rayleigh numbers of 7 x 10?, which suggests that turbulence has developed, and such a subrange was commonly observed above this Rayleigh number even with applied magnetic fields when the rolls oscillate. PMID:21230575

Yanagisawa, Takatoshi; Yamagishi, Yasuko; Hamano, Yozo; Tasaka, Yuji; Yano, Kanako; Takahashi, Jumpei; Takeda, Yasushi

2010-11-01

256

Wake-up effects in Si-doped hafnium oxide ferroelectric thin films  

SciTech Connect

Hafnium oxide based ferroelectric thin films have shown potential as a promising alternative material for non-volatile memory applications. This work reports the switching stability of a Si-doped HfO{sub 2} film under bipolar pulsed-field operation. High field cycling causes a “wake-up” in virgin “pinched” polarization hysteresis loops, demonstrated by an enhancement in remanent polarization and a shift of negative coercive voltage. The rate of wake-up is accelerated by either reducing the frequency or increasing the amplitude of the cycling field. We suggest de-pinning of domains due to reduction of the defect concentration at bottom electrode interface as origin of the wake-up.

Zhou, Dayu, E-mail: zhoudayu@dlut.edu.cn [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024 (China) [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024 (China); Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024 (China); State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054 (China); Xu, Jin [Department of Electronic Engineering, Dalian Neusoft University of Information, Dalian 116023 (China)] [Department of Electronic Engineering, Dalian Neusoft University of Information, Dalian 116023 (China); Li, Qing; Guan, Yan [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024 (China)] [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024 (China); Cao, Fei; Dong, Xianlin [Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 (China)] [Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 (China); Müller, Johannes [Fraunhofer IPMS-CNT, Koengisbruecker Strasse 180, 01109 Dresden (Germany)] [Fraunhofer IPMS-CNT, Koengisbruecker Strasse 180, 01109 Dresden (Germany); Schenk, Tony; Schröder, Uwe [Namlab gGmbH/TU Dresden, Noethnitzer Strasse 64, 01187 Dresden (Germany)] [Namlab gGmbH/TU Dresden, Noethnitzer Strasse 64, 01187 Dresden (Germany)

2013-11-04

257

Analysis of the Radar Reflectivity of Aircraft Vortex Wakes  

NASA Technical Reports Server (NTRS)

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

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

2000-01-01

258

[Sleep-wake regulation by prostaglandin D2 and adenosine].  

PubMed

Prostaglandin (PG) D2 and adenosine are potent endogenous somnogens that accumulate in the brain during prolonged wakefulness. Lipocalin-type PGD synthase (L-PGDS) catalyzes the isomerization of PGH2, a common precursor of various prostanoids, to produce PGD2. L-PGDS is localized in the leptomeninges, choroid plexus, and oligodendrocytes of the central nervous system. PGD2 stimulates DP1 receptors localized in the basal forebrain and increases the local extracellular concentration of adenosine, a paracrine signaling molecule, to promote sleep. Adenosine activates adenosine A2A receptor-expressing neurons in the basal forebrain and ventrolateral preoptic area (VLPO) and inhibits adenosine A1 receptor-possessing arousal neurons. Sleep-promoting neurons in the VLPO send inhibitory signals to suppress the histaminergic neurons in the tuberomammillary nucleus (TMN); the histaminergic neurons contribute to arousal through histamine H1 receptors. GABAergic inhibition of TMN is involved in the induction of non-rapid eye movement (non-REM) sleep by PGD2 and adenosine A2A agonists. The neural network between the VLPO and TMN is considered to play a key role in regulation of vigilance states. Administering an L-PGD inhibitor (SeCl4), DP1 antagonist (ONO-4127Na), or adenosine A2A receptor antagonist (caffeine) suppresses both non-REM and REM sleep, indicating that the PGD2-adenosine system is crucial for maintaining physiological sleep. Selective gene-deletion strategies based on Cre/loxP technology and focal RNA interference have been used for silencing the expression of the A2A receptor by local infection with adeno-associated virus carrying Cre-recombinase or short hairpin RNA. The results of these studies have shown that the A2Asubreceptors in the shell region of the nucleus accumbens are responsible for the effect of caffeine on wakefulness. PMID:22647469

Nagata, Nanae; Urade, Yoshihiro

2012-06-01

259

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 6, NO. 6, JUNE 2010 229 Color Breakup Suppression in Field-Sequential  

E-print Network

, the observer's eyes could integrate the sequential fields and perceive the original colored images. A big and observers' eyes, and the observer will see the color splitting patterns or rainbow effect at the boundary of consecutive frames along the trace of saccadic eye motion, we can eval- uate the color and brightness

Wu, Shin-Tson

260

Force estimation and turbulence in the wake of a freely flying European Starling  

NASA Astrophysics Data System (ADS)

Flapping wings are one of the most complex yet widespread propulsion method found in nature. Although aeronautical technology has advanced rapidly over the past 100 years, natural flyers, which have evolved over millions of years, still feature higher efficiency and represent one of nature's finest locomotion methods. One of the key questions is the role of the unsteady motion in the flow due to the wing flapping and its contribution to the forces acting on a bird during downstroke and upstroke. The wake of a freely flying European Starling is investigated as a case study of unsteady wing aerodynamics. Measurements of the near wake have been taken using long duration high-speed PIV in the wake behind a freely flying bird in a specially designed avian wind tunnel. The wake has been characterized by means of velocity and vorticity fields. The measured flow field is decomposed based on the wing position phases. Drag and lift have been estimated using the mean velocity deficit and the circulation at the wake region. In addition, kinematic analysis of the wing motion and the body has been performed using additional high-speed cameras that recorded the bird movement simultaneously with the PIV. Correlations between the wing kinematics and the flow field characteristics are presented as well as the time evolution of the velocity, vorticity and additional turbulence parameters.

Ben-Gida, Hadar; Kirchhefer, Adam; Kopp, Gregory; Gurka, Roi

2011-11-01

261

Enhancement of high-energy electron generation through suppression of Raman backscattering  

SciTech Connect

The effect of Raman backscattering (RBS) on high-energy electron generation in laser-plasma interaction has been investigated for laser intensities well above the wave breaking and electron trapping threshold. One-dimensional particle-in-cell simulations show that suppression of RBS increases the high-energy electron yield in this regime. RBS-induced heating causes heavy beam loading and damping of the laser wake. Its suppression leads to higher wake amplitudes and higher particle energies. RBS suppression through direct stimulation of Raman forward scatter is demonstrated. The implications for high-energy electron production through laser-plasma interaction are discussed.

Trines, R.M.G.M.; Kamp, L.P.J.; Schep, T.J.; Leemans, W.P.; Esarey, E.H.; Sluijter, F.W.

2004-05-27

262

Lift and wakes of flying snakes Lift and wakes of flying snakes  

E-print Network

Lift and wakes of flying snakes Lift and wakes of flying snakes Anush Krishnan,1 John J. Socha,2 in generating lift. This paper presents a computational investigation of the aerodynamics of the cross cross-section of the species Chrysopelea paradisi, showing that a significant enhancement in lift

Socha, Jake

263

Do trout swim better than eels? Challenges for estimating performance based on the wake of self-propelled bodies  

NASA Astrophysics Data System (ADS)

Engineers and biologists have long desired to compare propulsive performance for fishes and underwater vehicles of different sizes, shapes, and modes of propulsion. Ideally, such a comparison would be made on the basis of either propulsive efficiency, total power output or both. However, estimating the efficiency and power output of self-propelled bodies, and particularly fishes, is methodologically challenging because it requires an estimate of thrust. For such systems traveling at a constant velocity, thrust and drag are equal, and can rarely be separated on the basis of flow measured in the wake. This problem is demonstrated using flow fields from swimming American eels, Anguilla rostrata, measured using particle image velocimetry (PIV) and high-speed video. Eels balance thrust and drag quite evenly, resulting in virtually no wake momentum in the swimming (axial) direction. On average, their wakes resemble those of self-propelled jet propulsors, which have been studied extensively. Theoretical studies of such wakes may provide methods for the estimation of thrust separately from drag. These flow fields are compared with those measured in the wakes of rainbow trout, Oncorhynchus mykiss, and bluegill sunfish, Lepomis macrochirus. In contrast to eels, these fishes produce wakes with axial momentum. Although the net momentum flux must be zero on average, it is neither spatially nor temporally homogeneous; the heterogeneity may provide an alternative route for estimating thrust. This review shows examples of wakes and velocity profiles from the three fishes, indicating challenges in estimating efficiency and power output and suggesting several routes for further experiments. Because these estimates will be complicated, a much simpler method for comparing performance is outlined, using as a point of comparison the power lost producing the wake. This wake power, a component of the efficiency and total power, can be estimated in a straightforward way from the flow fields. Although it does not provide complete information about the performance, it can be used to place constraints on the relative efficiency and cost of transport for the fishes.

Tytell, Eric D.

264

Do trout swim better than eels? Challenges for estimating performance based on the wake of self-propelled bodies  

NASA Astrophysics Data System (ADS)

Engineers and biologists have long desired to compare propulsive performance for fishes and underwater vehicles of different sizes, shapes, and modes of propulsion. Ideally, such a comparison would be made on the basis of either propulsive efficiency, total power output or both. However, estimating the efficiency and power output of self-propelled bodies, and particularly fishes, is methodologically challenging because it requires an estimate of thrust. For such systems traveling at a constant velocity, thrust and drag are equal, and can rarely be separated on the basis of flow measured in the wake. This problem is demonstrated using flow fields from swimming American eels, Anguilla rostrata, measured using particle image velocimetry (PIV) and high-speed video. Eels balance thrust and drag quite evenly, resulting in virtually no wake momentum in the swimming (axial) direction. On average, their wakes resemble those of self-propelled jet propulsors, which have been studied extensively. Theoretical studies of such wakes may provide methods for the estimation of thrust separately from drag. These flow fields are compared with those measured in the wakes of rainbow trout, Oncorhynchus mykiss, and bluegill sunfish, Lepomis macrochirus. In contrast to eels, these fishes produce wakes with axial momentum. Although the net momentum flux must be zero on average, it is neither spatially nor temporally homogeneous; the heterogeneity may provide an alternative route for estimating thrust. This review shows examples of wakes and velocity profiles from the three fishes, indicating challenges in estimating efficiency and power output and suggesting several routes for further experiments. Because these estimates will be complicated, a much simpler method for comparing performance is outlined, using as a point of comparison the power lost producing the wake. This wake power, a component of the efficiency and total power, can be estimated in a straightforward way from the flow fields. Although it does not provide complete information about the performance, it can be used to place constraints on the relative efficiency and cost of transport for the fishes.

Tytell, Eric D.

2007-11-01

265

Vortex Wakes of Subsonic Transport Aircraft  

NASA Technical Reports Server (NTRS)

A historical overview will be presented of the research conducted on the structure and modification of the vortices generated by the lifting surfaces of subsonic transport aircraft. The seminar will describe the three areas of vortex research; namely, the magnitude of the hazard posed, efforts to reduce the hazard to an acceptable level, and efforts to develop a systematic means for avoiding vortex wakes. It is first pointed out that the characteristics of lift-generated vortices are related to the aerodynamic shapes that produce them and that various arrangements of surfaces can be used to produce different vortex structures. The largest portion of the research conducted to date has been directed at finding ways to reduce the hazard potential of lift-generated vortices shed by subsonic transport aircraft in the vicinity of airports during landing and takeoff operations. It is stressed that lift-generated vortex wakes are so complex that progress towards a solution requires application of a combined theoretical and experimental research program because either alone often leads to incorrect conclusions. It is concluded that a satisfactory aerodynamic solution to the wake-vortex problem at airports has not yet been found but a reduction in the impact of the wake-vortex hazard on airport capacity may become available in the foreseeable future through wake-vortex avoidance concepts currently under study. The material to be presented in this overview is drawn from articles published in aerospace journals that are available publicly.

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

1999-01-01

266

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

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

267

Separation control in low pressure turbines using plasma actuators with passing wakes  

NASA Astrophysics Data System (ADS)

A Dielectric Barrier Discharge (DBD) plasma actuator is operated in flow over the suction surface of a Pack-B Low Pressure Turbine (LPT) airfoil at a Reynolds number of 50,000 (based on exit velocity and suction surface length) and inlet free-stream turbulence intensity of 2.5%. Preliminary characterization studies were made of the effect of varying actuator pulsing frequency and duty cycle, actuator edge effects, and orientation of the actuator with the flow. Flow control was demonstrated with the actuator imparting momentum opposite to the stream-wise flow direction, showing that it is possible to use disturbances alone to destabilize the flow and effect transition. No frequencies of strong influence were found over the range tested, indicating that a broad band of effective frequencies exists. Edge effects were found to considerably enhance separation control. Total pressure measurements of the flow without passing wakes were taken using a glass total-pressure tube. Corrections for streamline displacement due to shear and wall effects were made, and comparisons with previous hot-wire measurements were used to validate data. Performance features of conventional two-electrode and a novel three-electrode actuator configuration were compared. Hot-wire anemometry was used to take time-varying ensemble-averaged near-wall velocity measurements of the flow with periodic passing wakes. Corrections were made for near-wall effects, temperature effects, and interference of the electric field. The wakes were generated by a wake generator mechanism located upstream of the airfoil passage. The near-suction-surface total pressure field (flow without wakes) and velocity field (flow with wakes) in the trailing part of the airfoil passage, and the wall-normal gradient of these quantities, were used to demonstrate effective prevention of flow separation using the plasma actuator. Both flows (with and without passing wakes) showed fully attached flow (or very thin separation zones) when the actuator was activated. The flow with passing wakes and the actuator on showed relatively little time variation in the boundary layer, and qualitative similarities to the corresponding flow without passing wakes and with the actuator on were noted.

Burman, Debashish

268

Wake Vortex Tracking Using a 35 GHz Pulsed Doppler Radar  

NASA Technical Reports Server (NTRS)

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

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

2005-01-01

269

Suppression of type I collagen in human scleral fibroblasts treated with extremely low-frequency electromagnetic fields  

PubMed Central

Purpose To investigate the expression differences of type I collagen (COL1A1) and its underlying mechanisms in human fetal scleral fibroblasts (HFSFs) that were treated with conditioned medium from retinal pigment epithelial (RPE) cells under extremely low-frequency electromagnetic fields (ELF-EMFs). Methods The ELF-EMFs used in this study were established by slidac and artificial coils. Growth of the treated HFSFs was evaluated by a cell-counting kit-8 assay. The expression of COL1A1 and matrix metalloproteinases-2 (MMP-2) in the treated HFSFs was detected by reverse transcription PCR (RT-PCR) and western blot, and the expression of transforming growth factor-?2 (TGF-?2) and basic fibroblast growth factor-2 (FGF-2) in RPE cells exposed to EMFs was detected by RT-PCR. The expression of COL1A1 and MMP-2 in HFSFs was further confirmed by immunofluorescence staining. Activation of extracellular signal-regulated kinase 1/2 (ERK1/2 also called p44/p42 mitogen-activated protein kinases [MAPK]) and p38 in HFSFs was measured by western blot. Results We found that exposure to ELF-EMFs resulted in a decreased proliferation rate of HFSFs and that addition of RPE supernatant medium could enhance this effect. Compared with that of the control cells, a significant decrease in collagen synthesis was detected in HFSFs under ELF-EMFs. However, the expression of MMP-2 was upregulated, which could be further enhanced via an RPE supernatant additive. The activities of ERK1/2 and p38 were significantly increased in HFSFs exposed to ELF-EMFs, and this effect could be enhanced by RPE supernatant medium additive. Conclusions Our results suggested that ELF-EMFs can inhibit the expression of type I collagen in HFSFs and contribute to the remodeling of the sclera. PMID:23592926

Wang, Jie; Cui, Jiefeng

2013-01-01

270

Experimental investigation about the effect of non-axisymmetric wake impact on a low speed axial compressor  

NASA Astrophysics Data System (ADS)

Non-axisymmetric wake impact experiments were carried out after the best exciting frequency for a low speed axial compressor had been found by axisymmetric wake impact experiments. When the number and circumferential distribution of inlet guide vanes (IGV) are logical the wakes of non-axisymmetric IGVs can exert beneficial unsteady exciting effect on their downstream rotor flow fields and improve the compressor’s performance. In the present paper, four non-axisymmetric wake impact plans were found working better than the axisymmetric wake impact plan. Compared with the base plan, the best non-axisymmetric plan increased the compressor’s peak efficiency, and the total pressure rise by 1.1 and 2%, and enhanced the stall margin by 4.4%. The main reason why non-axisymmetric plans worked better than the axisymmetric plan was explained as the change of the unsteady exciting signal arising from IGV wakes. Besides the high-frequency components, the non-axisymmetric plan generated a beneficial low-frequency square-wave exciting signal and other secondary frequency components. Compared with the axisymmetric plan, multi-frequency exciting wakes arising from the non-axisymmetric plans are easier to get coupling relation with complex vortices such as clearance vortices, passage vortices and shedding vortices.

Liu, Jianyong; Lu, Yajun; Li, Zhiping

2010-05-01

271

Ship wakes: Kelvin or Mach angle?  

E-print Network

From the analysis of a set of airborne images of ship wakes, we show that the wake angles decrease as $U^{-1}$ at large velocities, in a way similar to the Mach cone for supersonic airplanes. This previously unnoticed Mach-like regime is in contradiction with the celebrated Kelvin prediction of a constant angle of $19.47\\degree$ independent of the ship's speed. We propose here a model, confirmed by numerical simulations, in which the finite size of the disturbance explains this transition between the Kelvin and Mach regimes at a Froude number $Fr = U/\\sqrt{gL} \\simeq 0.5$, where $L$ is the hull ship length.

Rabaud, Marc

2013-01-01

272

Wake measurements in a strong adverse pressure gradient  

NASA Technical Reports Server (NTRS)

The behavior of wakes in adverse pressure gradients is critical to the performance of high-lift systems for transport aircraft. Wake deceleration is known to lead to sudden thickening and the onset of reversed flow; this 'wake bursting' phenomenon can occur while surface flows remain attached. Although 'wake bursting' is known to be important for high-lift systems, no detailed measurements of 'burst' wakes have ever been reported. Wake bursting has been successfully achieved in the wake of a flat plate as it decelerated in a two-dimensional diffuser, whose sidewalls were forced to remain attached by use of slot blowing. Pilot probe surveys, L.D.V. measurements, and flow visualization have been used to investigate the physics of this decelerated wake, through the onset of reversed flow.

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

1994-01-01

273

Laser Doppler velocimeter system simulation for sensing aircraft wake vortices  

NASA Technical Reports Server (NTRS)

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

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

1974-01-01

274

Full-potential circular wake solution of a twisted rotor blade in hover  

NASA Technical Reports Server (NTRS)

A solution for transonic flow past a twisted rotor blade in hover is obtained using a modified version of the full-potential code ROT22 and a circular wake. The flow is also evaluated for a fixed-wing-type straight wake. The solutions for the straight wake and circular wake, and the circular wake and a two-dimensional wake are compared. The data reveal that the circular wake and the general two-dimensional wake solutions have similar characteristics.

Aggarwal, Hans R.

1986-01-01

275

POD Analysis of Jet-Plume/Afterbody-Wake Interaction  

NASA Astrophysics Data System (ADS)

The understanding of the flow physics in the base region of a powered rocket is one of the keys to designing the next generation of reusable launchers. The base flow features affect the aerodynamics and the heat loading at the base of the vehicle. Recent efforts at the National Center for Physical Acoustics at the University of Mississippi have refurbished two models for studying jet-plume/afterbody-wake interactions in the NCPA's 1-foot Tri-Sonic Wind Tunnel Facility. Both models have a 2.5 inch outer diameter with a nominally 0.5 inch diameter centered exhaust nozzle. One of the models is capable of being powered with gaseous H2 and O2 to study the base flow in a fully combusting senario. The second model uses hi-pressure air to drive the exhaust providing an unheated representative flow field. This unheated model was used to acquire PIV data of the base flow. Subsequently, a POD analysis was performed to provide a first look at the large-scale structures present for the interaction between an axisymmetric jet and an axisymmetric afterbody wake. PIV and Schlieren data are presented for a single jet-exhaust to free-stream flow velocity along with the POD analysis of the base flow field.

Murray, Nathan E.; Seiner, John M.; Jansen, Bernard J.; Gui, Lichuan; Sockwell, Shuan; Joachim, Matthew

2009-11-01

276

Wake pattern and wave resistance for anisotropic moving disturbances  

NASA Astrophysics Data System (ADS)

We present a theoretical study of gravity waves generated by an anisotropic moving disturbance. We model the disturbance by an elliptical pressure field of given aspect ratio W. We study the wake pattern as a function of W and the longitudinal hull Froude number Fr = V/sqrt{gL}, where V is the velocity, g is the acceleration of gravity, and L is the size of the disturbance in the direction of motion. For large hull Froude numbers, we analytically show that the rescaled surface profiles for which sqrt{W}/Fr is kept constant coincide. In particular, the angle outside which the surface is essentially flat remains constant and equal to the Kelvin angle, and the angle corresponding to the maximum amplitude of the waves scales as sqrt{W}/Fr, thus showing that previous work on the wake's angle for isotropic objects can be extended to anisotropic objects of given aspect ratio. We then focus on the wave resistance and discuss its properties in the case of an elliptical Gaussian pressure field. We derive an analytical expression for the wave resistance in the limit of very elongated objects and show that the position of the speed corresponding to the maximum wave resistance scales as sqrt{gL}/sqrt{W}.

Benzaquen, Michael; Darmon, Alexandre; Raphaël, Elie

2014-09-01

277

Efficient Prediction of Helicopter BVI Noise under Different Conditions of Wake and Blade Deformation  

NASA Astrophysics Data System (ADS)

Predictions of helicopter BVI noise using three-dimensional Euler code with a single blade grid are conducted under three different conditions: BVI noise caused by (1) interaction between rotating blades and vortex shed from fixed wing vortex generator, (2) interaction between rotating blades and tip vortices shed from preceding blades, and (3) interaction between rotating blades with elastic deformation and shed tip vortices. In the CFD calculation, the Field Velocity Approach (FVA) and Scully’s vortex model are used to import the wake information into the calculation grid and to determine the induced velocity made by tip vortices, respectively (cases 1 3). Beddoes generalized wake model is used to prescribe the tip vortices position in the wake (cases 2 and 3). Information about blade elastic deformation is imported from HART II project experimental data into the calculation (case 3). Acoustic analyses based on Ffowcs-Williams and Hawkings (FW-H) equation are conducted subsequently in each case. The results from the calculations show good agreement with experiments in all three cases, indicating that application of FVA, Scully’s model, and Beddoes generalized wake model is effective for BVI noise prediction in this study, which is intended for low calculation cost using a single blade grid. Also, use of blade elastic deformation data in the calculation shows marked improvement in calculation precision. Consequently, the method used in this study can predict BVI noise under various conditions of wake or blade deformation with acceptable precision and low calculation cost.

Inada, Yoshinobu; Yang, Choongmo; Iwanaga, Noriki; Aoyama, Takashi

278

LOOK AT THE INFLUENCE OF BUILDING ORIENTATION ON PLUME DISPERSION IN THE WAKE OF A BUILDING  

EPA Science Inventory

Observations of mean pollutant concentration profiles downwind of a block-sized model building are reported. These data are part of a more comprehensive field model study of building wake effects conducted in the U.S. Environmental Protection Agency's Fluid Modeling Facility. The...

279

Negative wake behind a sphere rising in viscoelastic fluids: A lattice Boltzmann investigation  

Microsoft Academic Search

We investigate the complex flow field around a sphere rising in a Maxwell fluid by means of the lattice Boltzmann simulation to provide insights into the strange negative wake experimentally observed behind a bubble or particle in non Newtonian fluids. The influence of the rise velocity, sphere diameter, and fluid's rheology is considered through two dimensionless numbers: the Deborah number

Xavier Frank; Huai Z. Li

2006-01-01

280

Description of Wake Survey Rake Calibration Technique and Calibration Results of Rake 4.  

National Technical Information Service (NTIS)

The David Taylor Naval Ship R&D Center conducts wake surveys on models of surface ships and submarines on the towing carriages. The device used to measure the velocity field in way of the propeller disc of the model is called a pitot tube rake and is made...

D. R. Mullinix

1985-01-01

281

Sleep in the Unresponsive Wakefulness Syndrome and Minimally Conscious State  

E-print Network

Sleep in the Unresponsive Wakefulness Syndrome and Minimally Conscious State Victor Cologan,1 Laureys1,7 Abstract The goal of our study was to investigate different aspects of sleep, namely the sleep-wake cycle and sleep stages, in the vegetative state/unresponsive wakefulness syndrome (VS

Delorme, Arnaud

282

Free surface effects on the wake of a flat plate  

Microsoft Academic Search

Measurements of the mean velocity and the normal and tangential stresses in the wake of a rough flat plate near a free water surface are presented. The various distributions are compared with similar measurements taken in the wake of the plate when submerged in an effectively infinite fluid. The results reveal striking differences in the manner in which the wake

T. F. Swean Jr.; R. D. Peltzer

1984-01-01

283

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

NASA Technical Reports Server (NTRS)

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

Stewart, Eric C.

1991-01-01

284

Effect of object potentials on the wake of a flowing plasma  

NASA Astrophysics Data System (ADS)

The structure of the electric potential and ion density of the ion front in the near wake created by the flow of synthetic plasma past a conducting plate was investigated experimentally and numerically, with particular attention given to the effect of plate potential on the structure of the ion front. Results were obtained for a molecular nitrogen plasma with ambient electron densities of about 100,000/cu cm, ion temperatures of about 0.025 eV, electron temperatures of about 0.3 eV, and plasma flow velocities of about 10,000 m/s. Two-dimensional simulations of the laboratory experiments were performed by using a multiple waterbag technique. The calculated and experimental results show that wake closure is well described by the acceleration of ions in the plasma steady-state electric field. However, the ion-front motion is strongly affected by the imposed potential of the object creating the wake.

Katz, I.; Mandell, M. J.; Parks, D. E.; Wright, K.; Stone, N. H.

1987-10-01

285

Computation of rotor wake turbulence noise  

Microsoft Academic Search

A major source of fan broadband noise is the interaction of rotor wake turbulence with the fan outlet guide vanes. A broadband noise model that utilizes computed rotor flow turbulence from a Reynolds averaged Navier–Stokes code is used to predict fan broadband noise spectra. The noise model is employed to examine the broadband noise characteristics of the 22-in source diagnostic

M. Nallasamy; E. Envia

2005-01-01

286

Wake characterization downstream of a fog collector  

Microsoft Academic Search

The problem of wake geometry characterization downstream of a collector for getting water out of advection fog is investigated combining the results coming from wind tunnel trials and an experimental campaign in Peru (Lomas de Mejia), where a fog collection project was running. Results from a physical model of the fog collector at a 1:100 scale tested in a wind

E Bresci

2002-01-01

287

Wake structure of a deformable Joukowski airfoil  

Microsoft Academic Search

We examine the vortical wake structure shed from a deformable Joukowski airfoil in an unbounded volume of inviscid and incompressible fluid. The deformable airfoil is considered to model a flapping fish. The vortex shedding is accounted for using an unsteady point vortex model commonly referred to as the Brown–Michael model. The airfoil’s deformations and rotations are prescribed in terms of

Adam Ysasi; Eva Kanso; Paul K. Newton

2011-01-01

288

Ram side of Wake Shield Facility  

NASA Technical Reports Server (NTRS)

The ram side of the Wake Shield Facility (WSF) is in the grasp of the Space Shuttle Discovery's Remote Manipulator System (RMS) arm in this 70mm frame. Clouds over the Atlantic Ocean and the blackness of space share the backdrop for the picture.

1994-01-01

289

Radiative forcing over ocean by Ship Wakes  

NASA Astrophysics Data System (ADS)

Changes in surface albedo represent one of the main forcing agents that can counteract, to some extent, the positive forcing from increasing greenhouse gas concentrations. Here, we report on enhanced ocean reflectance from ship wakes over the Pacific Ocean near the California coast, where we determined, based on airborne radiation measurements that ship wakes can increase reflected sunlight by more than 100%. We assessed the importance of this increase to climate forcing, where we estimated the global radiative forcing of ship wakes to be -0.00014 ± 53% Wm^-2 assuming a global distribution of 32331 ships of size ?100000 gross tonnage. The forcing is smaller than the forcing of aircraft contrails (-0.007 to +0.02 Wm^-2), but considering that the global shipping fleet has rapidly grown in the last five decades and this trend is likely to continue because of the need of more inter-continental transportation as a result of economic globalization, we argue that the radiative forcing of wakes is expected to be increasingly important especially in harbors and coastal regions.

Gatebe, C. K.; Wilcox, E. M.; Poudyal, R.; Wang, J.

2011-12-01

290

Holographic flow visualization. [of aircraft wakes  

NASA Technical Reports Server (NTRS)

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

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

1976-01-01

291

Observing the Evolution of Typhoon Wakes.  

National Technical Information Service (NTIS)

The objective of our research program was to observe the temporal and spatial evolution of typhoon cold wakes, in particular we directly observed the mixing associated with turbulence generated by the strong air-sea interaction in a typhoon. These observa...

S. R. Jayne

2012-01-01

292

Experiments in Waking Hypnosis for Instructional Purposes  

Microsoft Academic Search

The author discusses the theory of hypnotism and the possibility of employing it for experimental purposes in the class-room. He distinguishes between sleeping and waking hypnosis, justifying his use of the latter term with a wealth of historical and contemporary evidence. He reports the success of class experiments in both types of hypnosis, particularly the latter which he describes minutely.

W. R. Wells

1924-01-01

293

Explicit Expressions of Impedances and Wake Functions  

SciTech Connect

Sections 3.2.4 and 3.2.5 of the Handbook of Accelerator Physics and Engineering on Landau damping are combined and updated. The new addition includes impedances and wakes for multi-layer beam pipe, optical model, diffraction model, and cross-sectional transition.

Ng, K.Y.; /Fermilab; Bane, K,; /SLAC

2012-06-11

294

Radiative Forcing Over Ocean by Ship Wakes  

NASA Technical Reports Server (NTRS)

Changes in surface albedo represent one of the main forcing agents that can counteract, to some extent, the positive forcing from increasing greenhouse gas concentrations. Here, we report on enhanced ocean reflectance from ship wakes over the Pacific Ocean near the California coast, where we determined, based on airborne radiation measurements that ship wakes can increase reflected sunlight by more than 100%. We assessed the importance of this increase to climate forcing, where we estimated the global radiative forcing of ship wakes to be -0.00014 plus or minus 53% Watts per square meter assuming a global distribution of 32331 ships of size of greater than or equal to 100000 gross tonnage. The forcing is smaller than the forcing of aircraft contrails (-0.007 to +0.02 Watts per square meter), but considering that the global shipping fleet has rapidly grown in the last five decades and this trend is likely to continue because of the need of more inter-continental transportation as a result of economic globalization, we argue that the radiative forcing of wakes is expected to be increasingly important especially in harbors and coastal regions.

Gatebe, Charles K.; Wilcox, E.; Poudyal, R.; Wang, J.

2011-01-01

295

Separation of circadian and wake duration-dependent modulation of EEG activation during wakefulness  

Microsoft Academic Search

The separate contribution of circadian rhythmicity and elapsed time awake on electroencephalographic (EEG) activity during wakefulness was assessed. Seven men lived in an environmental scheduling facility for 4 weeks and completed fourteen 42.85-h ‘days’, each consisting of an extended (28.57-h) wake episode and a 14.28-h sleep opportunity. The circadian rhythm of plasma melatonin desynchronized from the 42.85-h day. This allowed

C Cajochen; J. K Wyatt; C. A Czeisler; D. J Dijk

2002-01-01

296

Helicopter rotor wake geometry and its influence in forward flight. Volume 2: Wake geometry charts  

NASA Technical Reports Server (NTRS)

Isometric and projection view plots, inflow ratio nomographs, undistorted axial displacement nomographs, undistorted longitudinal and lateral coordinates, generalized axial distortion nomographs, blade/vortex passage charts, blade/vortex intersection angle nomographs, and fore and aft wake boundary charts are discussed. Example condition, in flow ratio, undistorted axial location, longitudinal and lateral coordinates, axial coordinates distortions, blade/tip vortex intersections, angle of intersection, and fore and aft wake boundaries are also discussed.

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

1983-01-01

297

An exploratory investigation of a wake disruption technique for studying wake reestablishment time  

NASA Technical Reports Server (NTRS)

An exploratory investigation was made of a wake disruption technique for studying the hypersonic-wake reestablishment time in a blowdown wind tunnel. In this technique, a highly underexpanded jet issuing from the base of a 10 deg half-angle cone totally disrupts and displaces the conventional wake. The jet was rapidly shut off by an explosively actuated valve and the time for wake reestablishment was measured. The tests were conducted in the Mach 6 high Reynolds number tunnel at a stagnation temperature of 506 K and stagnation pressure of 2.86 MPa. The model base jet stagnation pressure was 3.55 MPa at room temperature. High-speed schlieren motion pictures indicated that disappearance of the disrupting jet and reestablishment of the wake-recompression shock were probably occurring simultaneously and that the time disruptive-jet-air shutoff to wake recompression shock reestablishment was probably between 200 and 450 microseconds (flow lengths from 1.8 to 4.2). The values of flow lengths are about one-thord to one-half the values measured in impulse facilities in a previous study. This shorter time is believed to be largely due to difference in flow conditions between the jet disruption technique and impulse facilities.

Clark, L. E.; Jones, R. A.

1974-01-01

298

First results from ARTEMIS lunar wake crossing: observations and hybrid simulation  

NASA Astrophysics Data System (ADS)

The Moon does not have an intrinsic magnetic field and its conductivity is not sufficient to facilitate the development of an induced magnetosphere. The interaction of the Moon with the unperturbed solar wind (SW) is, hence, dominated by the absorption of SW particles on its surface and the consequent generation of a lunar wake on the night side. The SW magnetic field is basically convected through the Moon; the pressure imbalance in lunar wake, however, accounts for a slight increase in magnetic pressure in the lunar wake center. The wake is slowly filled up with SW particles due to their thermal motion, which generates a magnetohydrodynamic (MHD) rarefaction wave propagating away from the wake in the SW frame of reference. Over the last 3 years the Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission provided excellent data helping the scientific community in drawing a detailed picture of the physical processes associated with the development of substorms in the terrestrial magnetotail. Two of the five THEMIS spacecraft are currently being sent into stationary orbits around the Moon in a follow-up mission called Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS). The ARTEMIS P1 spacecraft (formerly THEMIS-B) has recently passed through the lunar wake in a flyby maneuver on February 13, 2010. We show first results of two hybrid code simulations with static and, for the first time, dynamically changing SW input. Adapted SW monitor data of the NASA OMNI database is used as input for the simulations. During the wake crossing the spin stabilized spacecraft P1 was in lunar shadow and, hence, its spin period cannot be determined from sun sensor data. Therefore, an eclipse-spin model is applied to bridge the gap of missing spin period data in order to recover vector measurements. A comparison of the simulation results with correctly despun magnetic field and particle measurements of ARTEMIS P1 allows for a separation of static lunar wake and, due to SW variations, transient features in the observations.

Plaschke, F.; Wiehle, S.; Angelopoulos, V.; Auster, H.; Georgescu, E.; Glassmeier, K.; Motschmann, U. M.; Sibeck, D. G.

2010-12-01

299

Manipulation of electromagnetic fields with plasmonic nanostructures: Nonlinear frequency mixing, optical manipulation, enhancement and suppression of photocurrent in a silicon photodiode, and surface-enhanced spectroscopy  

NASA Astrophysics Data System (ADS)

Metallic nanostructures are one of the most versatile tools available for manipulating light at the nanoscale. These nanostructures support surface plasmons, which are collective excitations of the conduction electrons that can exist as propagating waves at a metallic interface or as localized excitations of a nanoparticle or nanostructure. Plasmonic structures can efficiently couple energy from freely propagating electromagnetic waves to localized electromagnetic fields and vice-versa, essentially acting as an optical antenna. As a result, the intensity of the local fields around and inside the nanostructure are strongly enhanced compared to the incident radiation. In this thesis, this ability to manipulate electromagnetic fields on the nanoscale is employed to control a wide range of optical phenomena. These studies are performed using structures based on metallic nanoshells, which consist of a thin Au shell coating a silica nanosphere. To investigate the parameters controlling the plasmonic response of metallic nanoshells, two changes to the nanoshell composition are studied: (1) the Au shell is replaced with Cu which has interband transitions that strongly influence the plasmon resonance, and (2) the silica core is replaced by a semiconducting Cu 2O core which has a significantly higher dielectric constant and non-trivial absorbance. The focusing of electromagnetic energy into intense local fields by plasmonic nanostructures is then directly investigated by profiling the nanoshell near field using a Raman-based molecular ruler. Next, plasmons supported by Au nanoshells are used to control the fluorescence of near-infrared fluorophores placed at controlled distances from the nanoshell surface. In this context, the analogy of an optical antenna is very relevant: the enhanced field at the surface of the nanoshell increases the absorption of light by the fluorophore, or equivalently couples propagating electromagnetic waves into a localized receiver, while the large scattering cross section enhances the coupling of energy from a localized source, the fluorophore, to far-field radiation. Excellent agreement with models based on Mie theory is achieved for both Raman and fluorescence. Experimentally measured enhancements of the radiative decay rate for fluorophores on Au nanoshells and Au nanorods are also consistent with this model. Plasmonic nanostructures can also control the flow of light into larger structures. This is observed by measuring the nanoparticle-induced enhancement and suppression of photocurrent in a silicon photodiode is at the single particle level for silica nanospheres, Au nanospheres, and two types of Au nanoshell Finally, the simultaneous physical manipulation of an individual plasmonic nanostructure on the few-nanometer scale using light and detection of the local electromagnetic field during this ongoing process with the same incident beam is performed. For this experiment, a Au nanoshell is separated from a metallic surface by a few-nanometer thick polymer layer to form a nanoscale junction, or nanogap Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. An easily detectable four-wave mixing (FWM) signal is simultaneously generated by this illumination of the nanogap, providing a continuous, highly sensitive optical monitor of the nanogap spacing while it is being optically reduced. The dramatic increase in this signal upon contact provides a clear, unambiguous signal of the gap closing.

Grady, Nathaniel K.

300

Vortex shedding in high-speed compressor blade wakes  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

301

EEG Power During Waking and NREM Sleep in Primary Insomnia  

PubMed Central

Objective: Pathophysiological models of insomnia invoke the concept of 24-hour hyperarousal, which could lead to symptoms and physiological findings during waking and sleep. We hypothesized that this arousal could be seen in the waking electroencephalogram (EEG) of individuals with primary insomnia (PI), and that waking EEG power would correlate with non-REM (NREM) EEG. Methods: Subjects included 50 PI and 32 good sleeper controls (GSC). Five minutes of eyes closed waking EEG were collected at subjects' usual bedtimes, followed by polysomnography (PSG) at habitual sleep times. An automated algorithm and visual editing were used to remove artifacts from waking and sleep EEGs, followed by power spectral analysis to estimate power from 0.5–32 Hz. Results: We did not find significant differences in waking or NREM EEG spectral power of PI and GSC. Significant correlations between waking and NREM sleep power were observed across all frequency bands in the PI group and in most frequency bands in the GSC group. Conclusions: The absence of significant differences between groups in waking or NREM EEG power suggests that our sample was not characterized by a high degree of cortical arousal. The consistent correlations between waking and NREM EEG power suggest that, in samples with elevated NREM EEG beta activity, waking EEG power may show a similar pattern. Citation: Wu YM; Pietrone R; Cashmere JD; Begley A; Miewald JM; Germain A; Buysse DJ. EEG power during waking and NREM sleep in primary insomnia. J Clin Sleep Med 2013;9(10):1031-1037. PMID:24127147

Wu, You Meme; Pietrone, Regina; Cashmere, J. David; Begley, Amy; Miewald, Jean M.; Germain, Anne; Buysse, Daniel J.

2013-01-01

302

The near wake of a freely flying European starling  

NASA Astrophysics Data System (ADS)

The wake of a freely flying European starling (Sturnus vulgaris) has been measured using high speed, time-resolved, particle image velocimetry, simultaneously with high speed cameras which imaged the bird. These have been used to generate vector maps that can be associated with the bird's location and wing configuration in the wind tunnel. Time series of measurements have been expressed as composite wake plots which depict segments of the wing beat cycle for various spanwise locations in the wake. Measurements indicate that downwash is not produced during the upstroke, suggesting that the upstroke does not generate lift. As well, the wake velocities imply the presence of streamwise vortical structures, in addition to tip vortices. These two characteristics indicate similarities between the wake of a bird and the wake of a bat, which may be general features of the wakes of flapping wings.

Kirchhefer, Adam J.; Kopp, Gregory A.; Gurka, Roi

2013-05-01

303

Proceedings of the NASA First Wake Vortex Dynamic Spacing Workshop  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

304

Analytical and experimental studies of wakes behind circularly capped bubbles  

NASA Astrophysics Data System (ADS)

The wakes behind circularly capped bubbles are examined by means of an experimental and analytical study. A single two-inch diameter bubble is injected into a six by three foot fluid column, one half inch thick, producing an essentially two-dimensional flow. Aspirin powder placed in the fluid column just prior to bubble release highlights the structure of the flow field before dissolving. High speed film and sequenced photographs are taken to document the observed results. Pressure-time measurements are made with sensitive capacitive transducers mounted in the rear column wall and are synchronized with photographs using a high speed clock. Experimental fluids, prepared by mixing water and glycerine in varying proportions, are used to study the effects of viscosity on bubble shape and wake structure. Testing is performed over a range of Reynolds numbers from 14 to 29,655 which includes the transition from circularly capped to ellipsoidal bubble shape. Experimental data is reduced and summarized in convenient dimensionless form to permit comparison with analytical predictions.

Bessler, W. F.

305

Viscous effects on a vortex wake in ground effect  

NASA Technical Reports Server (NTRS)

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

Zheng, Z.; Ash, Robert L.

1992-01-01

306

Low-Dimensionality of the Turbulent Near-Wake  

NASA Astrophysics Data System (ADS)

We have performed a series of spectral DNS at high and reduced resolution (on unstructured grids) of the flow and heat transfer past a circular cylinder at Reynolds numbers 3,900 and 5,000. We found that an inertial range exists in the near-wake that spans more than half decade of wave number, in agreement with the experimental results of Wong and Wallace (1996). The surprising result is that a simulation employing only two modes in the span is able to predict mean and rms velocities quite accurately. For example, the drag coefficient at Re=3,900 predicted with 2 Fourier modes is 0.981 in agreement with the experimental value 0.98 ± 0.05 of Norberg (1987). In contrast, the corresponding two-dimensional value is 1.74. Similar results hold for the temperature field and the Nusselt number. Such findings suggest that the turbulent near-wake can be represented with only the top few most energetic modes, and this will be discussed in the context of proper orthogonal decomposition.

Ma, Xia; Karniadakis, George

1997-11-01

307

Fermionic suppression of dipolar relaxation  

E-print Network

We observe the suppression of inelastic dipolar scattering due to Fermi statistics in ultracold gases of the highly magnetic atom, dysprosium. Inelastic dipolar scattering in non-zero magnetic fields leads to heating or to loss of the trapped population, both detrimental to experiments intended to study quantum many-body physics with strongly dipolar gases. Fermi statistics, however, should lead to a kinematic suppression of these harmful reactions. Indeed, we observe a 120-fold suppression of dipolar relaxation in fermionic versus bosonic Dy, as expected from theory describing universal inelastic dipolar scattering. Similarly low inelastic cross sections are observed in spin mixtures, also with striking correspondence to theory predictions. The suppression of relaxation opens the possibility of employing fermionic dipolar species---atoms or molecules---in studies of quantum many-body physics involving, e.g., synthetic gauge fields and pairing.

Burdick, Nathaniel Q; Tang, Yijun; Lu, Mingwu; Lev, Benjamin L

2014-01-01

308

Effect of canopy and topography induced wakes on land-atmosphere fluxes of momentum and scalars  

NASA Astrophysics Data System (ADS)

Wakes shed from natural and anthropogenic landscape features affect land-atmosphere fluxes of momentum and scalars, including water vapor and trace gases (e.g. CO2). Canopies and bluff bodies, such as forests, buildings and topography, cause boundary layer flow separation, and lead to a break down of standard Monin-Obukhov similarity relationships in the atmospheric boundary layer (ABL). Wakes generated by these land surface features persist for significant distances (>100 typical length scales) and affect a large fraction of the Earth's terrestrial surface. This effect is currently not accounted for in land-atmosphere models, and little is known about how heterogeneity of wake-generating features affect land surface fluxes. Additionally flux measurements, made in wake-affected regions, do not satisfy the homogeneous flow requirements for the standard eddy correlation (EC) method. This phenomenon, often referred to as wind sheltering, has been shown to affect momentum and kinetic energy fluxes at the lake-atmosphere interface (Markfort et al. 2010). This presentation will highlight results from controlled wind tunnel experiments of neutral and thermally stratified boundary layers, using particle image velocimetry (PIV) and custom x-wire/cold-wire anemometry, to understand how the physical structure of upstream bluff bodies and porous canopies as well as how thermal stability affect the flow separation zone, boundary layer recovery and surface fluxes. We have found that there is a nonlinear relationship between canopy length/porosity and flow separation downwind of a canopy to clearing transition. Results will provide the basis for new parameterizations to account for wake effects on land-atmosphere fluxes and corrections for the EC measurements over open fields, lakes, and wetlands. Key words: Atmospheric boundary layer; Wakes; Stratification; Land-Atmosphere Parameterization; Canopy

Markfort, C. D.; Zhang, W.; Porté-Agel, F.; Stefan, H. G.

2012-04-01

309

Introduction to wakefields and wake potentials  

SciTech Connect

What are wakefields and wake potentials, and why are these concepts useful in the physics of linear accelerators and storage rings We approach this question by first reviewing the basic physical concepts which underlie the mathematical formalism. We then present a summary of the various techniques that have been developed to make detailed calculations of wake potentials. Finally, we give some applications to current problems of interest in accelerator physics. No attempt at completeness can be made in an introductory article of modest length. Rather, we try to give a broad overview and to list key references for more detailed study. It will also be apparent that the last chapter on this subject, with all the loose ends neatly tied up, has yet to be written. There are subtle points, there are controversial questions, and active calculations to resolve these questions are continuing at the time of this writing. 61 refs., 10 figs., 1 tab.

Wilson, P.B.

1989-01-01

310

[Migraine and "sleep-wake" cycle].  

PubMed

The results of the comparative analysis of patients with migraine attacks developing in the waking period (AP) and night sleep period are presented. Patients with sleep-related migraine (SM) had higher levels of depression and anxiety, marked autonomic dysfunction and stable sleep disturbances that resulted in significant maladaptation of these patients in the between-attacks period and low quality of life. Depression plays a role in the transformation of AP to SM and formation of stable sleep disturbances in the between-attacks period in patients with SM. It is suggested that common serotoninergic mechanisms involved in migraine attack may contribute to formation of "sleep-wake" cycle and depression development. The principles of optimal therapy of such patients are formulated. PMID:16768218

Osipova, V V; Levin, Ia I

2006-01-01

311

A Colorful Wake for Gerhard Soff  

E-print Network

We calculate the wake induced in a hot QCD plasma by a fast parton in the framework of linear response theory. We discuss two scenarios: ($i$) a weakly coupled quark-gluon plasma described by hard-thermal loop perturbation theory and ($ii$) a strongly coupled quark-gluon plasma which resembles a quantum liquid. We show that a Mach cone can appear in the second scenario, but not in the first one

Berndt Müller; Jörg Ruppert

2005-07-18

312

Wake patterns of the wings and tail of hovering hummingbirds  

NASA Astrophysics Data System (ADS)

The flow fields of slowly flying bats and fasterflying birds differ in that bats produce two vortex loops during each stroke, one per wing, and birds produce a single vortex loop per stroke. In addition, the circulation at stroke transition approaches zero in bats but remains strong in birds. It is unknown if these difference derive from fundamental differences in wing morphology or are a consequence of flight speed. Here, we present an analysis of the horizontal flow field underneath hovering Anna's hummingbirds (Calypte anna) to describe the wake of a bird flying at zero forward velocity. We also consider how the hummingbird tail interacts with the wake generated by the wings. High-speed image recording and analysis from three orthogonal perspectives revealed that the wing tips reach peak velocities in the middle of each stroke and approach zero velocity at stroke transition. Hummingbirds use complex tail kinematic patterns ranging from in phase to antiphase cycling with respect to the wings, covering several phase shifted patterns. We employed particle image velocimetry to attain detailed horizontal flow measurements at three levels with respect to the tail: in the tail, at the tail tip, and just below the tail. The velocity patterns underneath the wings indicate that flow oscillates along the ventral-dorsal axis in response to the down- and up-strokes and that the sideways flows with respect to the bird are consistently from the lateral to medial. The region around the tail is dominated by axial flows in dorsal to ventral direction. We propose that these flows are generated by interaction between the wakes of the two wings at the end of the upstroke, and that the tail actively defects flows to generate moments that contribute to pitch stability. The flow fields images also revealed distinct vortex loops underneath each wing, which were generated during each stroke. From these data, we propose a model for the primary flow structures of hummingbirds that more strongly resembles the bat model. Thus, pairs of unconnected vortex loops may be shared features of different animals during hovering and slow forward flight.

Altshuler, Douglas L.; Princevac, Marko; Pan, Hansheng; Lozano, Jesse

313

Wake patterns of the wings and tail of hovering hummingbirds  

NASA Astrophysics Data System (ADS)

The flow fields of slowly flying bats and faster-flying birds differ in that bats produce two vortex loops during each stroke, one per wing, and birds produce a single vortex loop per stroke. In addition, the circulation at stroke transition approaches zero in bats but remains strong in birds. It is unknown if these difference derive from fundamental differences in wing morphology or are a consequence of flight speed. Here, we present an analysis of the horizontal flow field underneath hovering Anna’s hummingbirds ( Calypte anna) to describe the wake of a bird flying at zero forward velocity. We also consider how the hummingbird tail interacts with the wake generated by the wings. High-speed image recording and analysis from three orthogonal perspectives revealed that the wing tips reach peak velocities in the middle of each stroke and approach zero velocity at stroke transition. Hummingbirds use complex tail kinematic patterns ranging from in phase to antiphase cycling with respect to the wings, covering several phase shifted patterns. We employed particle image velocimetry to attain detailed horizontal flow measurements at three levels with respect to the tail: in the tail, at the tail tip, and just below the tail. The velocity patterns underneath the wings indicate that flow oscillates along the ventral-dorsal axis in response to the down- and up-strokes and that the sideways flows with respect to the bird are consistently from the lateral to medial. The region around the tail is dominated by axial flows in dorsal to ventral direction. We propose that these flows are generated by interaction between the wakes of the two wings at the end of the upstroke, and that the tail actively defects flows to generate moments that contribute to pitch stability. The flow fields images also revealed distinct vortex loops underneath each wing, which were generated during each stroke. From these data, we propose a model for the primary flow structures of hummingbirds that more strongly resembles the bat model. Thus, pairs of unconnected vortex loops may be shared features of different animals during hovering and slow forward flight.

Altshuler, Douglas L.; Princevac, Marko; Pan, Hansheng; Lozano, Jesse

2009-05-01

314

Counterpropagating Rossby waves in confined plane wakes  

PubMed Central

In the present work, we revisit the temporal and the spatio-temporal stability of confined plane wakes under the perspective of the counterpropagating Rossby waves (CRWs). Within the context of broken line velocity profiles, each vorticity discontinuity can be associated to a counterpropagating Rossby wave. In the case of a wake modeled by a broken line profile, the interaction of two CRWs is shown to originate in a shear instability. Following this description, we first recover the stability results obtained by Juniper [J. Fluid Mech. 590, 163–185 (2007)]10.1017/S0022112007007975 and Biancofiore and Gallaire [Phys. Fluids 23, 034103 (2011)]10.1063/1.3554764 by means of the classical normal mode analysis. In this manner, we propose an explanation of the stabilizing influence of the confinement on the temporal stability properties. The CRW description further allows us to propose a new interpretation of the counterintuitive spatio-temporal destabilization in wake flows at moderate confinement noticed by Juniper [J. Fluid Mech. 565, 171–195 (2006)]10.1017/S0022112006001558: it is well predicted by the mean group velocity of the uncoupled CRWs. PMID:22865998

Biancofiore, L.; Gallaire, F.

2012-01-01

315

Molecular mechanisms of sleep and wakefulness.  

PubMed

Major questions on the biology of sleep include the following: what are the molecular functions of sleep; why can wakefulness only be sustained for defined periods before there is behavioral impairment; what genes contribute to the individual differences in sleep and the response to sleep deprivation? Behavioral criteria to define sleep have facilitated identification of sleep states in a number of different model systems: Drosophila, zebrafish, and Caenorhabditis elegans. Each system has unique strengths. Studies in these model systems are identifying conserved signaling mechanisms regulating sleep that are present in mammals. For example, the PKA-CREB signaling mechanism promotes wakefulness in Drosophila, mice, and C. elegans. Microarray studies indicate that genes whose expression is upregulated during sleep are involved in macromolecule biosynthesis (proteins, lipids [including cholesterol], heme). Thus, a key function of sleep is likely to be macromolecule synthesis. Moreover, in all species studied to date, there is upregulation of the molecular chaperone BiP with extended wakefulness. Sleep deprivation leads to cellular ER stress in brain and the unfolded protein response. Identification of genes regulating sleep has the potential for translational studies to elucidate the genetics of sleep and response to sleep deprivation in humans. PMID:18591493

Mackiewicz, Miroslaw; Naidoo, Nirinjini; Zimmerman, John E; Pack, Allan I

2008-01-01

316

[Sleep and wakefulness regulation: molecular players].  

PubMed

By combining brain section/lesion studies and sleep analysis, neurophysiologists have identified the brain areas responsible for regulating sleep and wakefulness during the first half of the 20th century. Identification of the phenotypic nature of the neurons underlying the regulation of vigilance, as well as their anatomical and functional connections led to a theoretical model based on mutual inhibitory interactions between sleep-on neurons (namely GABAergic neurons of the hypothalamic preoptic region) and wake-on neurons (mainly monoaminergic and cholinergic neurons). In addition to the corresponding neurotransmitters (serotonin, acetylcholine and GABA), other neuroactive molecules that play key roles in sleep and wakefulness regulation have recently been discovered, leading to an updated model. Hypocretin, also known as orexin, is a key neuropeptide involved in the sleep disorder narcolepsy. Extensive characterization of the respective roles of these neurotransmitters has led to the identification of novel therapeutic targets for the treatment of sleep disorders. For example, blockade of hypocretin receptors has hypnotic effects. PMID:22812160

Fabre, Véronique; Adrien, Joëlle; Bonnavion, Patricia; Hamon, Michel

2011-10-01

317

Identification of characteristic properties in different vessel wake signals  

NASA Astrophysics Data System (ADS)

The potential threat in terms of environmental protection and safe navigation posed by wake waves from high-speed ferries and fast conventional ships is well documented. Vessels that travel in the near-critical regime (depth Froude number ? 1) at some sections along their ship tracks can generate packets of large, solitonic, very long and long crested waves. The heights and periods of the leading waves, excited at near-critical speeds, may be much larger than those of conventional ferries or vessels travelling at even slightly slower speeds. However, it is difficult to determine a general characterization of such wakes at the coast, due to the transient and nonlinear nature of this phenomenon, and the fact that wake impact is influenced by the local bathymetry and coastline configuration. Such a characterization is required in order to set reasonable limits to wake wash that are sufficient for protection but not excessively restrictive for ship navigation. This paper investigates the potential benefits of wake analysis by means of a time-frequency technique (windowed Fourier transform), which is well known in signal analysis but has only recently been applied in wake analysis. Analysis of ship wakes have been performed based on instrumental data of sea surface elevation recorded at different sites in Tallinn Bay, the Baltic Sea, which is characterized by very intense ship traffic and provides a very rich collection of vessel-wake signals. Results show that the wake signals are easily identified in spectrograms. The method is particularly useful for identification of low frequency signals that may easily be masked by high frequency noise in the wave record. Furthermore, the spectrogram provides an image of the wake that makes it possible to associate wake events with individual ships at a given location. This approach also opens a new direction for the statistical description of wakes, applicable to the characterization of the "wake climate" for sites with intense vessel traffic.

Didenkulova, Ira; Sheremet, Alex; Torsvik, Tomas; Soomere, Tarmo

2013-04-01

318

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

319

Suppression of the metal-insulator transition by magnetic field in (Pr1-yYy)0.7Ca0.3CoO3 (y = 0.0625)  

NASA Astrophysics Data System (ADS)

The (Pr1-yYy)0.7Ca0.3CoO3 compound (y = 0.0625, TMI-SS=40 K), at the lower limit for occurrence of the first-order metal-insulator (MI) and simultaneous spin-state (SS) transitions, has been studied using electrical resistivity and magnetization measurements in magnetic fields up to 17 T. The isothermal experiments demonstrate that the low-temperature insulating phase can be destabilized by an applied field and the metallic phase returns well below the transition temperature TMI-SS. The reverse process with decreasing field occurs with a significant hysteresis. The temperature scans taken at fixed magnetic fields reveal a parabolic-like decrease in TMI-SS with increasing field strength and a complete suppression of the MI-SS transition in fields above 9 T.

Naito, Tomoyuki; Fujishiro, Hiroyuki; Nishizaki, Terukazu; Kobayashi, Norio; Hejtmánek, Ji?í; Knížek, Karel; Jirák, Zden?k

2014-06-01

320

Blockade of recruitment of ovarian follicles by suppression of the secondary surge of follicle-stimulating hormone with porcine follicular field.  

PubMed Central

The increased serum concentration of follicle-stimulating hormone (FSH; follitropin) between proestrus and estrus in the rat has been hypothesized to recruit the follicles destined to ovulate in the next cycle. Injection of porcine follicular fluid (PFF) late in proestrus suppresses the secondary FSH surge; injection early in proestrus suppresses the primary FSH surge without affecting the secondary FSH surge. Thus, it is possible to use PFF to test the FSH/follicular recruitment hypothesis and to distinguish between the contributions of the primary and secondary FSH surges to this recruitment. The normal recruitment of follicles occurs in the diameter range 350-499 microns between the day of proestrus and the day of estrus. When the secondary FSH surge was suppressed by injection of PFF late in proestrus, PFF, but not porcine serum (PS), blocked follicular recruitmenet into size groups of 350-499 microns on the morning of estrus. The number of ova ovulated did not differ between PFF- and PS-treated animals. When we suppressed only the primary FSH surge, by injecting PFF early in proestrus, there were no differences between PFF- and PS-treated animals in the number of ova ovulated, follicle size distribution, or hormones. In the last experiment, the secondary FSH surge was blocked with PFF but was replaced with exogenous ovine FSH which caused a dose-related increase in follicular recruitment, substantiating the interpretation that the follicular fluid suppressed recruitment by suppressing FSH secretion. Thus, in mammals with short reproductive cycles, the gonadotropin surges provide a "fail-safe" mechanism whereby luteinizing hormone triggers ovulation, thus ending one cycle, and the secondary increase in FSH levels recruits follicles for the next cycle. Images PMID:6776533

Hoak, D C; Schwartz, N B

1980-01-01

321

Detection and Behavior of Pan Wakes in Saturn's A Ring  

NASA Technical Reports Server (NTRS)

Six previously unseen Pan wakes are found interior and exterior to the Encke gap in Saturn's A ring, one in the Voyager 2 photopolarimeter (PPS) stellar occultation data and five in the Voyager 1 radio science (RSS) Earth occultation data. Pan orbits at the center of the Encke gap and maintains it...The detection of Pan wakes at longitudes greater than 360(deg) demonstrates that wakes persist for much longer than originally hypothesized and may interact with one another.

Horn, L. J.; Showalter, M. R.; Russell, C. T.

1996-01-01

322

Changing and shielded magnetic fields suppress c-Fos expression in the navigation circuit: input from the magnetosensory system contributes to the internal representation of space in a subterranean rodent  

PubMed Central

The neural substrate subserving magnetoreception and magnetic orientation in mammals is largely unknown. Previous experiments have demonstrated that the processing of magnetic sensory information takes place in the superior colliculus. Here, the effects of magnetic field conditions on neuronal activity in the rodent navigation circuit were assessed by quantifying c-Fos expression. Ansell's mole-rats (Fukomys anselli), a mammalian model to study the mechanisms of magnetic compass orientation, were subjected to natural, periodically changing, and shielded magnetic fields while exploring an unfamiliar circular arena. In the undisturbed local geomagnetic field, the exploration of the novel environment and/or nesting behaviour induced c-Fos expression throughout the head direction system and the entorhinal–hippocampal spatial representation system. This induction was significantly suppressed by exposure to periodically changing and/or shielded magnetic fields; discrete decreases in c-Fos were seen in the dorsal tegmental nucleus, the anterodorsal and the laterodorsal thalamic nuclei, the postsubiculum, the retrosplenial and entorhinal cortices, and the hippocampus. Moreover, in inactive animals, magnetic field intensity manipulation suppressed c-Fos expression in the CA1 and CA3 fields of the hippocampus and the dorsal subiculum, but induced expression in the polymorph layer of the dentate gyrus. These findings suggest that key constituents of the rodent navigation circuit contain populations of neurons responsive to magnetic stimuli. Thus, magnetic information may be integrated with multimodal sensory and motor information into a common spatial representation of allocentric space within this circuit. PMID:20219838

Burger, Tomas; Lucova, Marcela; Moritz, Regina E.; Oelschlager, Helmut H. A.; Druga, Rastislav; Burda, Hynek; Wiltschko, Wolfgang; Wiltschko, Roswitha; Nemec, Pavel

2010-01-01

323

Numerical investigation of laminar-turbulent transition in a flat plate wake  

NASA Technical Reports Server (NTRS)

Lamina-turbulent transition of high-deficit flat plate wakes is investigated by direct numerical simulations using the complete Navier Stokes equations. The simulations are based on a spatial model so that both the base flow and the disturbance flow can develop in the downstream direction. The Navier Stokes equations are used in a vorticity-velocity form and are solved using a combination of finite difference and spectral approximations. Fourier series are used in the spanwise direction. Second-order finite-differences are used to approximate the spatial derivatives in the streamwise and transverse directions. For the temporal discretion, a combination of ADI, Crank-Nicolson, and Adams-Bashforth methods is employed. The discretized velocity equations are solved using fast Helmholtz solvers. Code validation is accomplished by comparison of the numerical results to both linear stability and to experiments. Calculations of two- and/or three-dimensional sinuous and mode disturbances in the wake of flat plate are undertaken. For calculations of two-dimensional disturbances, the wake is forced at an amplitude level so that nonlinear disturbance development may be observed. In addition, the forcing amplitude is varied in order to determine its effect on the disturbance behavior. To investigate the onset of three-dimensionality, the wake is forced with a small-amplitude three-dimensional disturbance and a larger amplitude two-dimensional disturbance. The two-dimensional forcing amplitude is varied in order to determine its influence on the three-dimensional flow field.

Fasel, Hermann F.; Newell, Alan C.; Dratler, David I.

1990-01-01

324

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

NASA Technical Reports Server (NTRS)

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

Hall, G. F.

1975-01-01

325

Plasma wave turbulence due to the wake of an ionospheric sounding rocket  

NASA Astrophysics Data System (ADS)

A sounding rocket moving in the ionosphere generally interacts with surrounding plasma. Because this affects in-situ measurement data, it is very important to understand the physics of the interaction between the ionosphere and a moving vehicle. For instance, a rarefied plasma region called "plasma wake" is formed behind a sounding rocket. Several previous studies based on rocket experiments have suggested that upper-hybrid resonance (UHR) mode waves are excited in a rocket wake. A wake turbulence model has been proposed as a possible explanation for the waves where two stream instability occurs in the wake center owing to the incident plasma flow from the both sides of the wake edges. Thus, plasma waves are generated and have been observed by the wave receivers onboard the rockets. Plasma waves in a wake have been reported not only around sounding rockets but also around solar system bodies such as Moon. As for a rocket wake, the generation mechanism of the waves has been investigated by using wave receivers with time resolutions worse than 500 msec. They are, however, not enough for detailed investigations about the plasma wave generations and propagations. To discuss the properties of the plasma waves caused around a rocket wake, we have analyzed the data of electric fields and electron number density in the S-520-26 sounding rocket experiment, which was performed at Uchinoura, Japan, on January 12, 2012. The rocket reached an altitude of 298 km, and the data has been obtained four or five times in one spin period of the rocket by using a newly developed digital plasma wave monitor and an impedance probe, whose time resolutions are about 260 msec. In the observation, enhancement of plasma waves has been observed in two frequency ranges from 0.02 to 0.9 MHz (LF range), and from 1.3 to 2.4 MHz (MF range). The frequency range of the MF emissions is around the UHR frequency, which is determined based on the IGRF magnetic field model and electron number density measured by the impedance probe. However, the lowest frequency of the emissions is almost the same as the Z-mode cutoff frequency, particularly in higher altitude range than 280 km. The wave spectra are similar to those observed by the previous studies. The frequency range of the LF emissions is found to be that of whistler mode branch. Based on the rocket attitude, it is suggested that the electric fields of the LF and MF emissions are nearly perpendicular and parallel to the wake structure, respectively. If we can assume that the observed waves are generated around the rocket, they have to be electrostatic waves because the wave length should be shorter than the size of the disturbed region. We have performed calculations of plasma dispersion relations with assuming some anisotropic velocity distribution functions of electrons expected around the wake, and deduced the linear growth rates, group velocities, etc. We compare the observational results with calculated ones, and discuss the generation mechanisms of the plasma waves.

Endo, Ken; Kumamoto, Atsushi; Oya, Hiroshi; Ono, Takayuki; Katoh, Yuto

2013-04-01

326

Rotor wake characteristics of a transonic axial flow fan  

NASA Technical Reports Server (NTRS)

State of the art turbomachinery flow analysis codes are not capable of predicting the viscous flow features within turbomachinery blade wakes. Until efficient 3D viscous flow analysis codes become a reality there is therefore a need for models which can describe the generation and transport of blade wakes and the mixing process within the wake. To address the need for experimental data to support the development of such models, high response pressure measurements and laser anemometer velocity measurements were obtained in the wake of a transonic axial flow fan rotor.

Hathaway, M. D.; Gertz, J.; Epstein, A.; Strazisar, A. J.

1985-01-01

327

NASA aerial applications wake interaction research. [particle trajectories in aircraft induced wakes  

NASA Technical Reports Server (NTRS)

A computer code has been developed for predicting trajectories of particles released in the wakes of airplanes or helicopters. The code accounts for effects of turbulence, crosswind, propeller slipstream, terrain variations, and plant canopy density on particle trajectories. Comparisons are given between experiments and theory. Applications of the code for spray pattern improvement are illustrated.

Morris, D. J.; Croom, C. C.; Holmes, B. J.; Van Dam, C. P.

1982-01-01

328

Wakes from arrays of buildings. [flight safety  

NASA Technical Reports Server (NTRS)

Experiments were carried out in a small wind tunnel in which atmospheric flow around buildings was simulated. Arrays of one, two, three, and four model buildings were tested, and wake profiles of velocity and turbulence were measured. The data indicate the effect of the buildings on the wind environment encountered by aircraft during landing or takeoff operations. It was possible to use the results to locate the boundaries of the air regions affected by the obstacles and to recommend preferred arrangements of buildings to maximize light safety.

Logan, E., Jr.; Lin, S. H.

1982-01-01

329

Geometrical Wake of a Smooth Flat Collimator  

SciTech Connect

A transverse geometrical wake generated by a beam passing through a smooth flat collimator with a gradually varying gap between the upper and lower walls is considered. Based on generalization of the approach recently developed for a smooth circular taper we reduce the electromagnetic problem of the impedance calculation to the solution of two much simpler static problems - a magnetostatic and an electrostatic ones. The solution shows that in the limit of not very large frequencies, the impedance increases with the ratio h/d where h is the width and d is the distance between the collimating jaws. Numerical results are presented for the NLC Post Linac collimator.

Stupakov, G.V.; /SLAC

2011-09-09

330

Wake Forest University Physics Demonstration Videos  

NSDL National Science Digital Library

Physics is plenty exciting on its own, but this clutch of physics demonstration videos offered up by Wake Forest University's Physics departments will probably have students running out to learn more about string theory and cosmology. Teachers will definitely appreciate this resource, as they can use these videos in the classroom or just recommend to their students. Visitors can view the videos in their entirety by subject headings, which include "Motion", "Heat", "Optics", and not surprisingly, "Newton". All told there are dozens of videos, including "Bed of Nails", "Cartesian Diver", and the surreal yet appropriately titled "Marshmallow Man". Overall, this resource is a delightful find.

331

Thermal wake/vessel detection technique  

DOEpatents

A computer-automated method for detecting a vessel in water based on an image of a portion of Earth includes generating a thermal anomaly mask. The thermal anomaly mask flags each pixel of the image initially deemed to be a wake pixel based on a comparison of a thermal value of each pixel against other thermal values of other pixels localized about each pixel. Contiguous pixels flagged by the thermal anomaly mask are grouped into pixel clusters. A shape of each of the pixel clusters is analyzed to determine whether each of the pixel clusters represents a possible vessel detection event. The possible vessel detection events are represented visually within the image.

Roskovensky, John K. (Albuquerque, NM); Nandy, Prabal (Albuquerque, NM); Post, Brian N (Albuquerque, NM)

2012-01-10

332

Neuropharmacology of Sleep and Wakefulness: 2012 Update  

PubMed Central

Synopsis The development of sedative/hypnotic molecules has been empiric rather than rational. The empiric approach has produced clinically useful drugs but for no drug is the mechanism of action completely understood. All available sedative/hypnotic medications have unwanted side effects and none of these medications creates a sleep architecture that is identical to the architecture of naturally occurring sleep. This chapter reviews recent advances in research aiming to elucidate the neurochemical mechanisms regulating sleep and wakefulness. One promise of rational drug design is that understanding the mechanisms of sedative/hypnotic action will significantly enhance drug safety and efficacy. PMID:23162386

Watson, Christopher J.; Baghdoyan, Helen A.; Lydic, Ralph

2012-01-01

333

Melanin-concentrating hormone control of sleep-wake behavior.  

PubMed

The melanin-concentrating hormone (MCH) is a 19 aminoacid peptide found in mammals predominantly in neurons located in the lateral hypothalamus and incerto-hypothalamic area. The biological function of MCH is mediated by two G-protein-coupled receptors known as MCHR1 and MCHR2, although the latter is expressed only in carnivores, primates and man. The MCHR1 couples to Gi, Gq and Go proteins, with Gi leading to the inhibition of both excitatory and inhibitory synaptic events. Within the central nervous system (CNS) MCH participates in a number of functions including sleep-wake behavior. In this respect, MCHergic neurons project widely throughout the CNS to brain regions involved in the regulation of behavioral states. MCHergic neurons are silent during wakefulness (W), increase their firing during slow wave sleep (SWS) and still more during REM sleep (REMS). Studies in knockout mice for MCH (MCH(-/-)) have shown a reduction in SWS and an increase of W during the light and the dark phase of the light-dark cycle. Moreover, in response to food deprivation a marked reduction in REMS time was observed in these animals. Conflicting effects on sleep variables have been reported in MCHR1(-/-) mice by different authors. The i.c.v. administration of MCH increases REMS and SWS in the rat. In addition, an enhancement of REMS has been described following the microinjection of the neuropeptide into the nucleus pontis oralis of the cat, while its infusion into the dorsal raphe nucleus (DR) and the basal forebrain (horizontal limb of the diagonal band of Broca) is followed by an increase of REMS and a reduction of W in the rat. Immunoneutralization of MCH in the DR augmented W and suppressed REMS in the rat, as did the s.c. injection of selective MCHR1 antagonists. The robust REMS-inducing effect of MCH is likely related to the deactivation of monoaminergic, orexinergic, glutamatergic, cholinergic (W-on) and GABAergic (REM-off) neurons involved in the generation of W and the inhibition of REMS. On the basis of preclinical studies, it can be proposed that selective MCHR1 receptor agonists could constitute potential therapeutic modalities in the arsenal of insomnia pharmacotherapy. Due to the lack of adequate animal models, the role of the MCHR2 on sleep is still unknown. PMID:23477948

Monti, Jaime M; Torterolo, Pablo; Lagos, Patricia

2013-08-01

334

A vorticity-free approach to wake-based swimming/flying force estimation  

NASA Astrophysics Data System (ADS)

Traditional wake-based analyses of animal swimming and flying depend largely on knowledge of the vorticity field, which can be difficult or impossible to incorporate in the context of unsteady fluid-structure interactions. This talk will describe the development and application of a technique for estimating swimming/flying forces that does not require measurement of the vorticity field. The method is based on the identification of Lagrangian Coherent Structures in the wake, whose dynamics are governed by the theory for deformable bodies in potential flow (Peng and Dabiri, J. Exp. Biol. 2007). This paradigm for the analysis of unsteady fluid-structure interactions is integrated with existing DPIV measurement techniques to analyze medusan (jellyfish) swimming and the dynamics of the bluegill sunfish pectoral fin.

Dabiri, John O.; Peng, Jifeng

2006-11-01

335

Observing the multiverse with cosmic wakes  

NASA Astrophysics Data System (ADS)

Current theories of the origin of the Universe, including string theory, predict the existence of a multiverse with many bubble universes. These bubble universes may collide, and collisions with ours produce cosmic wakes that enter our Hubble volume, appear as unusually symmetric disks in the cosmic microwave background, and disturb large scale structure. There is preliminary evidence consistent with one or more of these disturbances on our sky. However, other sources can produce similar features in the cosmic microwave background, and so additional signals are needed to verify their extra-universal origin. Here we find, for the first time, the detailed three-dimensional shape, temperature, and polarization signals of the cosmic wake of a bubble collision consistent with current observations. The polarization pattern has distinct features that when correlated with the corresponding temperature pattern are a unique and striking signal of a bubble collision. These features represent a verifiable prediction of the multiverse paradigm and might be detected by current or future experiments. A detection of a bubble collision would confirm the existence of the multiverse, provide compelling evidence for the string theory landscape, and sharpen our picture of the Universe and its origins.

Kleban, Matthew; Levi, Thomas S.; Sigurdson, Kris

2013-02-01

336

Wake interaction of two disks falling in tandem  

Microsoft Academic Search

The fluid dynamics video illustrates the interaction of two disks falling in tandem at Reynolds number close to 100. Two fluorescent dyes were used to visualize the wake of each body. We can observe that the trailing body accelerates thanks to the entrainment provided by the wake of the leading body and eventually catches up the leadind body. Then, thick

N. Brosse; S. Cazin; P. Ern

2010-01-01

337

Simulation of wake-induced transition on a flat plate  

Microsoft Academic Search

We present results from the direct numerical simulation of the interaction between a laminar flat-plate boundary layer and the unsteady wake of a stationary circular cylinder placed above the plate leading edge. Interactions of this type occur in many applications, such as the flow past a multi-element airfoil, in which boundary layers on downstream elements develop underneath the wake from

Ugo Piomelli; Elias Balaras; Meelan Choudhari

2001-01-01

338

Vortical patterns in the wake of an oscillating airfoil  

Microsoft Academic Search

The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied in a low-speed water channel. It is shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude, and also the shape of the oscillation waveform. An important observation in

Manoochehr M. Koochesfahani

1989-01-01

339

Modeling the turbulent trailing ship wake in the infrared.  

PubMed

The sea surface turbulent trailing wake of a ship, which can be rather easily observed in the infrared by airborne surveillance systems, is a consequence of the difference in roughness and temperature between the wake and the sea background. We have developed a phenomenological model for the infrared radiance of the turbulent wake by assuming that the sea surface roughness is dependent upon the turbulent intensity near the sea surface. Describing the sea surface roughness with a Cox and Munk probability distribution function of slopes, we distinguish on the sea surface between the sea background and the turbulent wake by the variance of sea surface slopes, ?CM2=constant and ?TW2(x,y)?constant. The latter dependence is assumed to be inversely proportional to the turbulent intensity of the wake, Urms(x,y). Given the incident solar, atmospheric, and sky infrared radiances, we calculate the reflected and emitted sea surface radiance from both the wake and the background. We compare the infrared contrast of the wake with infrared image data obtained in an airborne trial. Our predictions and the measurements agree very well in trend over a significant range of observer zenith angles. Our calculations reveal the strong dependence of the wake radiance on the observer zenith angle, allowing for positive and negative contrasts with the background. PMID:25089992

Issa, Vivian; Daya, Zahir A

2014-07-01

340

Coupled wake boundary layer model of wind-farms  

E-print Network

We present and test a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall wind-farm boundary layer structure. The wake expansion/superposition model captures the effect of turbine positioning, while the top-down portion adds the interaction between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the model requires specification of a parameter that is not known a-priori. For the wake model the wake expansion coefficient is required, while the top-down model requires an effective span-wise turbine spacing within which the model's momentum balance is relevant. The wake expansion coefficient is obtained by matching the predicted mean velocity at the turbine from both approaches, while the effective span-wise turbine spacing depends on turbine positioning and thus can be determined from the wake expansion...

Stevens, Richard J A M; Meneveau, Charles

2014-01-01

341

Momentum transfer between the Io plasma wake and Jupiter's ionosphere  

Microsoft Academic Search

The interaction between Io and Jupiter is dramatically illustrated by recent ultraviolet and infrared imaging of Jupiter's ionosphere. Bright auroral emissions are observed at the base of Io's flux tube with emissions at the footprint of Io's wake extending large distances downstream (roughly 100° around Jupiter). We propose as a possible explanation for the persisting wake emissions a subcorotating torus

P. A. Delamere; F. Bagenal; R. Ergun; Y.-J. Su

2003-01-01

342

Proceedings of the ARO Rotorcraft Wake Prediction Basic Research Workshop  

E-print Network

In Hovering Rotor Tip Vortex Dynamics 3 H.Tadghighi Boeing Current Assessments Of Boeing-Mesa CFD Tools Vortex Calculations To Wind Tunnel Measurements 5 S. P´eron, C. Benoit, G. Jeanfaivre ONERA High Free-wake coupled with Vortex-lattice, Potential-panel and CFD method 9 A.Wissink AFDD Wake Prediction

343

Visualization of airflow in the wake of a ship superstructure  

Microsoft Academic Search

Helicopter landings on naval surface ships, such as cruisers and destroyers, must take place in the presence of an air wake created by flow over the ship superstructure. Wake turbulence over the flight deck makes piloted landings dangerous and difficult, and poses significant problems for the use of unmanned rotorcraft. To address this problem, a comprehensive set of experimental and

C. J. Brownell; W. P. Stillman; J. H. Golden; S. A. Simpson; L. Luznik; D. S. Miklosovic; G. White; J. S. Burks; M. R. Snyder

2009-01-01

344

Dynamics of the vortex wakes of flying and swimming vertebrates.  

PubMed

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

Rayner, J M

1995-01-01

345

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

346

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

347

Direct Numerical Simulation of a Temporally Evolving Incompressible Plane Wake: Effect of Initial Conditions on Evolution and Topology  

NASA Technical Reports Server (NTRS)

Direct numerical simulations have been used to examine the effect of the initial disturbance field on the development of three-dimensionality and the transition to turbulence in the incompressible plane wake. The simulations were performed using a new numerical method for solving the time-dependent, three-dimensional, incompressible Navier-Stokes equations in flows with one infinite and two periodic directions. The method uses standard Fast Fourier Transforms and is applicable to cases where the vorticity field is compact in the infinite direction. Initial disturbances fields examined were combinations of two-dimensional waves and symmetric pairs of 60 deg oblique waves at the fundamental, subharmonic, and sub-subharmonic wavelengths. The results of these simulations indicate that the presence of 60 deg disturbances at the subharmonic streamwise wavelength results in the development of strong coherent three-dimensional structures. The resulting strong three-dimensional rate-of-strain triggers the growth of intense fine scale motions. Wakes initiated with 60 deg disturbances at the fundamental streamwise wavelength develop weak coherent streamwise structures, and do not develop significant fine scale motions, even at high Reynolds numbers. The wakes which develop strong three-dimensional structures exhibit growth rates on par with experimentally observed turbulent plane wakes. Wakes which develop only weak three-dimensional structures exhibit significantly lower late time growth rates. Preliminary studies of wakes initiated with an oblique fundamental and a two-dimensional subharmonic, which develop asymmetric coherent oblique structures at the subharmonic wavelength, indicate that significant fine scale motions only develop if the resulting oblique structures are above an angle of approximately 45 deg.

Sondergaard, R.; Cantwell, B.; Mansour, N.

1997-01-01

348

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

349

Measuremants in the wake of an infinite swept airfoil  

NASA Technical Reports Server (NTRS)

This is a report of the measurements in the trailing edge region as well as in the report of the developing wake behind a swept NACA 0012 airfoil at zero incidence and a sweep angle of 30 degrees. The measurements include both the mean and turbulent flow properties. The mean flow velocities, flow inclination and static pressure are measured using a calibrated three-hole yaw probe. The measurements of all the relevant Reynolds stress components in the wake are made using a tri-axial hot-wire probe and a digital data processing technique developed by the authors. The development of the three dimensional near-wake into a nearly two dimensional far-wake is discussed in the light of the experimental data. A complete set of wake data along with the data on the initial boundary layer in the trailing edge region of the airfoil are tabulated in an appendix to the report.

Novak, C. J.; Ramaprian, B. R.

1982-01-01

350

Contrail ice particles in aircraft wakes and their climatic importance  

NASA Astrophysics Data System (ADS)

Measurements of gaseous (NO, NOy, SO2, HONO) and ice particle concentrations in young contrails in primary and secondary wakes of aircraft of different sizes (B737, A319, A340, A380) are used to investigate ice particle formation behind aircraft. The gas concentrations are largest in the primary wake and decrease with increasing altitude in the secondary wake, as expected for passive trace gases and aircraft-dependent dilution. In contrast, the measured ice particle concentrations were found larger in the secondary wake than in the primary wake. The contrails contain more ice particles than expected for previous black carbon (soot) estimates. The ice concentrations may result from soot-induced ice nucleation for a soot number emission index of 1015 kg-1. For a doubled ice particle concentration in young contrails, a contrail cirrus model computes about 60% increases of global radiative forcing by contrail cirrus because of simultaneous increases in optical depth, age, and cover.

Schumann, Ulrich; JeßBerger, Philipp; Voigt, Christiane

2013-06-01

351

Comparison of Engineering Wake Models with CFD Simulations  

NASA Astrophysics Data System (ADS)

The engineering wake models by Jensen [1] and Frandsen et al. [2] are assessed for different scenarios simulated using Large Eddy Simulation and the Actuator Line method implemented in the Navier-Stokes equations. The scenarios include the far wake behind a single wind turbine, a long row of turbines in an atmospheric boundary layer, idealised cases of an infinitely long row of wind turbines and infinite wind farms with three different spacings. Both models include a wake expansion factor, which is calibrated to fit the simulated wake velocities. The analysis highlights physical deficiencies in the ability of the models to universally predict the wake velocities, as the expansion factor can be fitted for a given case, but with not apparent transition between the cases.

Andersen, S. J.; Sørensen, J. N.; Ivanell, S.; Mikkelsen, R. F.

2014-06-01

352

Axisymmetric turbulent wakes with new nonequilibrium similarity scalings.  

PubMed

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

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

2013-10-01

353

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

354

On the generation of a reverse Von Krmn street for the controlled cylinder wake in the laminar regime  

E-print Network

On the generation of a reverse Von Kármán street for the controlled cylinder wake in the laminar in the laminar regime. For a Reynolds number equal to 200, we give numerical evidence that partial control that a mean flow correction field with negative drag is observable for this controlled flow configuration

Boyer, Edmond

355

Ferromagnetic resonance probe liftoff suppression apparatus  

DOEpatents

A liftoff suppression apparatus utilizing a liftoff sensing coil to sense the amount a ferromagnetic resonance probe lifts off the test surface during flaw detection and utilizing the liftoff signal to modulate the probe's field modulating coil to suppress the liftoff effects.

Davis, Thomas J. (Issaquah, WA); Tomeraasen, Paul L. (West Richland, WA)

1985-01-01

356

Quantifying error of remote sensing observations of wind turbine wakes using computational fluid dynamics  

NASA Astrophysics Data System (ADS)

Wind-profiling lidars are now regularly used in boundary-layer meteorology and in applications such as wind energy and air quality. Lidar wind profilers exploit the Doppler shift of laser light backscattered from particulates carried by the wind to measure a line-of-sight (LOS) velocity. The Doppler Beam Swinging (DBS) technique, used by many commercial systems, considers measurements of this LOS velocity in multiple radial directions in order to estimate horizontal and vertical winds. The method relies on the assumption of homogeneous flow across the region sampled by the beams. Using such a system in inhomogeneous flow, such as wind turbine wakes or complex terrain, will result in errors. To quantify the errors expected from such violation of the assumption of horizontal homogeneity, we simulate inhomogeneous flow in the atmospheric boundary layer, notably stably-stratified flow past a wind turbine. This slightly stable case results in 15° of wind direction change across the turbine rotor disk. The resulting flow field is sampled in the same fashion that a lidar samples the atmosphere with the DBS approach, enabling quantification of the error in the DBS observations. The observations from the instruments located upwind have small errors, which are ameliorated with time averaging. However, the downwind observations, particularly within the first two rotor diameters downwind from the wind turbine, suffer from errors due to the heterogeneity of the wind turbine wake. Errors in the stream-wise component of the flow are generally small, less than 0.5 m s-1. Errors in the cross-stream and vertical velocity components are much larger: cross-stream component errors are on the order of 1.0 m s-1 and errors in the vertical velocity exceed the actual measurements of the vertical velocity. DBS-based assessments of wake wind speed deficits based on the stream-wise velocity can be relied on even within the near wake within 0.5 m s-1, but cross-stream and vertical velocity estimates in the near wake are compromised. Measurements of inhomogeneous flow such as wind turbine wakes are susceptible to these errors, and interpretations of field observations should account for this uncertainty.

Lundquist, J. K.; Churchfield, M. J.; Lee, S.; Clifton, A.

2014-09-01

357

Rosetta - waking up and then some  

NASA Astrophysics Data System (ADS)

The Rosetta Mission is the third cornerstone mission (after XMM and Cluster/SOHO) of the ESA programme Horizon 2000. The aim of the mission is to map the comet 67-P/Churyumov-Gerasimenko by remote sensing, to examine its environment insitu and its evolution in the inner solar system. The lander Philae will be the first device to land on a comet and perform in-situ science on the surface. Launched in March 2004 and after a number of gravity assists and various asteroid fly -bys, the spacecraft entered deep space hibernation in June 2011. Nearly 10 years after launch on 20th January 2014 at 10:00 UTC the spacecraft will wake up for comet rendez-vous preparation. This presentation will provide a brief overview of the mission up to date and provide an insight into the exciting years we have ahead of us as Rosetta reaches and studies its target.

Taylor, M. G. G. T.; Altobelli, N.; Jansen, F.; Kueppers, M.; Schwehm, G.; Alexander, C.; Barthelemy, M.; Geiger, B.; Moissl, R.; Vallat, C.; Grieger, B.; Schmidt, A.

2013-09-01

358

Morphine Inhibits Sleep-Promoting Neurons in the Ventrolateral Preoptic Area Via Mu Receptors and Induces Wakefulness in Rats  

PubMed Central

Morphine is the most efficacious and widely prescribed treatment for pain. However, it decreases the total amount of deep sleep and rapid eye movement sleep in humans. Acute morphine administration at low doses causes wakefulness in animal models. To clarify the mechanism by which morphine affects sleep–wake behavior, we investigated the effects of morphine on the sleep-promoting neurons of the ventrolateral preoptic area (VLPO), a putative sleep-active nucleus, using in vitro brain slices by the patch-clamp technique. We also examined the effects of morphine on sleep–wake profiles after administration of opioid receptor antagonist to the VLPO using EEG and electromyogram recordings in freely moving rats. The results showed that morphine inhibited the firing rate of sleep-promoting neurons and hyperpolarized their membrane potentials without affecting interneurons in the VLPO. Morphine-induced hyperpolarization of membrane potentials could be reversed by, D-Phe-Cys-Thr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a mu receptor antagonist, in the presence of tetrodotoxin. However, after the mu receptors were blocked by CTOP, morphine still suppressed the firing of the sleep-promoting neurons. This effect was antagonized by nor-BIN, a kappa receptor antagonist. Activation of kappa receptor by U50488H inhibited the firing of the sleep-promoting neurons. These results indicate that morphine could inhibit the activity of sleep-promoting neurons in the VLPO through mu and kappa receptors. EEG recordings revealed that morphine injected subcutaneously induced arousal in a dose-dependent manner. CTOP microinjected into VLPO antagonized the arousal effects of morphine, but nor-BIN did not. However, CTOP alone was not associated with any changes in the physiological sleep–wake cycle. Taken together, these findings clearly indicate that morphine inhibits sleep-promoting neurons in the VLPO by affecting mu receptors and so induces wakefulness in rats. PMID:23303062

Wang, Qin; Yue, Xiao-Fang; Qu, Wei-Min; Tan, Rong; Zheng, Ping; Urade, Yoshihiro; Huang, Zhi-Li

2013-01-01

359

Morphine inhibits sleep-promoting neurons in the ventrolateral preoptic area via mu receptors and induces wakefulness in rats.  

PubMed

Morphine is the most efficacious and widely prescribed treatment for pain. However, it decreases the total amount of deep sleep and rapid eye movement sleep in humans. Acute morphine administration at low doses causes wakefulness in animal models. To clarify the mechanism by which morphine affects sleep-wake behavior, we investigated the effects of morphine on the sleep-promoting neurons of the ventrolateral preoptic area (VLPO), a putative sleep-active nucleus, using in vitro brain slices by the patch-clamp technique. We also examined the effects of morphine on sleep-wake profiles after administration of opioid receptor antagonist to the VLPO using EEG and electromyogram recordings in freely moving rats. The results showed that morphine inhibited the firing rate of sleep-promoting neurons and hyperpolarized their membrane potentials without affecting interneurons in the VLPO. Morphine-induced hyperpolarization of membrane potentials could be reversed by, D-Phe-Cys-Thr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a mu receptor antagonist, in the presence of tetrodotoxin. However, after the mu receptors were blocked by CTOP, morphine still suppressed the firing of the sleep-promoting neurons. This effect was antagonized by nor-BIN, a kappa receptor antagonist. Activation of kappa receptor by U50488H inhibited the firing of the sleep-promoting neurons. These results indicate that morphine could inhibit the activity of sleep-promoting neurons in the VLPO through mu and kappa receptors. EEG recordings revealed that morphine injected subcutaneously induced arousal in a dose-dependent manner. CTOP microinjected into VLPO antagonized the arousal effects of morphine, but nor-BIN did not. However, CTOP alone was not associated with any changes in the physiological sleep-wake cycle. Taken together, these findings clearly indicate that morphine inhibits sleep-promoting neurons in the VLPO by affecting mu receptors and so induces wakefulness in rats. PMID:23303062

Wang, Qin; Yue, Xiao-Fang; Qu, Wei-Min; Tan, Rong; Zheng, Ping; Urade, Yoshihiro; Huang, Zhi-Li

2013-04-01

360

Rotor Wake Development During the First Revolution  

NASA Technical Reports Server (NTRS)

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

McAlister, Kenneth W.

2003-01-01

361

Strong axiality and Ising exchange interaction suppress zero-field tunneling of magnetization of an asymmetric Dy2 single-molecule magnet.  

PubMed

The high axiality and Ising exchange interaction efficiently suppress quantum tunneling of magnetization of an asymmetric dinuclear Dy(III) complex, as revealed by combined experimental and theoretical investigations. Two distinct regimes of blockage of magnetization, one originating from the blockage at individual Dy sites and the other due to the exchange interaction between the sites, are separated for the first time. The latter contribution is found to be crucial, allowing an increase of the relaxation time by 3 orders of magnitude. PMID:21744848

Guo, Yun-Nan; Xu, Gong-Feng; Wernsdorfer, Wolfgang; Ungur, Liviu; Guo, Yang; Tang, Jinkui; Zhang, Hong-Jie; Chibotaru, Liviu F; Powell, Annie K

2011-08-10

362

Features of thermals in stable wake and neutral stratifications in the disturbed monsoon boundary layer  

NASA Astrophysics Data System (ADS)

Thermals, defined as humidity excesses, seem to exhibit distinct features in the disturbed Indian Monsoon Boundary Layer (BL) depending on whether they occur in the stable wake regions, called S in this study, of tropical squalls, or in the neutral regions of the advancing squalls, called N. The stable S region developed weak but gradually increasing ascending motion in the BL while the N region showed strong upward motion at the top of the surface layer (85 m) decreasing steadily thereafter. Similar differences also appear in the other mean thermal fields such as temperature and humidity. The present study agrees with that of Lenschow and Stephens in that there is an increase of thermal cover (area covered by thermals) with height. Our study points out additionally that this happens only within the stable wake region. In the neutral region, the thermal cover is fairly constant. It appears that thermal parameters such as vertical velocity are good indicators of the atmospheric BL structure.

Roadcap, J. R.; Rao, G. V.

1986-07-01

363

Molecular Tagging Velocimetry and Thermometry in the Wake of a Heated Circular Cylinder  

NASA Astrophysics Data System (ADS)

We report improvements to the Molecular Tagging Velocimetry and Thermometry (MTV)technique for simultaneous velocity and temperature measurements in liquids. Using phosphorescent molecules, a pulsed laser "tags" the regions of interest, and these regions are interrogated at two successive times within the emission lifetime of the molecules. The Lagrangian displacement of the tagged regions gives the estimate of the velocity. The temperature mapping relied on the temperature dependence of phosphorescence lifetime, estimated from the intensity ratio of the images. This technique is used to perform simultaneous velocity and temperature measurements in the wake of a heated cylinder. Significant modification of the wake structure is observed as the Richardson number increases toward unity. Results are discussed in terms of the mean and fluctuating velocity and temperature fields, the velocity-temperature correlation, and the shedding frequency.

Hu, H.; Koochesfahani, M.

2003-11-01

364

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

365

A Lagrangian approach to vortex identification in swimming and flying animal wakes.  

NASA Astrophysics Data System (ADS)

The fluid wakes of swimming and flying animals are generally time-dependent. The Eulerian velocity field, which can be measured by existing DPIV measurement techniques, does not directly indicate the flow geometry in this type of unsteady flows. In this study, a Lagrangian approach is developed to determine the Lagrangian Coherent Structures, which are physical boundaries separating flow regions with distinct dynamics, including vortices. The determination of morphology and kinematics of vortices is necessary in estimating time-dependent locomotive forces (Dabiri, J. Exp. Bio., 2006). It also provides information in studying fluid transport in animal swimming and flying. The application of the method is demonstrated by studying the wake of a bluegill sunfish pectoral fin and that of a free-swimming jellyfish.

Peng, Jifeng; Dabiri, John

2006-11-01

366

Vortex wake alleviation studies with a variable twist wing  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

367

Neuroligin-1 links neuronal activity to sleep-wake regulation.  

PubMed

Maintaining wakefulness is associated with a progressive increase in the need for sleep. This phenomenon has been linked to changes in synaptic function. The synaptic adhesion molecule Neuroligin-1 (NLG1) controls the activity and synaptic localization of N-methyl-d-aspartate receptors, which activity is impaired by prolonged wakefulness. We here highlight that this pathway may underlie both the adverse effects of sleep loss on cognition and the subsequent changes in cortical synchrony. We found that the expression of specific Nlg1 transcript variants is changed by sleep deprivation in three mouse strains. These observations were associated with strain-specific changes in synaptic NLG1 protein content. Importantly, we showed that Nlg1 knockout mice are not able to sustain wakefulness and spend more time in nonrapid eye movement sleep than wild-type mice. These changes occurred with modifications in waking quality as exemplified by low theta/alpha activity during wakefulness and poor preference for social novelty, as well as altered delta synchrony during sleep. Finally, we identified a transcriptional pathway that could underlie the sleep/wake-dependent changes in Nlg1 expression and that involves clock transcription factors. We thus suggest that NLG1 is an element that contributes to the coupling of neuronal activity to sleep/wake regulation. PMID:23716671

El Helou, Janine; Bélanger-Nelson, Erika; Freyburger, Marlène; Dorsaz, Stéphane; Curie, Thomas; La Spada, Francesco; Gaudreault, Pierre-Olivier; Beaumont, Éric; Pouliot, Philippe; Lesage, Frédéric; Frank, Marcos G; Franken, Paul; Mongrain, Valérie

2013-06-11

368

Armodafinil-induced wakefulness in animals with ventrolateral preoptic lesions  

PubMed Central

Armodafinil is the pharmacologically active R-enantiomer of modafinil, a widely prescribed wake-promoting agent used to treat several sleep-related disorders including excessive daytime sleepiness associated with narcolepsy, shift work sleep disorder, and obstructive sleep apnea/hypopnea syndrome. Remarkably, however, the neuronal circuitry through which modafinil exerts its wake-promoting effects remains unresolved. In the present study, we sought to determine if the wake-promoting effects of armodafinil are mediated, at least in part, by inhibiting the sleep-promoting neurons of the ventrolateral preoptic (VLPO) nucleus. To do so, we measured changes in waking following intraperitoneal administration of armodafinil (200 mg/kg) or the psychostimulant methamphetamine (1 mg/kg) in rats with cell-body specific lesion of the VLPO. Rats with histologically confirmed lesions of the VLPO demonstrated a sustained increase in wakefulness at baseline, but the increase in wakefulness following administration of both armodafinil and methamphetamine was similar to that of intact animals. These data suggest that armodafinil increases wakefulness by mechanisms that extend beyond inhibition of VLPO neurons. PMID:24833927

Vetrivelan, Ramalingam; Saper, Clifford B; Fuller, Patrick M

2014-01-01

369

Armodafinil-induced wakefulness in animals with ventrolateral preoptic lesions.  

PubMed

Armodafinil is the pharmacologically active R-enantiomer of modafinil, a widely prescribed wake-promoting agent used to treat several sleep-related disorders including excessive daytime sleepiness associated with narcolepsy, shift work sleep disorder, and obstructive sleep apnea/hypopnea syndrome. Remarkably, however, the neuronal circuitry through which modafinil exerts its wake-promoting effects remains unresolved. In the present study, we sought to determine if the wake-promoting effects of armodafinil are mediated, at least in part, by inhibiting the sleep-promoting neurons of the ventrolateral preoptic (VLPO) nucleus. To do so, we measured changes in waking following intraperitoneal administration of armodafinil (200 mg/kg) or the psychostimulant methamphetamine (1 mg/kg) in rats with cell-body specific lesion of the VLPO. Rats with histologically confirmed lesions of the VLPO demonstrated a sustained increase in wakefulness at baseline, but the increase in wakefulness following administration of both armodafinil and methamphetamine was similar to that of intact animals. These data suggest that armodafinil increases wakefulness by mechanisms that extend beyond inhibition of VLPO neurons. PMID:24833927

Vetrivelan, Ramalingam; Saper, Clifford B; Fuller, Patrick M

2014-01-01

370

Isoflurane suppresses early cortical activity  

PubMed Central

Objective Isoflurane and other volatile anesthetics are widely used in children to induce deep and reversible coma, but they may also exert neurotoxic actions. The effects of volatile anesthetics on the immature brain activity remain elusive, however. Methods The effects of isoflurane on spontaneous and sensory-evoked activity were explored using intracortical extracellular field potential and multiple unit recordings in the rat barrel cortex from birth to adulthood. Results During the first postnatal week, isoflurane suppressed cortical activity in a concentration-dependent manner. At surgical anesthesia levels (1.5–2%), isoflurane completely suppressed the electroencephalogram and silenced cortical neurons. Although sensory potentials evoked by the principal whisker deflection persisted, sensory-evoked early gamma and spindle-burst oscillations were completely suppressed by isoflurane. Isoflurane-induced burst-suppression pattern emerged during the second postnatal week and matured through the first postnatal month. Bursts in adolescent and adult rats were characterized by activation of entire cortical columns with a leading firing of infragranular neurons, and were triggered by principal and adjacent whiskers stimulation, and by auditory and visual stimuli, indicating an involvement of horizontal connections in their generation and horizontal spread. Interpretation The effects of isoflurane on cortical activity shift from total suppression of activity to burst-suppression pattern at the end of the first postnatal week. Developmental emergence of bursts likely involves a development of the intracortical short-and long-range connections. We hypothesize that complete suppression of cortical activity under isoflurane anesthesia during the first postnatal week may explain neuronal apoptosis stimulated by volatile anesthetics in the neonatal rats.

Sitdikova, Guzel; Zakharov, Andrei; Janackova, Sona; Gerasimova, Elena; Lebedeva, Julia; Inacio, Ana R; Zaynutdinova, Dilyara; Minlebaev, Marat; Holmes, Gregory L; Khazipov, Roustem

2014-01-01

371

Final Report: Experimental Investigation of Nonlinear Plasma Wake-Fields  

SciTech Connect

We discuss the exploration of the newly proposed blowout regime of the plasma wakefield accelerator and advanced photoinjector technology for linear collider applications. The plasma wakefield experiment at ANL produced several ground-breaking results in the physics of the blowout regime. The photoinjector R and D effort produced breakthroughs in theoretical, computational, and experimental methods in high brightness beam physics. Results have been published.

Rosenzweig, J.

1997-10-31

372

Effects of Sleep and Wake on Oligodendrocytes and Their Precursors  

PubMed Central

Previous studies of differential gene expression in sleep and wake pooled transcripts from all brain cells and showed that several genes expressed at higher levels during sleep are involved in the synthesis/maintenance of membranes in general and of myelin in particular, a surprising finding given the reported slow turnover of many myelin components. Other studies showed that oligodendrocyte precursor cells (OPCs) are responsible for the formation of new myelin in both the injured and the normal adult brain, and that glutamate released from neurons, via neuron–OPC synapses, can inhibit OPC proliferation and affect their differentiation into myelin-forming oligodendrocytes. Because glutamatergic transmission is higher in wake than in sleep, we asked whether sleep and wake can affect oligodendrocytes and OPCs. Using the translating ribosome affinity purification technology combined with microarray analysis in mice, we obtained a genome-wide profiling of oligodendrocytes after sleep, spontaneous wake, and forced wake (acute sleep deprivation). We found that hundreds of transcripts being translated in oligodendrocytes are differentially expressed in sleep and wake: genes involved in phospholipid synthesis and myelination or promoting OPC proliferation are transcribed preferentially during sleep, while genes implicated in apoptosis, cellular stress response, and OPC differentiation are enriched in wake. We then confirmed through BrdU and other experiments that OPC proliferation doubles during sleep and positively correlates with time spent in REM sleep, whereas OPC differentiation is higher during wake. Thus, OPC proliferation and differentiation are not perfectly matched at any given circadian time but preferentially occur during sleep and wake, respectively. PMID:24005282

Bellesi, Michele; Pfister-Genskow, Martha; Maret, Stephanie; Keles, Sunduz; Tononi, Giulio

2013-01-01

373

Population-based study of wake-up strokes  

PubMed Central

Objective: Previous studies have estimated that wake-up strokes comprise 8%to 28% of all ischemic strokes, but these studies were either small or not population-based. We sought to establish the proportion and event rate of wake-up strokes in a large population-based study and to compare patients who awoke with stroke symptoms with those who were awake at time of onset. Methods: First-time and recurrent ischemic strokes among residents of the Greater Cincinnati/Northern Kentucky region (population 1.3 million) in 2005 were identified using International Classification of Diseases–9 codes 430–436 and verified via study physician review. Ischemic strokes in patients aged 18 years and older presenting to an emergency department were included. Baseline characteristics were ascertained, along with discharge modified Rankin Scale scores and 90-day mortality. Results: We identified 1,854 ischemic strokes presenting to an emergency department, of which 273 (14.3%) were wake-up strokes. There were no differences between wake-up strokes and all other strokes with regard to clinical features or outcomes except for minor differences in age and baseline retrospective NIH Stroke Scale score. The adjusted wake-up stroke event rate was 26.0/100,000. Of the wake-up strokes, at least 98 (35.9%) would have been eligible for thrombolysis if arrival time were not a factor. Conclusions: Within our population, approximately 14% of ischemic strokes presenting to an emergency department were wake-up strokes. Wake-up strokes cannot be distinguished from other strokes by clinical features or outcome. We estimate that approximately 58,000 patients with wake-up strokes presented to an emergency department in the United States in 2005. PMID:21555734

Kleindorfer, D.; Sucharew, H.; Moomaw, C.J.; Kissela, B.M.; Alwell, K.; Flaherty, M.L.; Woo, D.; Khatri, P.; Adeoye, O.; Ferioli, S.; Khoury, J.C.; Hornung, R.; Broderick, J.P.

2011-01-01

374

Dexamethasone suppression test  

MedlinePLUS

DST; ACTH suppression test; Cortisol suppression test ... During this test, you will receive dexamethasone. This is a strong man-made (synthetic) glucocorticoid medication. Afterward, your blood is drawn ...

375

First Results from ARTEMIS, a New Two-Spacecraft Lunar Mission: Counter-Streaming Plasma Populations in the Lunar Wake  

NASA Technical Reports Server (NTRS)

We present observations from the first passage through the lunar plasma wake by one of two spacecraft comprising ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun), a new lunar mission that re-tasks two of five probes from the THEMIS magnetospheric mission. On Feb 13, 2010, ARTEMIS probe P1 passed through the wake at 3.5 lunar radii downstream from the Moon, in a region between those explored by Wind and the Lunar Prospector, Kaguya, Chandrayaan, and Chang'E missions. ARTEMIS observed interpenetrating proton, alpha particle, and electron populations refilling the wake along magnetic field lines from both flanks. The characteristics of these distributions match expectations from self-similar models of plasma expansion into vacuum, with an asymmetric character likely driven by a combination of a tilted interplanetary magnetic field and an anisotropic incident solar wind electron population. On this flyby, ARTEMIS provided unprecedented measurements of the interpenetrating beams of both electrons and ions naturally produced by the filtration and acceleration effects of electric fields set up during the refilling process. ARTEMIS also measured electrostatic oscillations closely correlated with counter-streaming electron beams in the wake, as previously hypothesized but never before directly measured. These observations demonstrate the capability of the comprehensively instrumented ARTEMIS spacecraft and the potential for new lunar science from this unique two spacecraft constellation.

Halekas, J. S.; Angelopoulos, V.; Sibeck, D. G.; Khurana, K. K.; Russell, C. T.; Delory, G. T.; Farrell, W. M.; McFadden, J. P.; Bonnell, J. W.; Larson, D.; Ergun, R. E.; Plaschke, F.; Glassmeier, K. H.

2011-01-01

376

Asymmetric vortex pair in the wake of a circular cylinder  

NASA Astrophysics Data System (ADS)

Stationary configurations of two asymmetric point vortices in the wake of an infinite circular cylinder, spinning or not about its axis, are analytically investigated using an ideal fluid approximation. Four different vortex configurations (patterns) in the wake of a spinning cylinder are found in the case when vortex asymmetry is weak; each configuration is associated with a certain direction of the Magnus force. The qualitative relation between a pattern and a direction of the Magnus force is in agreement with experimental data. Also obtained are asymmetrical vortex configurations in the wake of a nonspinning cylinder.

Iosilevskii, G.; Seginer, A.

1994-10-01

377

Assessment of a wake vortex flight test program  

NASA Technical Reports Server (NTRS)

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

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

1974-01-01

378

Numerical modeling studies of wake vortex transport and evolution within the planetary boundary layer  

NASA Technical Reports Server (NTRS)

The proposed research involves four tasks. The first of these is to simulate accurately the turbulent processes in the atmospheric boundary layer. TASS was originally developed to study meso-gamma scale phenomena, such as tornadic storms, microbursts and windshear effects in terminal areas. Simulation of wake vortex evolution, however, will rely on appropriate representation of the physical processes in the surface layer and mixed layer. This involves two parts. First, a specified heat flux boundary condition must be implemented at the surface. Using this boundary condition, simulation results will be compared to experimental data and to other model results for validation. At this point, any necessary changes to the model will be implemented. Next, a surface energy budget parameterization will be added to the model. This will enable calculation of the surface fluxes by accounting for the radiative heat transfer to and from the ground and heat loss to the soil rather than simple specification of the fluxes. The second task involves running TASS with prescribed wake vortices in the initial condition. The vortex models will be supplied by NASA Langley Research Center. Sensitivity tests will be performed on different meteorological environments in the atmospheric boundary layer, which include stable, neutral, and unstable stratifications, calm and severe wind conditions, and dry and wet conditions. Vortex strength may be varied as well. Relevant non-dimensional parameters will include the following: Richardson number or Froude number, Bowen ratio, and height to length scale ratios. The model output will be analyzed and visualized to better understand the transport, decay, and growth rates of the wake vortices. The third task involves running simulations using observed data. MIT Lincoln Labs is currently planning field experiments at the Memphis airport to measure both meteorological conditions and wake vortex characteristics. Once this data becomes available, it can be used to validate the model for vortex behavior under different atmospheric conditions. The fourth task will be to simulate the wake in a more realistic environment covering a wider area. This will involve grid nesting, since high resolution will be required in the wake region but a larger total domain will be used. During the first allocation year, most of the first task will be accomplished.

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

1994-01-01

379

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

380

Theoretical modelling of wakes from retractable flapping wings in forward flight  

PubMed Central

A free-wake method is used to simulate the wake from retractable, jointed wings. The method serves to complement existing experimental studies that visualise flying animal wakes. Simulated wakes are shown to be numerically convergent for a case study of the Rock Pigeon in minimum power cruising flight. The free-wake model is robust in simulating wakes for a range of wing geometries and dynamics without requiring changes to the numerical method. The method is found to be useful for providing low order predictions of wake geometries. However, it is not well suited to reconstructing 3d flowfields as solutions are sensitive to the numerical mesh node locations. PMID:23882442

Crowther, William J.

2013-01-01

381

Detailed investigation of thermal convection in a liquid metal under a horizontal magnetic field: Suppression of oscillatory flow observed by velocity profiles  

Microsoft Academic Search

Thermal convection experiments in a liquid gallium layer were carried out with various intensities of uniform horizontal magnetic fields. The gallium layer was in a rectangular vessel with a 4:1:1 length ratio (1 is the height), where the magnetic field is applied in the direction normal to the longest vertical wall. An ultrasonic velocity profiling method was used to visualize

Takatoshi Yanagisawa; Yasuko Yamagishi; Yozo Hamano; Yuji Tasaka; Kanako Yano; Jumpei Takahashi; Yasushi Takeda

2010-01-01

382

Resonance effects in wake shedding from compressor blading  

Microsoft Academic Search

Sounds omitted from a single stage experimental compressor at a sequence of frequencies have been recorded and investigated. They are found to be due to a series of acoustic resonances excited by periodic wakes shed from the rotor blades.

R. Parker

1967-01-01

383

Ionization-Induced Electron Trapping inUltrarelativistic Plasma Wakes  

SciTech Connect

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.

Oz, E.; Deng, S.; Katsouleas, T.; Muggli, P.; /UCLA; Barnes, C.D.; Blumenfeld, I.; Decker, F.J.; Emma, P.; Hogan, M.J.; Ischebeck, R.; Iverson, R.H.; Kirby, N.; Krejcik,; O'Connell, C.; Siemann, R.H.; Walz, D.; /SLAC; Auerbach, D.; Clayton, C.E.; Huang, C.; Johnson, D.K.; Joshi, C.; /UCLA

2007-04-06

384

The Wake of a Single Vertical Axis Wind Turbine  

NASA Astrophysics Data System (ADS)

Vertical axis wind turbines (VAWTs) pose various advantages over traditional horizontal axis wind turbines (HAWTs), including their smaller size and footprint, quiet operation, and ability to produce power under a greater variety of wind directions and wind speeds. To determine the optimal spacing of an array of VAWTs for maximum power output, an understanding of the fundamental wake structure of a single VAWT is needed. This study is among the first attempts to experimentally visualize the wake of a VAWT using stereo particle image velocimetry (PIV). A scale VAWT is placed inside a wind tunnel and a motor rotates the scale model at a constant rotational speed. Wake data at several Reynolds numbers and tip speed ratios indicate that vortices are shed by each blade of the spinning VAWT, demonstrating significant differences between the wake of a VAWT and a spinning cylinder.

Barsky, Danielle

385

32 CFR 707.10 - Wake illumination light.  

Code of Federal Regulations, 2012 CFR

... NAVIGATION SPECIAL RULES WITH RESPECT TO ADDITIONAL STATION AND SIGNAL LIGHTS § 707.10 Wake illumination light. Naval vessels may display a white spot light located near the stern to illuminate the...

2012-07-01

386

32 CFR 707.10 - Wake illumination light.  

Code of Federal Regulations, 2011 CFR

... NAVIGATION SPECIAL RULES WITH RESPECT TO ADDITIONAL STATION AND SIGNAL LIGHTS § 707.10 Wake illumination light. Naval vessels may display a white spot light located near the stern to illuminate the...

2011-07-01

387

32 CFR 707.10 - Wake illumination light.  

Code of Federal Regulations, 2013 CFR

... NAVIGATION SPECIAL RULES WITH RESPECT TO ADDITIONAL STATION AND SIGNAL LIGHTS § 707.10 Wake illumination light. Naval vessels may display a white spot light located near the stern to illuminate the...

2013-07-01

388

Wind turbine wake characterization using long-range Doppler lidar  

NASA Astrophysics Data System (ADS)

Wind turbines extract energy from the freestream flow, resulting in a waked region behind the rotor which is characterized by reduced wind speed and increased turbulence. The velocity deficit in the wake diminishes with distance, as faster-moving air outside is gradually entrained. In a concentrated group of turbines, then, downwind machines experience very different inflow conditions compared to those in the front row. As utility-scale turbines rarely exist in isolation, detailed knowledge of the mean flow and turbulence structure inside wakes is needed to correctly model both power production and turbine loading at modern wind farms. To this end, the Turbine Wake and Inflow Characterization Study (TWICS) was conducted in the spring of 2011 to determine the reduction in wind speeds downstream from a multi-MW turbine located at the National Renewable Energy Laboratory's National Wind Technology Center (NWTC) near Boulder, Colorado. Full-scale measurements of wake dynamics are hardly practical or even possible with conventional sensors, such as cup anemometers mounted on meteorological (met) masts. Accordingly, the High Resolution Doppler Lidar (HRDL) developed by the National Oceanic and Atmospheric Administration's Earth System Research Laboratory was employed to investigate the formation and propagation of wakes under varying levels of ambient wind speed, shear, atmospheric stability, and turbulence. HRDL remotely senses line-of-sight wind velocities and has been used in several previous studies of boundary layer aerodynamics. With a fully steerable beam and a maximum range up to about 5 km, depending on atmospheric conditions, HRDL performed a comprehensive survey of the wind flow in front of and behind the turbine to study the shape, meandering, and attenuation of wakes. Due in large part to limited experimental data availability, wind farm wake modeling is still subject to an unacceptable amount of uncertainty, particularly in complex terrain. Here, analytical techniques are developed to distinguish wakes from the background variability, and moreover, wakes are then classified by width, height, length, and velocity deficit based on atmospheric stability and inflow conditions. By integrating these advanced observational capabilities with innovative approaches to atmospheric modeling, this work will help to improve simulation tools used to quantify power loss and fatigue loading due to wake effects, thereby aiding the optimization of wind farm layouts.

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

2012-12-01

389

Hypothalamic contribution to sleep–wake cycle development  

Microsoft Academic Search

Infant mammals cycle rapidly between sleep and wakefulness and only gradually does a more consolidated sleep pattern develop. The neural substrates responsible for this consolidation are unknown. To establish a reliable measure of sleep-wake cyclicity in infant rats, nuchal muscle tone was measured in 2-, 5-, and 8-day-old rats, as were motor behaviors associated with sleep (i.e. myoclonic twitching) and

K. Æ. KARLSSON; J. C. KREIDER; M. S. BLUMBERG

2004-01-01

390

Mechanical forcing of the wake of a flat plate  

Microsoft Academic Search

We report experimental results of the forced wake of a thin symmetric flat plate, placed parallel to an uniform air stream, in the range of thickness-based Reynolds number 50Re e<200. External wake forcing was introduced by small harmonic oscillations of a moving flap, placed at the trailing-edge of the flat plate. When the flap remains in a fixed horizontal position,

M. Vial; L. Bellon; R. H. Hernández

2004-01-01

391

A model of shiftworker sleep\\/wake behaviour  

Microsoft Academic Search

Software-based biomathematical models of alertness provide a means to estimate fatigue-related risk in advance of a schedule being worked. Obtaining a good estimate of employees’ sleep\\/wake behaviour during non-work periods is critical in obtaining accurate estimates of alertness. This is because estimates of alertness are generated based on estimated sleep and wake times, not rest and work times per se.

David Darwent; Drew Dawson; Gregory D. Roach

392

The Effects of Aircraft Wake Dynamics on Contrail Development  

Microsoft Academic Search

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

D. C. Lewellen; W. S. Lewellen

2001-01-01

393

Molecular quantum wakes in the hydrodynamic plasma waveguide in air  

SciTech Connect

We demonstrate a modulated plasma guiding effect from the molecular alignment wakes in the hydrodynamic plasma waveguide. A properly time-delayed laser pulse can be spatially confined by the hydrodynamic expansion induced plasma waveguide of an advancing femtosecond laser pulse. The spatial confinement can be further strengthened or weakened by following the quantum wakes of the impulsively excited rotational wave packets of the molecules in the plasma waveguide.

Wu Jian; Cai Hua; Zeng Heping [State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062 (China); Milchberg, H. M. [Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 (United States)

2010-10-15

394

Wake structure and wing motion in bat flight  

Microsoft Academic Search

We report on experiments concerning the wake structure and kinematics of bat flight, conducted in a low-speed wind tunnel using time-resolved PIV (200Hz) and 4 high-speed cameras to capture wake and wing motion simultaneously. 16 Lesser dog-faced fruit bats (C. brachyotis) were trained to fly in the wind tunnel at 3-6.5m\\/s. The PIV recordings perpendicular to the flow stream allowed

Tatjana Hubel; Kenneth Breuer; Sharon Swartz

2008-01-01

395

Control of three-dimensional wakes using evolution strategies  

Microsoft Academic Search

We investigate three-dimensional cylinder wakes of incompressible fully developed flows at Re=300, resulting from control induced by tangential motions of the cylinder surface. The motion of the cylinder surface, in two dimensions, is optimized using evolution strategies, resulting in significant drag reduction and drastic modification of the wake as compared to the uncontrolled flow. The quasi-optimal velocity profile obtained in

Philippe Poncet; Georges-Henri Cottet; Petros Koumoutsakos

2005-01-01

396

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

397

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

NASA Technical Reports Server (NTRS)

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

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

2003-01-01

398

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

NASA Astrophysics Data System (ADS)

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

Ahmed, Anwar

2010-01-01

399

Free-wake computation of helicopter rotor flowfields in forward flight  

NASA Technical Reports Server (NTRS)

A new method has been developed for computing advancing rotor flows. This method uses the Vorticity Embedding technique, which has been developed and validated over the last several years for hovering rotor problems. In this work, the unsteady full potential equation is solved on an Eulerian grid with an embedded vortical velocity field. This vortical velocity accounts for the influence of the wake. Dynamic grid changes that are required to accommodate prescribed blade motion and deformation are included using a novel grid blending method. Free wake computations have been performed on a two-bladed AH-1G rotor at low advance ratios including blade motion. Computed results are compared with experimental data. The sudden variations in airloads due to blade-vortex interactions on the advancing and retreating sides are well captured. The sensitivity of the computed solution to various factors like core size, time step and grids has been investigated. Computed wake geometries and their influence on the aerodynamic loads at these advance ratios are also discussed.

Ramachandran, K.; Schlechtriem, S.; Caradonna, F. X.; Steinhoff, John

1993-01-01

400

The Plasma Wake Downstream of Lunar Topographic Obstacles: Preliminary Results from 2D Particle Simulations  

NASA Technical Reports Server (NTRS)

Anticipating the plasma and electrical environments in permanently shadowed regions (PSRs) of the moon is critical in understanding local processes of space weathering, surface charging, surface chemistry, volatile production and trapping, exo-ion sputtering, and charged dust transport. In the present study, we have employed the open-source XOOPIC code [I] to investigate the effects of solar wind conditions and plasma-surface interactions on the electrical environment in PSRs through fully two-dimensional pattic1e-in-cell simulations. By direct analogy with current understanding of the global lunar wake (e.g., references) deep, near-terminator, shadowed craters are expected to produce plasma "mini-wakes" just leeward of the crater wall. The present results (e.g., Figure I) are in agreement with previous claims that hot electrons rush into the crater void ahead of the heavier ions, fanning a negative cloud of charge. Charge separation along the initial plasma-vacuum interface gives rise to an ambipolar electric field that subsequently accelerates ions into the void. However, the situation is complicated by the presence of the dynamic lunar surface, which develops an electric potential in response to local plasma currents (e.g., Figure Ia). In some regimes, wake structure is clearly affected by the presence of the charged crater floor as it seeks to achieve current balance (i.e. zero net current to the surface).

Zimmerman, Michael I.; Farrell, W. M.; Snubbs, T. J.; Halekas, J. S.

2011-01-01

401

Increased coherence among striatal regions in the theta range during attentive wakefulness  

PubMed Central

The striatum, the largest component of the basal ganglia, is usually subdivided into associative, motor and limbic components. However, the electrophysiological interactions between these three subsystems during behavior remain largely unknown. We hypothesized that the striatum might be particularly active during exploratory behavior, which is presumably associated with increased attention. We investigated the modulation of local field potentials (LFPs) in the striatum during attentive wakefulness in freely moving rats. To this end, we implanted microelectrodes into different parts of the striatum of Wistar rats, as well as into the motor, associative and limbic cortices. We then used electromyograms to identify motor activity and analyzed the instantaneous frequency, power spectra and partial directed coherence during exploratory behavior. We observed fine modulation in the theta frequency range of striatal LFPs in 92.5 ± 2.5% of all epochs of exploratory behavior. Concomitantly, the theta power spectrum increased in all striatal channels (P < 0.001), and coherence analysis revealed strong connectivity (coefficients >0.7) between the primary motor cortex and the rostral part of the caudatoputamen nucleus, as well as among all striatal channels (P < 0.001). Conclusively, we observed a pattern of strong theta band activation in the entire striatum during attentive wakefulness, as well as a strong coherence between the motor cortex and the entire striatum. We suggest that this activation reflects the integration of motor, cognitive and limbic systems during attentive wakefulness. PMID:22735177

Lepski, G.; Arevalo, A.; do Valle, A.C.; Ballester, G.; Gharabaghi, A.

2012-01-01

402

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

NASA Technical Reports Server (NTRS)

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

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

2003-01-01

403

Periodicity of the density wake past a vortex ring in a stratified liquid  

NASA Astrophysics Data System (ADS)

Spatial coherent structure of the density wake past a vortex ring moving horizontally in viscid stratified liquid is experimentally revealed. It follows from analysis that repetition period of the structure is determined by rotation radial frequency (or mean vorticity) of the vortex core and toward speed of the vortex ring. The wake formation of the ring is considered in respect to vorticity shedding which produces velocity disturbances in ambient medium. In case of stratified liquid velocity fluctuations, in their turn, cause density field distortion. This process is superimposed by vortex core oscillations, and, in result, vorticity shedding will be not monotonous but modulated at some frequency. So, the density wake is periodically structured, and the spatial period is defined by intrinsic frequency of the core and forward speed of the ring. To support analysis, experiments were conducted in which vortex rings excited by spring-piston generator were observed with high-sensitive Schlieren instrument and computer-controlled camera. Experimental tank was filled with salt-stratified water of constant buoyancy period, vortex ring velocities range from 3 to 16 cm/s. Spatial period is derived from schlieren image using two independent methods, both 2D spectral analysis and geometry calculations of the vortex core. Spatial periods and vortex intrinsic frequencies calculated by both algorithms are in good agreement; they vary in power lows depending on vortex speed

Prokhorov, V.

2009-04-01

404

Riding the wake of a merging galaxy cluster  

NASA Astrophysics Data System (ADS)

Using William Herschel Telescope (WHT) Optically Adaptive System for Imaging Spectroscopy (OASIS) integral field unit observations, we report the discovery of a thin plume of ionized gas extending from the brightest cluster galaxy (BCG) in Abell 2146 to the subcluster X-ray cool core which is offset from the BCG by ˜37 kpc. The plume is greater than 15 kpc long and less than 3 kpc wide. This plume is unique in that the cluster it is situated in is currently undergoing a major galaxy cluster merger. The BCG is unusually located behind the X-ray shock front and in the wake of the ram-pressure-stripped X-ray cool core, and evidence for recent disruption to the BCG is observed. We examine the gas and stellar morphology, the gas kinematics of the BCG and their relation to the X-ray gas. We propose that a causal link between the ionized gas plume and the offset X-ray cool core provides the simplest explanation for the formation of the plume. An interaction or merger between the BCG and another cluster galaxy is probably the cause of the offset.

Canning, R. E. A.; Russell, H. R.; Hatch, N. A.; Fabian, A. C.; Zabludoff, A. I.; Crawford, C. S.; King, L. J.; McNamara, B. R.; Okamoto, S.; Raimundo, S. I.

2012-03-01

405

Development and characterization of a wake-controlled exterior hood.  

PubMed

A wake-controlled exterior hood was developed to overcome the negative influence of cross draft on an exterior hood and avoid the operation inconvenience caused by the enclosure of an airflow capture booth. This new type of local exterior hood used the hood suction flow to stabilize the dynamic vortex shedding that was induced when a crossflow passed over a blockage plate, and therefore formed a hydrodynamics-stabilized local isolation area for efficient removing of the contaminant. The development process was performed in a test section of an open-circuit wind tunnel. The blockage plate and the exterior hood model were placed in a wind-tunnel test section so that the crossflow could be freely supplied by the airstream of the wind tunnel. The laser light sheet flow visualization method and the laser Doppler velocimeter were employed to reveal the characteristics of the flow field. Primary influential parameters were factored out of the measured velocity results so that a design procedure was proposed. Experiments using hot-wire type alcohol sensors to measure the toluene vapor concentration distributions showed that the capture efficiency of this type of actively controlled hood was remarkably higher than that of an uncontrolled hood. PMID:15742706

Huang, Rong Fung; Liu, Gene Shin; Lin, Shin Yi; Chen, Yu-Kang; Wang, Shun-Chih; Peng, Chiung-Yu; Yeh, Wen-Yu; Chen, Chun-Wann; Chang, Cheng-Ping

2004-12-01