Note: This page contains sample records for the topic wake field suppression from Science.gov.
While these samples are representative of the content of Science.gov,
they are not comprehensive nor are they the most current set.
We encourage you to perform a real-time search of Science.gov
to obtain the most current and comprehensive results.
Last update: August 15, 2014.
1

Wake fields and wake field acceleration  

SciTech Connect

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

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

1984-12-01

2

Collinear wake field acceleration  

NASA Astrophysics Data System (ADS)

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

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

1985-04-01

3

Collinear wake field acceleration  

SciTech Connect

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

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

1985-04-01

4

Wake field acceleration  

SciTech Connect

We are investigating the possibility of accelerating particles with high gradients in a ''Wake Field Transformer'' (1,2). The progress of this experiment will be described. The development of the high current hollow beam electron gun was continued. In the conventional linac, the hollow beam was accelerated to about 6 MeV. Beam monitors came into operation, two gap monitors, two fluorescent monitors and a C-hacekerenkov monitor. Calculations with the computer code WAKTRACK(3) gave the final details for the high energy section of the accelerator that will be installed during 1986.

Bialowons, W.; Bremer, H.D.; Decker, F.h.; Hartrott, M.v.; Lewin, H.C.; Voss, G.h.; Weiland, T.; Wilhelm, P.; Chengde, X.; Yokoya, K.; and others

1987-05-05

5

Visualization of wake fields  

SciTech Connect

Rapid advancements in computing capability, such as supercomputers, are enabling scientists and engineers to use numerical modeling tools to analyze large scale problems of volumetric complexity never considered before, and which are impossible to solve analytically. The Finite Difference Time Domain method (FDTD) is widely used to model different electromagnetic interaction problems. The large amount of information obtained from 3-D simulations makes global data analysis difficult. Visualization allows this analysis to be done in a more efficient manner. This technique is implemented here, using the FDTD code. For this purpose, we have generated a short video to observe the 3-D time evolution of the wake fields, as a beam travelling in a cylindrical pipe encounters two structures: a cylindrically shaped pillbox cavity with and without parallel flat plates. 6 refs., 2 figs., 2 tabs.

Harfoush, F.; Jurgens, T.

1991-05-01

6

Conformal FDTD modeling wake fields  

SciTech Connect

Many computer codes have been written to model wake fields. Here we describe the use of the Conformal Finite Difference Time Domain (CFDTD) method to model the wake fields generated by a rigid beam traveling through various accelerating structures. The non- cylindrical symmetry of some of the problems considered here requires the use of a three dimensional code. In traditional FDTD codes, curved surfaces are approximated by rectangular steps. The errors introduced in wake field calculations by such an approximation can be reduced by increasing the mesh size, therefore increasing the cost of computing. Another approach, validated here, deforms Ampere and Faraday contours near a media interface so as to conform to the interface. These improvements of the FDTD method result in better accuracy of the fields at asymptotically no computational cost. This method is also capable of modeling thin wires as found in beam profile monitors, and slots and cracks as found in resistive wall motions. 4 refs., 5 figs.

Jurgens, T.; Harfoush, F.

1991-05-01

7

Argonne plasma wake-field acceleration experiments  

SciTech Connect

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 wake-fields are of interest both in the laboratory, for acceleration and focusing of electrons and positrons in future linear colliders, and in nature as a possible cosmic ray acceleration mechanism. The purpose of the present work is to review the recent experimental advances made in PWFA research at Argonne National Laboratory. Some of the topics discussed are: the Argonne Advanced Accelerator Test Facility; linear plasma wake-field theory; measurement of linear plasma wake-fields; review of nonlinear plasma wave theory; and experimental measurement of nonlinear plasma wake-fields. 25 refs., 11 figs.

Rosenzweig, J.B.; Cole, B.; Gai, W.; Konecny, R.; Norem, J.; Schoessow, P.; Simpson, J.

1989-03-14

8

Wake field in electron-positron plasmas.  

National Technical Information Service (NTIS)

We study the creation of wake field in cold electron positron plasma by electron bunches. In the resulting plasma inhomogeneity we study the propagation of short electromagnetic pulse. It is found that wake fields can change the frequency of the radiation...

K. Avinash V. I. Berezhiani

1993-01-01

9

Vorticity Field Evolution in a Forced Wake.  

National Technical Information Service (NTIS)

The purpose of this work is to quantify the vorticity evolution in the flow field of the forced wake of a splitter plate inside a confining geometry. The interest in this flow stems from the fact that forcing a low Reynolds number 2-D wake can lead to a h...

R. Cohn, M. Koochesfahani

1999-01-01

10

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

11

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

12

Influence of the ion wake-field on the collisional entanglement fidelity in complex dusty plasmas  

NASA Astrophysics Data System (ADS)

The ion wake-field effects on the entanglement fidelity for the elastic electron-dust grain collisions are investigated in complex dusty plasmas. The screened effective potential and partial wave analysis are employed to obtain the entanglement fidelity in dusty plasmas as a function of the Mach number, Debye length, and collision energy. It is shown that the ion wake-field effect suppresses the entanglement fidelity in dusty plasmas. It is also found that the ion wake-field effect on the entanglement fidelity decreases with an increase of the collision energy.

Jung, Young-Dae; Hong, Woo-Pyo

2012-03-01

13

Measurements of harmonic wake fields excited by rough surfaces.  

PubMed

An experiment has been carried out at the TESLA Test Facility linac to investigate the wake fields generated by picosecond electron bunches in narrow beam pipes with an artificially roughened inner surface. The energy structure imposed on the bunches by the wake fields has been analyzed with a magnetic spectrometer. Strong harmonic-wake-field effects are observed as expected from simulations in which the rough surface is modeled by a dielectric layer. PMID:11863903

Hüning, M; Schlarb, H; Schmüser, P; Timm, M

2002-02-18

14

Wake Fields in the Super B Factory Interaction Region  

SciTech Connect

The geometry of storage ring collider interaction regions present an impedance to beam fields resulting in the generation of additional electromagnetic fields (higher order modes or wake fields) which affect the beam energy and trajectory. These affects are computed for the Super B interaction region by evaluating longitudinal loss factors and averaged transverse kicks for short range wake fields. Results indicate at least a factor of 2 lower wake field power generation in comparison with the interaction region geometry of the PEP-II B-factory collider. Wake field reduction is a consderation in the Super B design. Transverse kicks are consistent with an attractive potential from the crotch nearest the beam trajectory. The longitudinal loss factor scales as the -2.5 power of the bunch length. A factor of 60 loss factor reduction is possible with crotch geometry based on an intersecting tubes model.

Weathersby, Stephen; /SLAC; Novokhatski, Alexander; /SLAC

2011-06-02

15

Plasma wake field XUV radiation source  

DOEpatents

A XUV radiation source uses an interaction of electron beam pulses with a gas to create a plasma radiator. A flowing gas system (10) defines a circulation loop (12) with a device (14), such as a high pressure pump or the like, for circulating the gas. A nozzle or jet (16) produces a sonic atmospheric pressure flow and increases the density of the gas for interacting with an electron beam. An electron beam is formed by a conventional radio frequency (rf) accelerator (26) and electron pulses are conventionally formed by a beam buncher (28). The rf energy is thus converted to electron beam energy, the beam energy is used to create and then thermalize an atmospheric density flowing gas to a fully ionized plasma by interaction of beam pulses with the plasma wake field, and the energetic plasma then loses energy by line radiation at XUV wavelengths Collection and focusing optics (18) are used to collect XUV radiation emitted as line radiation when the high energy density plasma loses energy that was transferred from the electron beam pulses to the plasma.

Prono, Daniel S. (Los Alamos, NM); Jones, Michael E. (Los Alamos, NM)

1997-01-01

16

Collimator wake fields in the SLC final focus.  

National Technical Information Service (NTIS)

The SLC final focus system accommodates 29 fixed or adjustable collimators for machine protection and background reduction. By amplifying pulse to pulse orbit variations and by generating emittance growth, collimator wake fields may degrade the beam quali...

F. Zimmermann K. L. F. Bane C. K. Ng

1996-01-01

17

Experimental Measurement of Nonlinear Plasma Wake-Fields.  

National Technical Information Service (NTIS)

We report direct high resolution observation of nonlinear steepened plasma waves excited in the wake of an intense, self-pinched electron beam. Oscillators in both accelerating and deflecting fields are measured, and analyzed in the context of linear and ...

J. B. Rosenzweig P. Schoessow B. Cole W. Gai R. Konecny

1989-01-01

18

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

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

19

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

20

Localized wake-field excitation and relativistic wave-breaking  

SciTech Connect

The localized wake-field behavior behind a short laser pulse is analyzed numerically as well as analytically in one space dimension. A laser pulse propagating in an underdense plasma oscillates in amplitude and width. Within a certain parameter regime, the variations may be near the threshold for wake-field generation. During the times when the width becomes small enough, localized electrostatic wave-packets are generated. The latter may have amplitudes in the relativistic regime. The breaking of the wake-field wave packets is investigated. Known criteria for wave-breaking are generalized to the relativistic and inhomogeneous cases. It is found that relativistic, inhomogeneous packets break without threshold; however, not, in general, on the first oscillation. The time for wave-breaking can be estimated. The numerical findings are proven analytically by making use of a Lagrangian coordinate formulation.

Lehmann, G.; Laedke, E. W.; Spatschek, K. H. [Institut fuer Theoretische Physik, Heinrich-Heine-Universitaet Duesseldorf, D-40225 Duesseldorf (Germany)

2007-10-15

21

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

22

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

23

Longitudinal wake field corrections in circular machines.  

National Technical Information Service (NTIS)

In computations of longitudinal particle motions in accelerators and storage rings, the fields produced by the interactions of the beam with the cavity in which it circulates are usually calculated by multiplying Fourier components of the beam current by ...

K. R. Symon

1996-01-01

24

Suppression of wake-induced vibration of tandem cylinders with free-to-rotate control plates  

NASA Astrophysics Data System (ADS)

Experiments have been carried out on a pair of circular cylinders to investigate the effectiveness of pivoting parallel plates as wake-induced vibration suppressors. Measurements of amplitude of vibration and average drag are presented for a circular cylinder, free to respond in the cross-flow direction, with mass ratio 2 and a damping level of 0.7% of critical damping. Reduced velocities were up to nearly 30, with associated Reynolds numbers up to 2.3×104 and the results presented are for a centre-to-centre separation of cylinders of 4 diameters. It is shown how vortex-induced vibration and wake-induced vibration of the downstream cylinder of a tandem pair can be practically eliminated by using free to rotate parallel plates. The device achieves vibration suppression with a substantial drag reduction when compared to a pair of fixed tandem cylinders at the same Reynolds number. Results for a single splitter plate and helical strakes are also presented for comparison and were found not to be effective in suppressing wake-induced vibration.

Assi, G. R. S.; Bearman, P. W.; Kitney, N.; Tognarelli, M. A.

2010-10-01

25

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

26

Electron Injection into Plasma Wake Fields by Colliding Laser Pulses  

SciTech Connect

An injector and accelerator is analyzed that uses three collinear laser pulses in a plasma: an intense pump pulse, which generates a large wake field ({ge}20 GV/m), and two counterpropagating injection pulses. When the injection pulses collide, a slow phase velocity beat wave is generated that injects electrons into the fast wake field for acceleration. Particle tracking simulations in 1D with injection pulse intensities near 10{sup 17} W/cm{sup 2} indicate the production of relativistic electrons with bunch durations as short as 3 fs, energy spreads as small as 0.3{percent}, and densities as high as 10{sup 18} cm{sup {minus}3} . {copyright} {ital 1997} {ital The American Physical Society}

Esarey, E.; Hubbard, R.F.; Ting, A.; Sprangle, P. [Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory, Washington, D.C. 20375-5346 (United States)] [Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory, Washington, D.C. 20375-5346 (United States); Leemans, W.P. [Ernest Orlando Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, California 94720 (United States)] [Ernest Orlando Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, California 94720 (United States)

1997-10-01

27

Fourier spectral simulations for wake fields in conducting cavities.  

SciTech Connect

We investigate Fourier spectral time-domain simulations applied to wake field calculations in two-dimensional cylindrical structures. The scheme involves second-order explicit leap-frogging in time and Fourier spectral approximation in space, which is obtained from simply replacing the spatial differentiation operator of the YEE scheme by the Fourier differentiation operator on nonstaggered grids. This is a first step toward investigating high-order computational techniques with the Fourier spectral method, which is relatively simple to implement.

Min, M.; Chin, Y.-H.; Fischer, P. F.; Chae, Y.-Chul; Kim, K.-J.; KEK High Energy Accelerator Research Organization

2007-01-01

28

Plasma wake field acceleration: A proposed experimental test  

SciTech Connect

The prospect of achieving very high accelerating fields has led to proposals for using electrostatic plasma waves to accelerate charged particles for high energy physics. It has been predicted theoretically that these plasma waves can be driven by the wake fields of short bunches, or trains of bunches, of charged particles, to accelerate a subsequent bunch; the longitudinal electric fields possible could be of the order of a few GV/m. This note presents an outline of a proposed experimental test of this principle.

Rosenzweig, J.B.; Cline, D.B.; Dexter, R.N.; Larson, D.J.; Leonard, A.W.; Mengelt, K.R.; Sprott, J.C.; Mills, F.E.; Cole, F.T.

1985-07-15

29

Nonlinear plasma and beam physics in plasma wake-fields  

SciTech Connect

In experimental studies of the Plasma Wake-field Accelerator performed to date at the Argonne Advanced Accelerator Test Facility, significant nonlinearities in both plasma and beam behavior have been observed. The plasma waves driven in the wake of the intense driving beam in these experiments exhibit three-dimensional nonlinear behavior which has as yet no quantitative theoretical explanation. This nonlinearity is due in part to the self-pinching of the driving beam in the plasma, as the denser self-focused beam can excite larger amplitude plasma waves. The self-pinching is a process with interesting nonlinear aspects: the initial evolution of the beam envelope and the subsequent approach to Bennett equilibrium through phase mixing. 35 refs., 10 figs.

Rosenzweig, J.B.

1990-02-12

30

Influence of fabrication errors on wake function suppression in NC X-band accelerating structures for linear colliders  

NASA Astrophysics Data System (ADS)

Wake function suppression is effected by ensuring that the mode frequencies of an X-band normal conducting (NC) accelerating structure of multiple cells are detuned and moderately damped by waveguide manifolds attached to the outer wall of the accelerator. We report on the dilution in the wake function suppression that occurs due to errors resulting from the fabrication process. After diffusion bonding 206 cells a non-uniform expansion in the cell geometry forces a substantial shift in the frequencies of select cells. We remap all circuit parameters to these shifted cell frequencies to predict the wake function. Experiments performed on the SLC at the SLAC National Accelerator Laboratory indicate that the wake function is well predicted by the circuit model.

Jones, R. M.; Adolphsen, C. E.; Miller, R. H.; Wang, J. W.; Higo, T.

2009-03-01

31

Two-Channel Rectangular Dielectric Wake Field Accelerator Structure Experiment  

NASA Astrophysics Data System (ADS)

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 (? = 4.7) supported within a rectangular copper block, forming a drive channel 12 mm×6 mm, and an accelerator channel 2 mm×6 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.; Marshall, T. C.; Shchelkunov, S. V.; Didenko, A.; Hirshfield, J. L.

2009-01-01

32

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

33

Field measurements and modeling of dilution in the wake of a US navy frigate  

Microsoft Academic Search

A field measurement and computer modeling effort was made to assess the dilution field of pulped waste materials discharged into the wake of a US Navy frigate. Pulped paper and fluorescein dye were discharged from the frigate’s pulper at known rates. The subsequent particle and dye concentration field was then measured throughout the wake by a following vessel using multiple

C. N Katz; D. B Chadwick; J Rohr; M Hyman; D Ondercin

2003-01-01

34

Multi-mode, multi-bunch dielectric wake field resonator accelerator  

NASA Astrophysics Data System (ADS)

We describe a multi-mode, dielectric-lined cylindrical resonator equipped with end reflectors in which wake fields are built up by a sequence of compact drive bunches. The parameters of the resonator are chosen such that the period of the wake fields is the same as the spacing of the drive bunches, and the length of the resonator is taken to be a half-integer multiple of the wake field period. Thus the wake field of a passing charge bunch will travel down the resonator and back so as to arrive at the front reflector just as the next bunch enters. Wake fields remain well defined because, excepting the lowest frequency TM mode, the resonator length is very nearly an integer multiple of the individual mode half-wavelengths. The device thus resembles a mode-locked laser resonator equipped with an ``optical switch'' (the passing bunches here). By numerical simulation, we find that the wake field amplitudes will increase with additional bunches, and show an example for an experiment to be done at the Yale Beam Physics Laboratory. For the first time we show how wake fields are reflected from boundary surfaces, an effect that should occur in every dielectric wake field apparatus and which we now exploit to advantage. The resonator concept permits a staged accelerator system, and could reduce the severity of beam bunch breakup due to charge asymmetries. .

Marshall, T. C.; Fang, J.-M.; Hirshfield, J. L.; Park, S.-Y.

2001-05-01

35

On the field anomaly of near wakes in a collisionless plasma.  

NASA Technical Reports Server (NTRS)

A variational approach is presented for determining the electric field induced by charge separation in the near wake of a large negatively charged body moving at mesothermal speeds in a tenuous plasma. It is shown that the presence of a potential well in the wake is due to the charge separation resulting from the unequal mass motions with which the ambient electrons and ions move into the wake.

Liu, V. C.; Jew, H.

1973-01-01

36

Accelerating field step-up transformer in wake-field accelerators  

SciTech Connect

In the wake-field scheme of particle acceleration, a short, intense drive bunch of electrons passes through a slow-wave structure, leaving behind high rf power in its wake field. The axial accelerating electric field associated with the rf can be quite large, > 100 MeV/m, and is used to accelerate a much less intense witness'' beam to eventual energies > 1 TeV. The rf power is deposited predominantly in the fundamental mode of the structure, which, for dielectric-lined waveguide as used at Argonne, is the TM{sub 01} mode. In all likelihood on the field amplitude will be limited only by rf breakdown of the dielectric material, the limit of which is currently unknown in the short time duration, high frequency regime of wake-field acceleration operation. To obtain such strong electric fields with given wake-field rf power, the dimensions of the dielectric-lined waveguide have to be fairly small, OD of the order of a cm and ID of a few mm, and this gives rise to the generation of strong deflection modes with beam misalignment. While a scheme exists to damp such deflection modes on a bunch-to-bunch time scale, head-tail beam deflection could still be a problem and BNS damping as well as FODO focusing are incomplete cures. Presented here are details of a scheme by which the rf power is generated by in a large-diameter wake-field tube, where deflection mode generation by the intense drive beam is tolerable, and then fed into a small-diameter acceleration tube where the less intense witness beam is accelerated by the greatly enhanced axial electric field. The witness beam generates little deflection-mode power itself, even in the small acceleration tube, thus a final high-quality, high-energy electron beam is produced.

Chojnacki, E.; Gai, W.; Schoessow, P.; Simpson, J.

1991-01-01

37

Analysis of a high brightness photo electron beam with self field and wake field effects  

SciTech Connect

High brightness sources are the basic ingredients in the new accelerator developments such as Free-Electron Laser experiments. The effects of the interactions between the highly charged particles and the fields in the accelerating structure, e.g. R.F., Space charge and Wake fields can be detrimental to the beam and the experiments. We present and discuss the formulation used, some simulation and results for the Brookhaven National Laboratory high brightness beam that illustrates effects of the accelerating field, space charge forces (e.g. due to self field of the bunch), and the wake field (e.g. arising from the interaction of the cavity surface and the self field of the bunch).

Parsa, Z.

1991-01-01

38

Analysis of a high brightness photo electron beam with self field and wake field effects  

SciTech Connect

High brightness sources are the basic ingredients in the new accelerator developments such as Free-Electron Laser experiments. The effects of the interactions between the highly charged particles and the fields in the accelerating structure, e.g. R.F., Space charge and Wake fields can be detrimental to the beam and the experiments. We present and discuss the formulation used, some simulation and results for the Brookhaven National Laboratory high brightness beam that illustrates effects of the accelerating field, space charge forces (e.g. due to self field of the bunch), and the wake field (e.g. arising from the interaction of the cavity surface and the self field of the bunch).

Parsa, Z.

1991-12-31

39

High Power Wake-Fields in a Dielectric-lined Waveguide  

Microsoft Academic Search

When a bunched electron beam traverses a dielectric-lined cylindrical waveguide, multiple mode Cerenkov wake-field radiation can arise.(T-B. Zhang, J. L. Hirshfield, T. C. Marshall and B. Hafizi, to be published, Physical Review E.) The composite TM_on wake-field of such a structure can include spatiotemporally localized fields and high total rf power. For a waveguide of inner radius 0.13 cm; outer

T.-B. Zhang; Jay L. Hirshfield; T. C. Marshall

1997-01-01

40

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

41

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

42

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

43

Proton entry into the near-lunar plasma wake for magnetic field aligned flow  

NASA Astrophysics Data System (ADS)

We report the first observation of protons in the near-lunar (100-200 km from the surface) and deeper (near anti-subsolar point) plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity (vsw) are parallel (aligned flow; angle between IMF and vsw?10°). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the Solar Wind Monitor sensor of the Sub-keV Atom Reflecting Analyser experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back tracing of the observed protons suggests that their source is the solar wind. The larger gyroradii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyroradii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons into the wake.

Dhanya, M. B.; Bhardwaj, A.; Futaana, Y.; Fatemi, S.; HolmströM, M.; Barabash, S.; Wieser, M.; Wurz, P.; Alok, A.; Thampi, R. S.

2013-06-01

44

Multibeam echo-sounding measurement of the microbubble field in a ship's wake  

NASA Astrophysics Data System (ADS)

Acoustic data collected with the U.S. Navy's 68 kHz Toroidal Volume Search Sonar (TVSS) system were used to measure the microbubble field generated by the wake of the towing vessel at a constant distance of 18 ship lengths astern. Swath bathymetry beamforming and echo detection techniques were adapted to map the sea surface and the spatial distribution of bubble fields observable in the ocean volume with the TVSS. The sonar was towed at 78 m depth yielding a sea surface measurement swath roughly 550 m wide thus allowing observations of wake bubble fields across three parallel ship tracks spaced 200 m apart and over 22 min after generation. Measurements in microbubble fields about 3 min after generation by the ship's wake yielded an average vertical attenuation of 2.1 dB/m and a maximum acoustic volume scattering strength of -17.1 dB. [Work supported by the U.S. Naval Research Laboratory.

Gallaudet, Timothy C.; de Moustier, Christian P.

2002-05-01

45

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

46

Effect of wake potential on Coulomb crystallization in the presence of magnetic field  

SciTech Connect

The formation of dust crystal in plasma under the influence of repulsive Yukawa (Debye-Hueckel) potential is a well known phenomenon. The regular structure of dust particles is affected by anisotropic ion flow near the sheath region. The bombardment of the ions over dust grains distorts their Debye sphere by overshielding the dust cloud and gives rise to an attractive oscillatory wake potential. In this paper, we have obtained an expression for wake potential along with the Yukawa type of potential in a complex plasma in the presence of magnetic field, for subsonic ion flow towards the plasma sheath. In the presence of magnetic field, interaction potential gets modified and becomes anisotropic. We have studied the combined effect of the attractive wake potential as well as repulsive Yukawa potential on a 2D dust crystal, both in the presence and absence of magnetic field, using molecular dynamic simulation.

Bhattacharjee, Saurav; Das, Nilakshi [Department of Physics, Tezpur University, Tezpur, Assam 784 028 (India)

2012-10-15

47

TE/TM alternating direction scheme for wake field calculation in 3D  

NASA Astrophysics Data System (ADS)

In the future, accelerators with very short bunches will be used. It demands developing new numerical approaches for long-time calculation of electromagnetic fields in the vicinity of relativistic bunches. The conventional FDTD scheme, used in MAFIA, ABCI and other wake and PIC codes, suffers from numerical grid dispersion and staircase approximation problem. As an effective cure of the dispersion problem, a numerical scheme without dispersion in longitudinal direction can be used as it was shown by Novokhatski et al. [Transition dynamics of the wake fields of ultrashort bunches, TESLA Report 2000-03, DESY, 2000] and Zagorodnov et al. [J. Comput. Phys. 191 (2003) 525]. In this paper, a new economical conservative scheme for short-range wake field calculation in 3D is presented. As numerical examples show, the new scheme is much more accurate on long-time scale than the conventional FDTD approach.

Zagorodnov, Igor; Weiland, Thomas

2006-03-01

48

Autoinjection of electrons into a wake field using a capillary with attached cone  

SciTech Connect

By using a cone attached to a capillary, electrons generated through a laser interaction were autoinjected and accelerated in a low-density wake field. The cone attached to the entrance of the capillary serves as an electron supplier. It increases the number of electrons from below the detection limit to 1.1 pC and the energy from 4 to 30 MeV. A two-dimensional particle-in-cell simulation reveals that a significant number of energetic electrons are extracted from the surface of the cone and are subsequently trapped in the wake field and accelerated in the capillary.

Mori, Y.; Kitagawa, Y. [Graduate School for the Creation of New Photonics, 1955-1 Kurematsu-cho, Hamamatsu, Shizuoka 431-1202 (Japan); Sentoku, Y. [Department of Physics, University of Nevada, 5625 Fox Avenue, Reno, Nevada 89506 (United States); Kondo, K.; Tsuji, K.; Nakanii, N.; Fukumochi, S.; Kashihara, M.; Kimura, K.; Tanaka, K. A.; Norimatsu, T.; Tanimoto, Tsuyoshi; Nakamura, H.; Kodama, R. [Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871 (Japan); Graduate School of Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871 (Japan); Takeda, K.; Tampo, M.; Mima, K. [Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871 (Japan); Miura, E. [National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568 (Japan)

2009-12-15

49

Fast ion surface energy loss and straggling in the surface wake fields.  

PubMed

We have measured the stopping powers and straggling of fast, highly ionized atoms passing through thin bilayer targets made up of metals and insulators. We were surprised to find that the energy losses as well as the straggling depend on the ordering of the target and have small but significantly different values on bilayer reversal. We ascribe this newly found difference in energy loss to the surface energy loss field effect due to the differing surface wake fields as the beam exits the target in the two cases. This finding is validated with experiments using several different projectiles, velocities, and bilayer targets. Both partners of the diatomic molecular ions also display similar results. A comparison of the energy loss results with those of previous theoretical predictions for the surface wake potential for fast ions in solids supports the existence of a self-wake. PMID:23821777

Nandi, T; Haris, K; Hala; Singh, Gurjeet; Kumar, Pankaj; Kumar, Rajesh; Saini, S K; Khan, S A; Jhingan, Akhil; Verma, P; Tauheed, A; Mehta, D; Berry, H G

2013-04-19

50

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

51

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

52

Field measurements and modeling of dilution in the wake of a US navy frigate.  

PubMed

A field measurement and computer modeling effort was made to assess the dilution field of pulped waste materials discharged into the wake of a US Navy frigate. Pulped paper and fluorescein dye were discharged from the frigate's pulper at known rates. The subsequent particle and dye concentration field was then measured throughout the wake by a following vessel using multiple independent measures. Minimum dilution of the pulped paper reached 3.2 x 10(5) within 1900 m behind the frigate, or about 8 min after discharge. Independent measures typically agreed within 25% of one another and within 20% of model predictions. Minimum dilution of dye reached 2.3 x 10(5) at a down-wake distance of approximately 3500 m, or roughly 15 min. Comparison to model measurements were again within 20%. The field test was not only successful at characterizing wake dilution under one set of at-sea conditions, but was successful at validating the computer model used for assessing a wide range of ships and conditions. PMID:12907193

Katz, C N; Chadwick, D B; Rohr, J; Hyman, M; Ondercin, D

2003-08-01

53

Investigation on wind turbine wakes: wind tunnel tests and field experiments with LIDARs  

NASA Astrophysics Data System (ADS)

An investigation on the interaction between atmospheric boundary layer flow and wind turbines is carried out with wind tunnel and LIDAR measurements. The former were carried out using hot-wire anemometry and multi-hole pressure probes in the wake of a three-bladed miniature wind turbine. The wind turbine wake is characterized by a strong velocity defect in the proximity of the rotor, and its recovery is found to depend on the characteristics of the incoming atmospheric boundary layer (mean velocity and turbulence intensity profiles). Field experiments were performed using three wind LIDARs. Bi-dimensional scans are performed in order to analyse the wake wind field with different atmospheric boundary layer conditions. Furthermore, simultaneous measurements with two or three LIDARs allow the reconstruction of multi-component velocity fields. Both LIDAR and wind tunnel measurements highlight an increased turbulence level at the wake boundary for heights comparable to the top-tip of the blades; this flow feature can produce dangerous fatigue loads on following wind turbines.

Valerio Iungo, Giacomo; Wu, Ting; Cöeffé, Juliette; Porté-Agel, Fernando

2011-11-01

54

Characterizing rotor stator interaction (RSI) using CFD and experimentally obtained wake flow fields  

NASA Astrophysics Data System (ADS)

RSI is a major reason for noise and vibration, and reduced performance of turbomachinery. The stationary cascade upstream of the impeller stage is a source of variations in velocity due to angular momentum transfer, creating a cascade blade-to-blade variation. In addition a number of secondary flow fields due to boundary layer dynamics, such as wake flows, emerge from the cascade. At UMN a number of TR PIV fields have been captured downstream of a hydrofoil in liquid water, c=81mm and Re,c= (5 to 8)e5, for different AoAs and for selected passive flow control techniques. The wake trailing the foil is characterized by swirling structures, albeit far from regular shedding. One line of analysis of the captured wake flow fields has been to characterize the structures by a statistical averaged energy analysis over the structures. A second approach has been to use the experimentally obtained data as input in CFD analysis of the impingement of the wake on a rotating vane. Both the procedure and results are described.

Kjeldsen, Morten; Finstad, Pal H. E.; Arndt, Roger E. A.

2010-11-01

55

Suppressing drift chamber diffusion without magnetic field  

Microsoft Academic Search

The spatial resolution in drift chamber detectors for ionizing radiation is limited by diffusion of the primary electrons. A strong magnetic field along the drift direction is often applied (Fancher et al., Nucl. Instr. and Meth. A 161 (1979) 383) because it suppresses the transverse diffusion, improving the resolution but at considerable increase in cost and complexity. Here we show

C. J. Martoff; D. P. Snowden-Ifft; T. Ohnuki; N. Spooner; M. Lehner

2000-01-01

56

Silicon oxynitride: A field emission suppression coating  

NASA Astrophysics Data System (ADS)

We have studied coatings deposited using our inductively-coupled RF plasma ion implantation and desposition system to suppress field emission from large, 3-D electrode structures used in high voltage applications, like those used by Thomas Jefferson National Accelerator Facility in their DC-field photoelectron gun. Currently time and labor-intensive hand-polishing procedures are used to minimize field emission from these structures. Previous work had shown that the field emission from polished stainless steel (27 muA of field-emitted current at 15 MV/m) could be drastically reduced with simultaneous deposition of sputtered silicon dioxide during nitrogen implantation (167 pA of field-emitted current at 30 MV/m). We have determined that this unique implantation and deposition procedure produces high-purity silicon oxynitride films that can suppress field emission from stainless steel regardless of their initial surface polish. However, when this implantation procedure was applied to large, 3-D substrates, arcs occurred, damaging the coating and causing unreliable and unrepeatable field emission suppression. We have developed a novel reactive sputtering procedure to deposit high-purity silicon oxynitride coatings without nitrogen ion implantation. We can control the stoichometry and deposition rate of these coatings by adjusting the nitrogen pressure and incident RF-power. Using profilometry, Auger electron spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection analysis, and current-voltage measurements, we have determined that the elemental composition, chemical bonding, density, and electrical properties of the reactively-sputtered silicon oxynitride coatings are similar to those produced by nitrogen implantation during silicon dioxide deposition. Furthermore, high voltage tests determined that both coatings similarly suppress field emission from 6" diameter, polished stainless steel electrodes. We determined a quantitative, predictive electron emission model to describe electron emission from our silicon oxynitride coatings. Although Fowler-Nordheim theory adequately describes field emission from metals, it does not apply to our dielectric coatings. Several models exist in the literature to describe electron emission from dielectrics. Based upon our high voltage field emission results, electron emission from our silicon oxynitride coatings is described by the Schottky and Poole-Frenkel emission models. These models predict that increasing the band gap, dielectric constant, and electron affinity of our silicon oxynitride coatings would further reduce field emission.

Theodore, Nimel D.

57

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 /Moscow, Inst. General Phys.; Nemoto, K.; /CRIEPI, Tokyo

2006-06-27

58

Suppressing drift chamber diffusion without magnetic field  

NASA Astrophysics Data System (ADS)

The spatial resolution in drift chamber detectors for ionizing radiation is limited by diffusion of the primary electrons. A strong magnetic field along the drift direction is often applied (Fancher et al., Nucl. Instr. and Meth. A 161 (1979) 383) because it suppresses the transverse diffusion, improving the resolution but at considerable increase in cost and complexity. Here we show that transverse track diffusion can be strongly suppressed without any magnetic field. This is achieved by using a gas additive which reversibly captures primary ionization electrons, forming negative ions. The ions drift with thermal energies even at very high drift fields and low pressures ( E/P=28.5 V/ cm torr), and the diffusion decreases with increasing drift field. Upon arrival at the avalanche region of the chamber the negative ions are efficiently stripped and ordinary avalanche gain is obtained. Using this technique, r.m.s. transverse diffusion less than 200 ?m has been achieved over a 15 cm drift path at 40 torr with zero magnetic field. The method can provide high spatial resolution in detectors with long drift distances and zero magnetic field. Negative ion drift chambers would be particularly useful at low pressures and in situations such as space-based or underground experiments where detector size scaleability is important and cost, space, or power constraints preclude the use of a magnetic field.

Martoff, C. J.; Snowden-Ifft, D. P.; Ohnuki, T.; Spooner, N.; Lehner, M.

2000-02-01

59

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

60

Design considerations for a density-channel-guided laser wake-field accelerator. Interim report  

SciTech Connect

A self-modulated laser wake-field accelerator configuration in which the laser pulse is optically guided by a plasma density channel is considered. Preliminary experiments on the generation of a plasma channel by a slow capillary discharge are described. It is shown that homogeneous channels with length L sub channel >> lambda{sub p} can be produced, where lambda{sub p} is the plasma wavelength. Key issues are addressed, including phase detuning between the accelerated electron bunch and the wake field, beam-plasma and laser-plasma instabilities, and the effect of density variations that might occur over the length of the plasma channel. Numerical simulations, using present experimental parameters, show accelerating gradients in excess of 50 GV/m.

Krall, J.; Zigler, A.

1995-04-14

61

Saturation of a longitudinal instability due to nonlinearity of the wake field  

SciTech Connect

Self-sustained synchrotron oscillations are observed in electron storage rings. In general the theoretical description of the saturation of an instability for large oscillation amplitude is a difficult problem, and techniques have not yet been developed which yield analytic approximations to the appropriate nonlinear Vlasov or Fokker-Planck equations. In this paper, a single point bunch interacting with the wake field from a single resonant mode of an rf cavity is considered, and the averaging method of Bogoliubov and Mitropolsky is used to study the saturation of the initial exponential growth of the oscillation amplitude, due to the nonlinearity of the wake field. The determination of the limiting amplitude of oscillation is discussed both in the presence and in the absence of radiation damping.

Krinsky, S.

1985-01-01

62

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

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

63

Unsteady RANS method for surface ship boundary layer and wake and wave field  

Microsoft Academic Search

Results are reported of an unsteady Reynolds-averaged Navier-Stokes (RANS) method for simulation of the boundary layer and wake and wave field for a surface ship advancing in regular head waves, but restrained from body motions. Second-order finite differences are used for both spatial and temporal discretization and a Poisson equation projection method is used for velocity-pressure coupling. The exact kinematic

Shin Hyung Rhee; Fred Stern

2001-01-01

64

The Junction Point Model: A Field Model of Waking, Sleeping, and Dreaming, Relating Dream Witnessing, the Waking\\/Sleeping Transition, and Transcendental Meditation in Terms of a Common Psychophysiologic State  

Microsoft Academic Search

A field model of waking, sleeping, and dreaming, called the junction point model, portrays waking, NREM-sleep, and REM-dreaming as expressions of a single, undifferentiated state. EEG data are presented that support this model: (1) in the transitions between waking, NREM-sleep, and REM-dreaming, bursts of similar frequency EEG—7 to 9 Hz—are seen that do not seem generated by known sleep mechanisms,

Frederick Travis

1994-01-01

65

Investigation of Beam Instability Under the Effects of Long-Range Transverse Wake Fields in the Berkeley Future Light Source  

SciTech Connect

An ultra-relativistic charged particle bunch moving through a resonator cavity leaves behind a wake field that will affect subsequent bunches (if the bunch is not ultra-relativistic, the wake field will not be exclusively behind it). If the initial bunch enters the cavity off-axis, it will produce a transverse wake field that can then kick later bunches off the axis. Thus, even bunches that were initially traveling on axis could be displaced and, in turn, produce their own transverse wake fields, affecting following bunches. The offsets obtained by bunches could increase along the bunch train, leading to the so-called multi-bunch beam break-up instability [1]. The purpose of our investigation is to see whether such instability will occur in the superconducting, 1.3 GHz, 2.5GeV linac (see Table 1) planned for the Berkeley future light source (BFLS). We assume an initial steady-state situation established for machine operation; i.e. a continuous process where every bunch follows the same trajectory through the linac, with only small deviations from the axis of the rf structures. We will look at a possible instability arising from a bunch having a small deviation from the established trajectory. Such a deviation would produce a wake field that is slightly different from the one produced by the bunches following the established trajectory. This could lead to subsequent bunches deviating further from the established trajectory. We will assume the deviations are small (at first) and so the difference in the wake field caused by a bunch not traveling along the established trajectory is well approximated by a long-range transverse dipole wake. We are concerned only with deviations from the established trajectory; thus, in our models, a transverse position of zero corresponds to the bunch traveling along the established trajectory. Under this assumption, only the additional long-range transverse dipole wake remains in our models.

Kur, Eugene; Zholents, Alexander A.

2008-08-31

66

Amplification of a Wake Field Generated by a Charged Bunch in a Resonant Medium  

SciTech Connect

A bunch of electrons moving in an active medium excites a wake that is amplified by the medium. The intense radiation field generated in this process reduces the population inversion and, as a result, the field-medium interaction reaches saturation. We show that the accelerating gradient at saturation may reach the 1 GV/m level before the medium is ionized. When ionization occurs, higher gradients may develop provided that we excite resonant sates of a partially stripped atom. {copyright} {ital 1999} {ital The American Physical Society}

Schaechter, L. [Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000 (Israel)] [Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000 (Israel)

1999-07-01

67

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

68

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

69

Clean beams from laser wake-field accelerators via optical injection with a cleanup pulse  

NASA Astrophysics Data System (ADS)

Multiple colliding-pulse injection schemes have been proposed as means for trapping electrons in the ultrashort acceleration buckets of laser-generated wake fields. The primary goal of this paper is to present a parameter study to determine the beams that can be obtained through collisions of collinear laser pulses in uniform plasma. The parameter study is through fully self-consistent, two-dimensional, particle-in-cell simulations, as previous work used only test-particle computations. To remove the multiple beams that can commonly be generated in colliding pulse injection, we use a cleanup pulse, a trailing laser pulse that absorbs the wake. The wake then no longer exists in the region where the trailing beamlets would be, and so the trailing beamlets no longer form. A series of simulations predicts that with such one can obtain single, short (<=10 fs) beams with a bunch charge of order 10 pC, normalized emittance of order 2? ?m, and energy spread of the order of 10%. The parameters of the beams are insensitive to the amplitude of the backward pulse above normalized amplitudes of abw~0.4.

Cary, John R.; Giacone, R. E.; Nieter, C.; Bruhwiler, D. L.

2005-05-01

70

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

71

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

72

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

73

Quasimonoenergetic GeV electron bunch generation by the wake-field of the chirped laser pulse  

SciTech Connect

One-dimensional nonlinear analysis of wake-field generation and electron bunch acceleration by a chirped laser pulse were investigated numerically. It was found that the optimum linear chirp parameter leads to the wake-field amplitude increase by one order of magnitude and accordingly the acceleration gradient. In our external injection scheme, electrons were accelerated using the initial energy of 100 KeV (gamma{sub in}=1.2). When the pulse passes through the electron bunch most part of the electrons trapped in the first cycle of the laser wake-field and accelerate to about 1 GeV in 1.8 mm. We concluded that the expensive electron preacceleration mechanism could be omitted in a laser-aided electron acceleration scheme.

Mirzanejhad, Saeed; Sohbatzadeh, Farshad; Asri, Mehdi; Ghanbari, Kobra [Department of Physics, Faculty of Science, Mazandaran University, Babolsar, P.O. Box 47415 (Iran, Islamic Republic of)

2010-03-15

74

Observation of high-resolution wind fields and offshore wind turbine wakes using TerraSAR-X imagery  

NASA Astrophysics Data System (ADS)

1. Introduction Numerous large-scale offshore wind farms have been built in European waters and play an important role in providing renewable energy. Therefore, knowledge of behavior of wakes, induced by large wind turbines and their impact on wind power output is important. The spatial variation of offshore wind turbine wake is very complex, depending on wind speed, wind direction, ambient atmospheric turbulence and atmospheric stability. In this study we demonstrate the application of X-band TerraSAR-X (TS-X) data with high spatial resolution for studies on wind turbine wakes in the near and far field of the offshore wind farm Alpha Ventus, located in the North Sea. Two cases which different weather conditions and different wake pattern as observed in the TS-X image are presented. 2. Methods The space-borne synthetic aperture radar (SAR) is a unique sensor that provides two-dimensional information on the ocean surface. Due to their high resolution, daylight and weather independency and global coverage, SARs are particularly suitable for many ocean and coastal applications. SAR images reveal wind variations on small scales and thus represent a valuable means in detailed wind-field analysis. The general principle of imaging turbine wakes is that the reduced wind speed downstream of offshore wind farms modulates the sea surface roughness, which in turn changes the Normalized Radar Cross Section (NRCS, denoted by ?0) in the SAR image and makes the wake visible. In this study we present two cases at the offshore wind farm Alpha Ventus to investigate turbine-induced wakes and the retrieved sea surface wind field. Using the wind streaks, visible in the TS-X image and the shadow behind the offshore wind farm, induced by turbine wake, the sea surface wind direction is derived and subsequently the sea surface wind speed is calculated using the latest generation of wind field algorithm XMOD2. 3. Case study alpha ventus Alpha Ventus is located approximately 45 km from the coast of Borkum, Germany, and consists of twelve 5-Megawatt wind power turbines. The retrieved results are validated by comparing with QuikSCAT measurements, the results of the German Weather Service (DWD) atmospheric model and in-situ measurements of wind speed and wind direction, obtained from the research platform FiNO1, installed 400 m west of Alpha Ventus. 4. Conclusion In the presented case study we quantify the wake characteristics of wake length, wake width, maximum velocity de?cit, wake merging and wake meandering. We show that SAR has the capability to map the sea surface two-dimensionally in high spatial resolution which provides a unique opportunity to observe spatial characteristics of offshore wind turbine wakes. The SAR derived information can support offshore wind farming with respect to optimal siting and design and help to estimate their effects on the environment.

Gies, Tobias; Jacobsen, Sven; Lehner, Susanne; Pleskachevsky, Andrey

2014-05-01

75

Powerful terahertz emission from laser wake fields excited in inhomogeneous plasmas  

SciTech Connect

Powerful coherent emission of broadband few-terahertz radiation can be produced from a laser wake field by linear mode conversion. This occurs when the laser pulse is incident obliquely to the density gradient of inhomogeneous plasmas. The emission spectrum and conversion efficiency predicted by mode conversion theory are in agreement with particle-in-cell simulations. The energy conversion efficiency from laser pulses to this low-frequency emission scales proportional to their frequency ratio by ({omega}/{omega}{sub 0}){sup 3} and increases with the laser intensity and the plasma density scale length. By adjusting the laser pulse duration and plasma density profiles, one can control the emission frequency, bandwidth, and duration. In two- and three-dimensional geometry, conical wake emission is found in the backward direction when the laser pulse propagates along the density gradient. This can be explained well by the linear mode conversion. To avoid conical emission, a laser pulse incident obliquely to the density gradient can be deployed so that collimated emission becomes dominant in the 'specular reflection' direction, suitable for practical applications.

Sheng Zhengming; Mima, Kunioki; Zhang Jie [Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080 (China); Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871 (Japan); Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080 (China)

2005-12-15

76

Electromagnetic wake-field due to surface roughness in an optical structure  

NASA Astrophysics Data System (ADS)

In this study we investigate the properties of the electromagnetic wake-field generated by an electron bunch moving in the vicinity of an optical structure of finite roughness. The model employed consists of a metallic cylindrical waveguide to which grooves of random width, height, and location are attached. Based on this model analytic expressions have been developed for the average energy emitted per groove and for its standard deviation. As expected, both quantities are virtually independent of the momentum in a highly relativistic regime and the average energy emitted per groove is proportional to the roughness parameter. Moreover, it has been found that the standard deviation of the energy emitted per groove is proportional to the standard deviation of the roughness parameter to the power of 1/4. The cumulative effect of surface roughness was studied resorting to both periodic and quasiperiodic structures-significant differences in the spectrum have been observed only for low frequencies.

Banna, S.; Schieber, D.; Schächter, L.

2004-04-01

77

Stochastic Heating in High Intensity Laser-Plasma Interaction. Application to the Wake Field Acceleration Process  

NASA Astrophysics Data System (ADS)

Recently, PIC simulations results published by Tajima et al. and Sheng et al. have shown that chaos can play an important role in the efficient electron heating observed in laser-plasma interaction at very high intensities. These results led us to investigate the conditions under which significant stochastic heating is likely to take place. First, we shall consider the dynamics of a single charged particle in the field of a high intensity wave propagating in an unmagnetized vacuum or plasma. In a second part, the effect of a constant homogeneous magnetic field will be discussed. Third, in the case of a plasma interacting with several electromagnetic waves, the use of Chirikov's criterion to predict the conditions favoring stochastic heating will be presented. Finally, it will be shown that when considering a low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field and efficient acceleration.

Bourdier, Alain; Davoine, Xavier; Drouin, Mathieu; Gremillet, Laurent; Lefebvre, Erik

2007-11-01

78

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

79

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

80

Measurements of surface-pressure and wake-flow fluctuations in the flow field of a whitcomb supercritical airfoil  

NASA Technical Reports Server (NTRS)

Measurements of surface pressure and wake flow fluctuations were made as part of a transonic wind tunnel investigation into the nature of a supercritical airfoil flow field. Emphasis was on a range of high subsonic Mach numbers and moderate lift coefficients corresponding to the development of drag divergence and buffeting. Fluctuation data were analyzed statistically for intensity, frequency content, and spatial coherence. Variations in these parameters were correlated with changes in the mean airfoil flow field.

Roos, F. W.; Riddle, D. W.

1977-01-01

81

The Role of Turbulence in Chemical and Dynamical Processes in the Near-Field Wake of Subsonic Aircraft  

NASA Technical Reports Server (NTRS)

During this grant, covering the period from September 1998 to December 2001, we continued the investigation of the role of turbulent mixing in the wake of subsonic aircraft initiated in 1994 for NASA's Atmospheric Effects of Aviation Project. The goal of the research has been to provide sufficient understanding and quantitative analytical capability to assess the dynamical, chemical, and microphysical interactions in the near-field wake that have the greatest potential to influence the global atmospheric impact of the projected fleet of subsonic aircraft. Through large-eddy simulations we have shown that turbulence in the early wake dynamics can have a strong effect on both the ice microphysics of contrail evolution and on wake chemistry. The wake vortex dynamics are the primary determinant of the vertical extent of the contrail; this together with the local wind shear largely determines the horizontal extent. The fraction of the initial ice crystals surviving the wake vortex dynamics, their spatial distribution, and the ice mass distribution are all sensitive to the aircraft type, assumed initial ice crystal number, and ambient humidity and turbulence conditions. Our model indicates that there is a significant range of conditions for which a smaller aircraft such as a B737 produces as significant a persistent contrail as a larger aircraft such as a B747, even though the latter consumes almost five times as much fuel. Large-eddy simulations of the near wake of a B757 provided a fine-grained chemical-dynamical representation of simplified NOx - HOx chemistry in wakes of ages from a few seconds to several minutes. By sampling the simulated data in a manner similar to that of in situ aircraft measurements it was possible to provide a likely explanation for a puzzle uncovered in the 1996 SUCCESS flight measurements of OH and HO2 The results illustrate the importance of considering fluid dynamics effects in interpreting chemistry results when mixing rates and species fluctuations are large, and demonstrate the feasibility of using 3D unsteady LES with coupled chemistry to study such phenomena.

Lewellen, D. C.; Lewellen, W. Steve

2002-01-01

82

Ionization effects in the generation of wake-fields by ultra-high contrast femtosecond laser pulses in argon gas  

NASA Astrophysics Data System (ADS)

Difference in mechanisms of wake-field generation and electron self-injection by high contrast femtosecond laser pulses in an initially neutral Argon gas and in pre-ionized plasma without ionization is studied via 2D particle-in-cell simulations including optical ionization of the media. For shorter laser pulses, 40 fs, ionization results only in an increase of the charge of accelerated electrons by factor of ~3 with qualitatively the same energy distribution. For longer pulses, 80 fs, a more stable wake field structure is observed in the neutral gas with the maximal energy of the accelerated electrons exceeding that in the fixed density plasma. In higher density Argon, an ionizing laser pulse converts itself to a complex system of solitons at a self-induced, critical density ramp.

Makito, K.; Zhidkov, A.; Hosokai, T.; Shin, J.-H.; Masuda, S.; Kodama, R.

2012-10-01

83

Preliminary Analysis on Linac Oscillation Data LI05-19 and Wake Field Energy Loss in FACET Commissioning 2012  

SciTech Connect

In this note, preliminary analysis on linac ocsillation data in FACET linac LI05-09 plus LI11-19 is presented. Several quadrupoles are identified to possibly have different strength, compared with their designed strength in the MAD optics model. The beam energy loss due to longitudinal wake fields in the S-band linac is also analytically calculated, also by LITRACK numerical simulations.

Sun, Yipeng; /SLAC

2012-07-23

84

Suppression of pair creation due to a steady magnetic field  

NASA Astrophysics Data System (ADS)

We investigate the electron-positron pair creation process in a supercritical static electric field in the presence of a static magnetic field that is perpendicular. If both fields vary spatially in one direction the dynamics can be reduced to a set of one-dimensional systems. Using a generalized computational quantum field theoretical procedure, we calculate the time dependence of the spatial density for the created electrons. In the presence of the magnetic field, a significant amount of suppression of pair creation is observed in the simulations and confirmed by an analytical analysis for the limits of short-range fields and long interaction times. This suppression might be interpreted in terms of Pauli blocking by the electron during its return to the creation region as it performs a cyclotronlike motion in the magnetic field.

Su, W.; Jiang, M.; Lv, Z. Q.; Li, Y. J.; Sheng, Z. M.; Grobe, R.; Su, Q.

2012-07-01

85

Suppression of edge-localized modes by magnetic field perturbations  

SciTech Connect

Transport bursts in simulations of edge-localized modes (ELMs) in tokamaks are suppressed by the application of magnetic field perturbations. The amplitude of the applied magnetic field perturbations is characterized by a stochasticity parameter S. When S>1, magnetic flux surfaces are destroyed and the magnetic field lines diffuse in minor radius. As S increases in the simulations, the magnitude of the ELM bursts decreases. The size of bursts is reduced to a very small value while S is still less than unity and most of the magnetic flux surfaces are still preserved. Magnetic field line stochasticity is not a requirement for the stabilization of ELMs by the magnetic field perturbations. The magnetic field perturbations act by suppressing the growth of the resistive ballooning instability that underlies the ELM bursts.

Kleva, Robert G.; Guzdar, Parvez N. [Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742-3511 (United States)

2010-11-15

86

Scaling of far-field wake angle of nonaxisymmetric pressure disturbance  

NASA Astrophysics Data System (ADS)

It has been recently emphasized that the angle of maximum wave amplitude ? in the wake of a disturbance of finite size can be significantly narrower than the maximum value ?K=sin-1(1/3)?19.47? predicted by the classical analysis of Kelvin. For axisymmetric disturbance, a simple argument based on the Cauchy-Poisson initial-value problem suggests that the wake angle decreases following a Mach-like law at large velocity, ? ?FrL-1, where FrL=U/?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 nonaxisymmetric disturbances, relevant to real ships. We find that, for intermediate Froude numbers, the wake angle follows an intermediate scaling law ? ?FrL-2, in agreement with the recent prediction of Noblesse et al. [Eur. J. Mech. B/Fluids 46, 164 (2014), 10.1016/j.euromechflu.2014.03.012]. We show that beyond a critical Froude number, which scales as A1/2 (where A is the length-to-width aspect ratio of the disturbance), the asymptotic scaling ? ?FrB-1 holds, where now FrB=A1/2FrL is the Froude number based on the disturbance width. We propose a simple model for this transition, and provide a regime diagram of the scaling of the wake angle as a function of parameters (A,FrL).

Moisy, F.; Rabaud, M.

2014-06-01

87

Laser-doppler studies of the wake-effected flow field in a turbine cascade  

SciTech Connect

The interaction between consecutive blade rows can be expected to have important effects on the heat transfer in cooled gas turbine cascades. In determining the local heat transfer under the influence of wake flow, nonintrusive optical measuring techniques were used to obtain the flow velocities and turbulence structures in the cascade inlet flow as well as along the test blade's surface. The main purpose of the measurements is to provide accurate experimental data for the development of predictive codes. The applicability primarily of the laser-Doppler technique is discussed and problems arising from the use of laser-dual-focus anemometry are reported. In simulating the effects of wake flow, a plane airfoil was traversed in front of the cascade.

Wittig, S.; Dullenkopf, K.; Schulz, A.; Hestermann, R.

1987-04-01

88

Wake flowfields for Jovian probe  

NASA Technical Reports Server (NTRS)

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

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

1980-01-01

89

The Wake of St. Vincent.  

NASA Astrophysics Data System (ADS)

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

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

1997-03-01

90

Beam emittance and the effects of the rf, space charge and wake fields: Application to the ATF photoelectron beam  

SciTech Connect

Laser driven photoelectron guns are of interest for use in new methods of accelerations, future development of Linear Colliders and new experiments such as Free Electron laser (IFEL). Such guns are potential source of low emittance-high current and short bunch length electron beams, where the emitted electrons are accelerated quickly to a relativistic energy by a strong rf, electric field in the cavity. We present a brief overview of the beam dynamic studies, e.g. emittance for the Brookhaven National Laboratory (BNL) ATF high brightness photocathode radio frequency gun (now in operation), and show the effects of the rf, Space Charge, and Wake fields on the photoelectrons. 4 refs., 7 figs.

Parsa, Z.

1991-01-01

91

A large-domain approach for calculating ship boundary layers and wakes and wave fields for nonzero Froude number  

SciTech Connect

A large-domain approach is developed for calculating ship boundary layers and wakes and wave fields for nonzero Froude number. The Reynolds-averaged Navier-Stokes and continuity equations are solved with the Baldwin-Lomax turbulence model, exact nonlinear kinematic and approximate dynamic free-surface boundary conditions, and a body/free-surface conforming grid. The results are validated through comparisons with data for the Series 60 C{sub B} = 0.6 ship model at low and high Froude numbers and results of a precursory interactive approach. Both approaches yield satisfactory results; however, the large-domain results indicate improved resolution of the flow close to the hull and wake centerplane and of the Froucle number differences due to near-wall turbulence modeling and non-linear free-surface boundary conditions. Additional evaluation is provided through discussion of the recent CFD Workshop Tokyo 1994, where both methods were among the best. Last, some concluding remarks are made. 20 refs., 7 figs.

Tahara, Y.; Stern, F. [Univ. of Iowa, Iowa City, IA (United States)] [Univ. of Iowa, Iowa City, IA (United States)

1996-09-01

92

The Wake of St. Vincent  

Microsoft Academic Search

The island of St. Vincent and the other Windward Islands in the southeastern Caribbean were chosen as a field site for the study of weak mountain wakes. By the authors' definition, a `weak wake' forms when the potential vorticity generated by a mountain is not strong enough to advect itself into eddies; rather, it is simply advected downstream by the

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

1997-01-01

93

Generation and Suppression of E Region Artificial Field Aligned 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 (HAARP) facility during campaigns in May and August of 2012 and were quantified using a 30 MHz coherent scatter radar in Homer, Alaska. The purpose of the experiment was to analyze the X-mode suppression of FAIs generated from O-mode heating and to measure the threshold required to excite thermal parametric instabilities. The irregularities were excited by gradually increasing the power of a zenith pointing O-mode emission transmitted at a frequency of 2.75 MHz. To suppress the irregularities, a second X-mode emission at a higher frequency was added on alternating power cycles. The Homer radar measured the signal-to-noise ratio, Doppler shift, and spectral width of echoes reflected from the irregularities. We will calculate the threshold electric field required to excite the irregularities and compare with similar experiments in order to better understand the thermal parametric instability.

Miceli, R. J.; Hysell, D. L.; Munk, J.; Han, S.

2012-12-01

94

Jovian Plasmas Torus Interaction with Europa. Plasma Wake Structure and Effect of Inductive Magnetic Field: 3D Hybrid Kinetic Simulation  

NASA Technical Reports Server (NTRS)

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect to a 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 background ions). Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider the models with Oþ þ and Sþ þ background plasma, and various betas for background ions and electrons, and pickup electrons. The majority of O2 atmosphere is thermal with an extended non-thermal population (Cassidy et al., 2007). In this paper, we discuss two tasks: (1) the plasma wake structure dependence on the parameters of the upstream plasma and Europa's atmosphere (model I, cases (a) and (b) with a homogeneous Jovian magnetosphere field, an inductive magnetic dipole and high oceanic shell conductivity); and (2) estimation of the possible effect of an induced magnetic field arising from oceanic shell conductivity. This effect was estimated based on the difference between the observed and modeled magnetic fields (model II, case (c) with an inhomogeneous Jovian magnetosphere field, an inductive magnetic dipole and low oceanic shell conductivity).

Lipatov, A. S.; Cooper, J F.; Paterson, W. R.; Sittler, E. C., Jr.; Hartle, R. E.; Simpson, David G.

2013-01-01

95

Wake-field and space charge effects on high brightness beams calculations and measured results for the laser driven photoelectrons at BNL-ATF  

SciTech Connect

We discuss the formalism used to study the effects of the interactions between the highly charged particles and the fields in the accelerating structure, including space charge and wake fields. Some of our calculations and numerical simulation results obtained for the Brookhaven National Laboratory (BNL) high-brightness photoelectron beam at the Accelerator Test Facility (ATF) and the measured data at ATF are also included.

Parsa, Z.

1993-05-01

96

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

97

Space charge and wake field analysis for a high brightness electron source  

SciTech Connect

We present a brief overview of the formalism used, and some simulation results for transverse and longitudinal motion of a bunch of particles moving through a cavity (e.g., the Brookhaven National Laboratory high brightness photocathode gun), including effects of the accelerating field, space charge forces (e.g., arising from the interaction of the cavity surface and the self field of the bunch). 3 refs., 12 figs.

Parsa, Z.

1991-01-01

98

Flow-Field Survey of an Empennage Wake Interacting with a Pusher Propeller.  

National Technical Information Service (NTIS)

The flow field between a model empennage and a 591-mm-diameter pusher propeller was studied in the Ames 7- by 10-Foot Wind Tunnel with directional pressure probes and hot-wire anemometers. The region probed was bounded by the empennage trailing edge and d...

W. C. Horne P. T. Soderman

1988-01-01

99

Nonlinear Optics in Relativistic Plasmas and Laser Wake Field Acceleration of Electrons  

Microsoft Academic Search

When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10^9 total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of

D. Umstadter; S.-Y. Chen; A. Maksimchuk; G. Mourou; R. Wagner

1996-01-01

100

Three-Dimensional Simulation of Exhaust Particle Dispersion and Concentration Fields in the Near-Wake Region of the Studied Ground Vehicle  

Microsoft Academic Search

In the present study, the interaction effects of different vehicle speeds and exhaust tailpipe exit velocity and temperature conditions on the three-dimensional flow structure, exhaust particle dynamic behavior, formation and evolution processes (i.e., nucleation, coagulation, condensation, and dispersion), number and volume concentration, and nucleation rate fields in the near-wake region behind the studied ground vehicle in urban road microenvironment were

Y. H. Liu; Z. He; T. L. Chan

2011-01-01

101

Effect of wakes from moving upstream rods on boundary layer separation from a high lift airfoil  

NASA Astrophysics Data System (ADS)

Highly loaded airfoils in turbines allow power generation using fewer airfoils. High loading, however, can cause boundary layer separation, resulting in reduced lift and increased aerodynamic loss. Separation is affected by the interaction between rotating blades and stationary vanes. Wakes from upstream vanes periodically impinge on downstream blades, and can reduce separation. The wakes include elevated turbulence, which can induce transition, and a velocity deficit, which results in an impinging flow on the blade surface known as a ``negative jet.'' In the present study, flow through a linear cascade of very high lift airfoils is studied experimentally. Wakes are produced with moving rods which cut through the flow upstream of the airfoils, simulating the effect of upstream vanes. Pressure and velocity fields are documented. Wake spacing and velocity are varied. At low Reynolds numbers without wakes, the boundary layer separates and does not reattach. At high wake passing frequencies separation is largely suppressed. At lower frequencies, ensemble averaged velocity results show intermittent separation and reattachment during the wake passing cycle.

Volino, Ralph J.

2011-11-01

102

Study of electron trapping by a transversely ellipsoidal bubble in the laser wake-field acceleration  

SciTech Connect

We present electron trapping in an ellipsoidal bubble which is not well explained by the spherical bubble model by [Kostyukov et al., Phys. Rev. Lett. 103, 175003 (2009)]. The formation of an ellipsoidal bubble, which is elongated transversely, frequently occurs when the spot size of the laser pulse is large compared to the plasma wavelength. First, we introduce the relation between the bubble size and the field slope inside the bubble in longitudinal and transverse directions. Then, we provide an ellipsoidal model of the bubble potential and investigate the electron trapping condition by numerical integration of the equations of motion. We found that the ellipsoidal model gives a significantly less restrictive trapping condition than that of the spherical bubble model. The trapping condition is compared with three-dimensional particle-in-cell simulations and the electron trajectory in test potential simulations.

Cho, Myung-Hoon [School of Natural Science, UNIST, BanYeon-Ri 100, Ulju-gun, Ulsan 689-798 (Korea, Republic of)] [School of Natural Science, UNIST, BanYeon-Ri 100, Ulju-gun, Ulsan 689-798 (Korea, Republic of); Kim, Young-Kuk; Hur, Min Sup [School of Electrical and Computer Engineering, UNIST, BanYeon-Ri 100, Ulju-gun, Ulsan 689-798 (Korea, Republic of)] [School of Electrical and Computer Engineering, UNIST, BanYeon-Ri 100, Ulju-gun, Ulsan 689-798 (Korea, Republic of)

2013-09-15

103

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.

104

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

105

Edge Suppression by Gradient Field Transformation Using Cross-Projection Tensors  

Microsoft Academic Search

We propose a new technique for edge-suppressing operations on images. We introduce cross projection tensors to achieve affine transformations of gradient fields. We use these tensors, for example, to remove edges in one image based on the edge-information in a second image. Tra- ditionally, edge suppression is acieved by setting image gradients to zero based on thresholds. A common application

Amit K. Agrawal; Ramesh Raskar; Rama Chellappa

2006-01-01

106

Euromech 160 on Periodic Flow and Wake Phenomena  

NASA Astrophysics Data System (ADS)

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

107

Background Suppression in Near-field Optical Imaging  

PubMed Central

In several recent studies, antenna-based optical microscopy (e.g. TENOM) has demonstrated its potential to resolve features as small as 10nm. Most studies are concerned with well-separated features on flat surfaces and there are only few studies that deal with samples of high feature density or even three-dimesional objects. The reason is that the external laser irradiation of the optical antenna (e.g. tip or particle) also directly irradiates the sample and therefore gives rise to a background. Here we introduce an efficient background suppression scheme that makes use of feedback modulation. The method is widely applicable and not restricted to cantilever-based scanning schemes. We apply this technique to both dense samples of dye molecules and ion channel proteins in plasma membranes and demonstrate effective background suppression and strongly improved sensitivity. The feedback modulation scheme is expected to find application for biological studies in liquid environments and for investigations of subsurface features in material science.

Hoppener, Christiane; Beams, Ryan; Novotny, Lukas

2010-01-01

108

Brain mechanisms that control sleep and waking  

Microsoft Academic Search

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

Jerome Siegel

2004-01-01

109

Dynamics of wakes downstream of wind turbine towers  

NASA Technical Reports Server (NTRS)

The near field wakes downstream of circular cylinders and of 12 sided cylinders were surveyed in a wind tunnel. Local velocity and velocity deficit diagrams are presented. The variation of turbulence in the wake was surveyed and the frequency of the periodic component of wake motion was determined. Differences between wakes of circular cylinders and of 12 sided cylinders are discussed. Also effects of strakes, orientation of the 12 sided cylinders, and rounding of the corners are noted.

Snyder, M. H.; Wentz, W. H., Jr.

1981-01-01

110

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

111

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

112

Suppression of magnetic relaxation by a transverse alternating magnetic field  

SciTech Connect

The evolution of the spatial distribution of the magnetic induction in a superconductor after the action of the alternating magnetic field perpendicular to the trapped magnetic flux has been analyzed. The observed stabilization of the magnetic induction profile is attributed to the increase in the pinning force, so that the screening current density becomes subcritical. The last statement is corroborated by direct measurements.

Voloshin, I. F.; Kalinov, A. V.; Fisher, L. M. [All-Russia Electrical Engineering Institute (Russian Federation)], E-mail: fisher@vei.ru; Yampol'skii, V. A. [National Academy of Sciences of Ukraine, Institute of Radiophysics and Electronics (Ukraine)], E-mail: yam@vk.kharkov.ua

2007-07-15

113

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

114

A study of rotor wake development and wake/body interactions in hover  

NASA Technical Reports Server (NTRS)

Experiments were conducted to document the wake geometry and interactional effects between a hovering rotor and a body representing a simplified helicopter fuselage. The wide-field shadowgraph technique was used to visualize the rotor wake vortices and to obtain quantitative information on the wake trajectories, with and without the presence of the body. Particular attention was paid to visualizing and quantifying the wake geometry during the direct impingement of tip vortices on the body surface. The rotor performance with and without the fuselage is also discussed.

Bagai, A.; Leishman, J. G.; Samak, D. K.

1991-01-01

115

Suppression of tunnel transport by magnetic field in polypyrrole  

NASA Astrophysics Data System (ADS)

The anomalous charge transport in conducting polymer polypyrrole from the resistivity measurements at low temperature is due to the presence of polarons and bipolarons in these materials. The deviation from the hopping fit in the resistivity in the region below 4 K is attributed to the tunneling contribution to the conductivity. The number of bipolarons is larger in heavily doped samples leading to higher resistivity values at low temperatures, than in lightly doped samples, due to the low mobility of bipolarons. Tunnel transport becomes more difficult in the presence of a magnetic field due to the shrinkage of the Bohr radius of localised states.

Reghu, M.; Subramanyam, S. V.

1990-04-01

116

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

117

Biological Suppression and Natural Population Decline of Heterodera schachtii in a California Field.  

PubMed

ABSTRACT Soil suppressiveness to Heterodera schachtii was demonstrated in a field at the research station of the University of California, Riverside. In two field trials planted to Swiss chard (Beta vulgaris), introduced H. schachtii multiplied 2.7 and 1.7 times more in preplant metam sodium-fumigated plots than in nontreated plots in 1994 and 1995, respectively. In greenhouse experiments, preplant treatments with metam sodium, methyl bromide, methyl iodide, formaldehyde, and aerated steam reduced suppressiveness of soil against H. schachtii to undetectable levels. H. schachtii multiplied significantly less in nontreated soil than in treated soil on Swiss chard. At harvest, the number of infective second-stage juveniles in suppressive soil was close to the lowest detection level, whereas high numbers were encountered in soils initially treated. In a crop rotation trial with host crops of H. schachtii, introduced H. schachtii populations were monitored for five cropping periods over 30 months in initially fumigated versus nontreated suppressive field plots. In fumigated plots, H. schachtii population levels increased in the first and second cropping periods and then declined in the third cropping period. In the fourth and fifth cropping periods, the nematode reproduction factor in the initially fumigated plots was not significantly different from that in suppressive plots. PMID:18944757

Westphal, A; Becker, J O

1999-05-01

118

Suppression of Cross Phase by Flow Shear in Inhomogeneous Magnetic Field  

Microsoft Academic Search

Flux measurements in probe-induced shear layers typically show flux reversal at some point in the shear layer. To understand these observations, we examine suppression of scalar transport due to the reduction of cross phase by strongly sheared flow in a sheared magnetic field. Of interest is the interplay of spatial variations for flow shear and magnetic shear, which go as

P. W. Terry; A. S. Ware; D. E. Newman

2001-01-01

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

Building wake diffusion  

SciTech Connect

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

Ramsdell, J.V.

1989-01-01

121

The Reykjavik wake  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

122

Suppression of excess noise in Transition-Edge Sensors using magnetic field and geometry  

Microsoft Academic Search

We report recent progress at NIST on Mo\\/Cu Transition-Edge Sensors (TESs). While the signal-band noise of our sensors agrees with theory, we observe excess high-frequency noise. We describe this noise and demonstrate that it can be strongly suppressed by a magnetic field perpendicular to the plane of the sensor. Both the excess noise and ?=(T\\/R)(dR\\/dT) depend strongly on field so

J. N. Ullom; W. B. Doriese; G. C. Hilton; J. A. Beall; S. Deiker; K. D. Irwin; C. D. Reintsema; L. R. Vale; Y. Xu

2004-01-01

123

Wake effects on the aerodynamic performance of horizontal axis wind turbines  

SciTech Connect

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

Afjeh, A.A.

1984-08-01

124

Magnetic tension in the Titan magnetic wake  

NASA Astrophysics Data System (ADS)

Magnetic field structure was observed in numerous Cassini flybys in the region of Titan interaction with the corotating flow of Kronian magnetosheric plasma. We consider the magnetic field tension near Titan and show that the draping is observed only upstream the Titan, but not in the Titan magnetic wake, where the magnetic field direction is not consistent with the induced magnetosphere produced by magnetic field lines draping and contradicts the classical picture of the ideal induced magnetosphere. We arrive at the conclusion that the mechanisms alternative to the induced magnetosphere formation due to the field line draping should be considered for the formation of Titan magnetic wake.

Israelevich, Peter; Ershkovich, Alexander

2014-05-01

125

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

126

Cosmic string wakes  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

127

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

128

Vortex interactions in multiple vortex wakes behind aircraft  

NASA Technical Reports Server (NTRS)

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

Ciffone, D. L.

1976-01-01

129

Trailing edge wake flow characteristics of upper surface blown configurations. [noise generators  

NASA Technical Reports Server (NTRS)

Mean and fluctuating flow characteristics in the wake of upper surface blown flap configurations are presented. Relative importance of the longitudinal and the transverse components of the wake flow turbulence for noise generation are evaluated using correlation between the near-field noise and the wake turbulence. Effects of the jet velocity, the initial turbulence in the jet, and the flap deflection angle on noise and wake flow characteristics are studied. The far-field noise data is compared with the existing empirical prediction method. The measured wake flow properties are compared with an analytical model used in the existing USB wake flow noise theory. The detailed wake flow profiles, wake flow turbulence space-time correlations, wake flow turbulence cross-power spectra, and near-field noise third octave band spectra are presented in the appendices.

Reddy, N. N.

1978-01-01

130

Wake effects of the aerodynamic performance of horizontal axis wind turbines  

SciTech Connect

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

Afjeh, A.A.A.K.

1984-01-01

131

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

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

Cavities of weak magnetic field strength in the wake of FTEs: Results from global magnetospheric MHD simulations  

NASA Astrophysics Data System (ADS)

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.

2009-05-01

134

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

135

Cooperative wake vortex instabilities  

NASA Astrophysics Data System (ADS)

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

Bristol, Robert Lindsey

136

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

137

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

138

Simultaneous suppression of disturbing fields and localization of magnetic markers by means of multipole expansion  

PubMed Central

Background Magnetically marked capsules serve for the analysis of peristalsis and throughput times within the intestinal tract. Moreover, they can be used for the targeted disposal of drugs. The capsules get localized in time by field measurements with a superconducting quantum interference device (SQUID) magnetometer array. Here it is important to ensure an online localization with high speed and high suppression of disturbing fields. In this article we use multipole expansions for the simultaneous localization and suppression of disturbing fields. Methods We expand the measurement data in terms of inner and outer multipoles. Thereby we obtain directly a separation of marker field and outer disturbing fields. From the inner dipoles and quadrupoles we compute the magnetization and position of the capsule. The outer multipoles get eliminated. Results The localization goodness has been analyzed depending on the order of the multipoles used and depending on the systems noise level. We found upper limits of the noise level for the usage of certain multipole moments. Given a signal to noise ratio of 40 and utilizing inner dipoles and quadrupoles and outer dipoles, the method enables an accuracy of 5 mm with a speed of 10 localizations per second. Conclusion The multipole localization is an effective method and is capable of online-tracking magnetic markers.

Hilgenfeld, Bernd; Haueisen, Jens

2004-01-01

139

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

140

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

141

Magnetic-field generation and turbulence suppression due to cross-helicity effects  

NASA Astrophysics Data System (ADS)

The regeneration mechanism of magnetic fields in turbulent motion is studied using a turbulent dynamo model, in which effects of global plasma rotation are incorporated through the turbulent cross helicity (velocity/magnetic-field correlation). The turbulent model consists of the mean-field equations and the transport equations for turbulent quantities such as the turbulent energy, its dissipation rate, and the turbulent cross helicity. Prominent properties of the cross-helicity effect, such as the direct linkage to the mean vortical motion, the alignment of the mean electric current and the mean vorticity, etc., are discussed in comparison with the helicity or ? effect. This model is applied to simple plasma flows, such as a cylindrical pipe and an accretion disk. The numerical results show that the turbulence suppression and magnetic-field generation due to the cross-helicity effects occur in the presence of mean vortical motion. The possible cross-helicity production mechanisms are also discussed.

Yokoi, Nobumitsu

1999-08-01

142

NASA wake vortex research  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

143

Plasma wake-field effects on high-current relativistic electron beam transport in the ion-focused regime  

NASA Astrophysics Data System (ADS)

Modulation of the beam current has recently been observed during ion-focused regime (IFR) transport of a high-power relativistic electron beam propagating through a low-density background plasma. Injecting a high-current, high-energy electron beam into an IFR channel immersed in a background plasma induces plasma oscillations. These background plasma oscillations, induced by the rise-time portion of the beam ejecting plasma electrons from the vicinity of the beam into the background plasma, give rise to a modulated axial electric field. This field travels with the beam leading to beam energy and current oscillations. In the experiment, a 1.7 MeV, 1 kA, rise-time-sharpened electron beam is propagated on a KrF excimer laser-produced IFR channel in trimethylamine (TMA) gas, which is immersed in a low-density plasma-filled transport tube. Experimental measurements, analytical theory, and detailed computer simulations are presented demonstrating modulation of this high-current relativistic electron beam near the low-density background plasma frequency.

Miller, J. D.; Schneider, R. F.; Weidman, D. J.; Nguyen, K. T.

1992-12-01

144

Benzocyclobutene barrier layer for suppressing conductance in nonlinear optical devices during electric field poling  

NASA Astrophysics Data System (ADS)

We measured the electro-optic (EO) coefficients (r33) of thin-film devices made from several monolithic, high number density organic EO chromophores with and without additional charge barrier layers. We found that a cross-linkable benzocyclobutene layer was very effective in suppressing unwanted, leakage current, keeping the effective poling voltage nearly identical to the applied voltage. This barrier layer proved to be superior to a titanium dioxide (TiO2) barrier layer. The suppression of the leakage current in combination with a new chromophore enabled the construction of EO devices that had r33 values in the range of 400-500 pm V-1 with poling fields ? 85 V ?m-1.

Jin, Wenwei; Johnston, Peter V.; Elder, Delwin L.; Tillack, Andreas F.; Olbricht, Benjamin C.; Song, Jinsheng; Reid, Philip J.; Xu, Ruimin; Robinson, Bruce H.; Dalton, Larry R.

2014-06-01

145

Wake vortex technology  

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

146

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

147

Suppression of spin fluctuations in TiBe/sub 2/ by high magnetic fields  

SciTech Connect

Measurement of the low-temperature specific heat of a well-characterized 15.6-mg sample of TiBe/sub 2/ was performed in magnetic fields of 0, 6.5, 11.4, 14.2, and 17.0 T. The results indicate a striking depression of the spin-fluctuation-caused upturn with increasing field in the lower-temperature specific heat and very little change at higher temperatures where the spin fluctuations are less predominant. A field for full suppression of the spin fluctuations is extrapolated to be above about 25 T. The field at which the onset of spin-fluctuation depression occurs is 5.2 +- 0.3 T, suggesting that the previously observed anomalies in the susceptibility and differential susceptibility of TiBe/sub 2/ at 5.5 T are connected to the onset of the depression of spin fluctuations. Furthermore, this onset of spin-fluctuation depression at 5.2 +- 0.3 T coupled with the extrapolation to full suppression above 25 T serves to unify the interpretations of previous data on TiBe/sub 2/ by Wohlfarth, by Acker et al., and by van Deursen et al. which were previously thought to be in contradiction.

Stewart, G.R.; Smith, J.L.; Brandt, B.L.

1982-10-01

148

Aircraft Wake RCS Measurement  

NASA Technical Reports Server (NTRS)

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

Gilson, William H.

1994-01-01

149

Turbulent Plane Wakes Subjected to Successive Strains  

NASA Technical Reports Server (NTRS)

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

Rogers, Michael M.

2003-01-01

150

Reduced field of view MRI with rapid, B1-robust outer volume suppression.  

PubMed

MRI scans are inefficient when the size of the anatomy under investigation is small relative to the subject's full extent. The field of view must be expanded, and acquisition times accordingly prolonged. Shorter scans are feasible with reduced field of view imaging (rFOV) using outer volume suppression (OVS), a magnetization preparation sequence that attenuates signal outside a region of interest (ROI). This work presents a new OVS sequence with a cylindrical ROI, short duration, and improved tolerance for B(1)+ inhomogeneity. The sequence consists of a nonselective adiabatic tipdown pulse, which provides B(1)+-robust signal suppression, and a fast 2D spiral cylindrical tipback pulse. Analysis of the Bloch equations with transverse initial magnetization reveals a conjugate symmetric constraint for tipback pulses with small flip angles. This property is exploited to achieve two-shot performance from the single-shot tipback pulse. The OVS sequence is validated in phantoms and in vivo with multislice spiral imaging at 3 T. The relative signal-to-noise ratio efficiency of the proposed sequence was 98% in a phantom and 75-90% in vivo. The effectiveness is demonstrated with cardiovascular rFOV imaging, which exhibits improved resolution and reduced artifacts compared to conventional, full field of view imaging. PMID:22083545

Smith, Travis B; Nayak, Krishna S

2012-05-01

151

Experimental study of the suppression of Rayleigh-Bénard instability in a cylinder by combined rotating and static magnetic fields  

Microsoft Academic Search

We consider experimentally transitions in a liquid metal cylinder heated from below and subject to superimposed rotating and static magnetic fields. The applied static magnetic fields are too weak to influence the characteristic velocity of the rotating field driven basic flow. Being itself turbulent, a strong enough magnetically driven flow suppresses considerably the temperature fluctuations due to the thermogravitational convection.

I. Grants; A. Pedchenko; G. Gerbeth

2006-01-01

152

Free wake analysis of helicopter rotor blades in hover using a finite volume technique  

NASA Technical Reports Server (NTRS)

A fully compressible method for determining helicopter rotor wake effects is described which computes the wake without requiring external specification of the wake, or separate computations for the wake and blade region. The method is a modification of a compressible finite volume Potential Flow technique, and it has been implemented in a program, HELIX I, for computing compressible rotor flow fields in hover with free wakes. Wake positions in substantial agreement with experiment have been calculated for cases including subsonic and transonic flows, high and low aspect ratios, and two- and four-bladed rotors.

Ramachandran, K.; Steinhoff, John; Tung, C.

1987-01-01

153

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

154

Wakes of Maneuvering Bodies in Stratified Fluids  

NASA Astrophysics Data System (ADS)

We present the results of experimental/theoretical studies on large momentum eddies generated in late wakes of unsteady moving self-propelled bodies in stratified fluids. The experiments were conducted with scaled submarine model at high Reynolds numbers (50,000), corresponding to the fully turbulent flow regime. Dye visualization and PIV were used for flow diagnostics. When a self-propelled body makes a maneuver, e.g. accelerates, it imparts net momentum on the surrounding fluid. We show that in a stratified fluid this leads to impulsive momentum wakes with large, long-lived coherent vortices in the late flows, which may be used as a signature for identification of submarine wakes in oceanic thermocline. First, we consider dynamics and properties of such wakes in a linearly stratified fluid and present a model that permits to predict the main flow characteristics. Second, we consider wakes in a two layer stratified fluid (analog of the upper ocean) and show that such wakes may penetrate to the water surface; we present a model for this phenomenon and propose criteria for the penetration of wake signatures to the water surface in terms of main governing parameters (signature contrast versus confinement number). Finally, we consider the evolution of such momentum wake eddies in the field of decaying background turbulence, which mimics the oceanic thermocline, and show that for the flow configuration studied the contrast number remains sufficiently large and detectable wake imprints survive for a long period of time. Some pertinent estimates for submarines cruising in the upper ocean are also given. For more details see [1-3]. This study was supported by grant from the Office of Naval Research. 1. Voropayev S.I., Fernando H.J.S., Smirnov S.A. & Morrison R.J. 2006. On surface signatures generated by submersed momentum sources. Phys. Fluids, under revision. 2. Voropayev S.I., Fernando H.J.S. & Morrison R.J. 2006. Dipolar eddies in a stratified turbulent flow. J. Fluid Mech., submitted. 3. Voropayev S.I., Smirnov S.A. & Fernando H.J.S. 2007. Late-wake vortices of maneuvering bodies in stratified fluids. J. Fluid Mech., submitted.

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

2007-05-01

155

Study on the effects of ion motion on laser-induced plasma wakes  

SciTech Connect

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 10{sup 21} W/cm{sup 2} and plasma background density below 10{sup 19} cm{sup -3}. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density.

Zhou Suyun [Institute of Modern Optical Technologies, Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province and Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006 (China); School of Materials Sciences and Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013 (China); Yu Wei [Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800 (China); Yuan Xiao [Institute of Modern Optical Technologies, Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province and Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006 (China); Xu Han [National Laboratory for Parallel and Distributed Processing, School of Computer Science, National University of Defense Technology, Changsha 410073 (China); Cao, L. H.; Cai, H. B.; Zhou, C. T. [Institute of Applied Physics and Computational Mathematics, Beijing 100088 (China)

2012-09-15

156

Contour detection based on the contextual modulation of non-classical receptive field facilitation and suppression  

NASA Astrophysics Data System (ADS)

Outside the classical receptive field (CRF), there exists a broad non-classical receptive field (NCRF). The response of the central neuron is affected not only by the stimulus inside the CRF, but also modulated by the stimulus surrounding it. The contextual modulation is mediated by horizontal connections across the visual cortex. In this paper, a contour detection method inspired by the visual mechanism in the primary visual cortex (V1) is proposed. The method is divided in three steps. Firstly, the response of every single visual neuron in V1 is computed by local energy. Secondly, the facilitation and suppression (the contextual influence) on a neuron through horizontal interactions are obtained by constructing a two neighbor modulating functions. Finally, the total output response of one neuron to complex visual stimuli is acquired by combing the influence of local visual context on the neuron and energy response by itself. We tested it on natural image and encouraging results were acquired.

Xiao, Jie; Guo, Zhaoli; Cai, Chao

2013-10-01

157

Wake effects on the aerodynamic performance of horizontal axis wind turbines  

NASA Astrophysics Data System (ADS)

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

Afjeh, A. A.

1984-08-01

158

Waking with the hypothalamus.  

PubMed

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

Haas, Helmut L; Lin, Jian-Sheng

2012-01-01

159

Hypersonic rarefied wake characterization  

NASA Technical Reports Server (NTRS)

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

Brewer, E. B.

1993-01-01

160

Three Dimensional Lunar Wake Reconstructed by the ARTEMIS Data  

NASA Astrophysics Data System (ADS)

Thanks to large data coverage of the ARTEMIS mission within the lunar wake, we reconstructed the lunar wake in a three dimensional manner by presenting distributions of key plasma and field parameters: the ion density, parallel and perpendicular temperatures, ion thermal pressure, magnetic pressure (field magnitude), total pressure, and field and flow perturbations. Our observations suggest that the wake is confined within a rarefaction front, which propagates at fast mode velocities in the rest frame of solar wind. When solar wind plasma is absorbed by the dayside lunar surface, a diamagnetic current system with thickness of ~2 ri forms on the surface of the Moon and initiates field disturbances there (bend and compression). These disturbances are controlled by the solar wind ion beta. In the wake behind the Moon, due to force unbalance, plasma reenters into the wake through perpendicular and parallel ways. In the perpendicular way, the inward flowing plasma continues to squeeze flux tubes in the wake, and thus enhances field magnitude there. In the parallel way, the refilling process presents more kinetic features, and leads to higher perpendicular temperature inside the wake. The refilling plasma from opposite sides of the wake may mix with each other significantly at ~6 RM downstream from the Moon, and the lunar wake is found to have fine structure within that distance. In addition, our data also shows that plasma may be decelerated by a total pressure gradient force in the direction against the background solar wind. Our observations thus establish a global lunar wake picture to be tested by theories or simulations.

Zhang, H.; Khurana, K. K.; Kivelson, M. G.; Angelopoulos, V.; Zong, Q.; Pu, Z.; Hsu, T.; Wan, W.; Shi, Q.; Liu, W.

2012-12-01

161

Cylinder wakes in flowing soap films  

SciTech Connect

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

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

1999-09-01

162

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

163

THE MAGNETOHYDRODYNAMIC WAKE OF THE MOON  

Microsoft Academic Search

The possible detection of the lee wake of the magnetohydrodynamie interaction of the solar wind with the moon as observed by the Imp 1 satellite is discussed. The inter- planetary magnetic field was found to fluctuate very rapidly and reach anomalously large values when the satellite was approximately eclipsed by the moon in December 1963. Later data on the interplanetary

Norman F. Ness

1965-01-01

164

Wake Properties of a Stripline Beam Kicker.  

National Technical Information Service (NTIS)

The transport of a high current relativistic electron beam in a stripline beam kicker is strongly dependent on the wake properties of the structure. The effect of the beam-induced fields on the steering of the beam must be determined for a prescribed traj...

B. R. Poole G. J. Caporaso W. C. Ng

1997-01-01

165

Cancellation of the ion deflection due to electron-suppression magnetic field in a negative-ion acceleratora)  

NASA Astrophysics Data System (ADS)

A new magnetic configuration is proposed for the suppression of co-extracted electrons in a negative-ion accelerator. This configuration is produced by an arrangement of permanent magnets embedded in one accelerator grid and creates an asymmetric local magnetic field on the upstream and downstream sides of this grid. Thanks to the "concentration" of the magnetic field on the upstream side of the grid, the resulting deflection of the ions due to magnetic field can be "intrinsically" cancelled by calibrating the configuration of permanent magnets. At the same time, the suppression of co-extracted electrons can be improved.

Chitarin, G.; Agostinetti, P.; Aprile, D.; Marconato, N.; Veltri, P.

2014-02-01

166

Waking Up to Waste  

ERIC Educational Resources Information Center

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

Vrdlovcova, Jill

2005-01-01

167

Diffusion in building wakes  

SciTech Connect

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

Ramsdell, J.V.

1988-03-01

168

An electronic device for artefact suppression in human local field potential recordings during deep brain stimulation  

NASA Astrophysics Data System (ADS)

The clinical efficacy of high-frequency deep brain stimulation (DBS) for Parkinson's disease and other neuropsychiatric disorders likely depends on the modulation of neuronal rhythms in the target nuclei. This modulation could be effectively measured with local field potential (LFP) recordings during DBS. However, a technical drawback that prevents LFPs from being recorded from the DBS target nuclei during stimulation is the stimulus artefact. To solve this problem, we designed and developed 'FilterDBS', an electronic amplification system for artefact-free LFP recordings (in the frequency range 2-40 Hz) during DBS. After defining the estimated system requirements for LFP amplification and DBS artefact suppression, we tested the FilterDBS system by conducting experiments in vitro and in vivo in patients with advanced Parkinson's disease undergoing DBS of the subthalamic nucleus (STN). Under both experimental conditions, in vitro and in vivo, the FilterDBS system completely suppressed the DBS artefact without inducing significant spectral distortion. The FilterDBS device pioneers the development of an adaptive DBS system retroacted by LFPs and can be used in novel closed-loop brain-machine interface applications in patients with neurological disorders.

Rossi, L.; Foffani, G.; Marceglia, S.; Bracchi, F.; Barbieri, S.; Priori, A.

2007-06-01

169

Modeling Flow Suppression of Error-field-induced Magnetic Islands in Tokamaks  

NASA Astrophysics Data System (ADS)

Small deviations from axisymmetry in applied tokamak magnetic fields can induce island formation at magnetic surfaces whose rotational transform resonates with the perturbation. These islands have a braking effect on plasma rotation that can destabilize resistive wall modes. The IPEC codefootnotetextJ.K. Park, et al., Phys. Plasmas 14, 052110 (2007). is useful for computing plasma response to harmonic perturbations in the infinite-conducting limit, assuming perfect shielding at the resonant surface, but cannot predict the nonlinear effects of finite-sized islands. Using the nonlinear extended MHD code M3D,footnotetextW. Park, et al., Phys. Plasmas 6, 1796 (1999). we explore the effects of a 2,1 perturbation on the nonlinear evolution of a family of equilibria with finite resistivity. Particular attention is paid to the effects of toroidal flow on suppressing island formation, making contact with the analytic theory of Fitzpatrick.footnotetextR. Fitzpatrick, Phys. Plasmas 5, 3325 (1998). Island suppression is shown to depend strongly on the tearing mode stability properties of the equilibrium.

Breslau, J. A.; Park, W.

2009-11-01

170

X-mode suppression of artificial E region field-aligned plasma density irregularities  

NASA Astrophysics Data System (ADS)

Ionospheric modification experiments have been performed at the High frequency Active Auroral Research Program involving the creation and suppression of artificial field-aligned density irregularities (AFAIs) using O-mode and X-mode continuous wave emissions, respectively. The emission frequencies were offset so that the O-mode upper hybrid interaction height nearly matched the X-mode reflection height in the ionospheric E region. AFAIs created by O-mode heating were observed using a 30 MHz coherent scatter radar imager. Simultaneous X-mode heating was found to suppress the AFAI intensity and increase the threshold power for excitation by approximately a factor of 5 in our experiments. The effects are attributed in part to the broadening of the upper hybrid interaction region and in part to increased O-mode absorption, which reduces the amplitude of the standing wave pattern in that region. Preliminary estimates based on local calculations suggest that the electron temperature in the E region was increased by a factor of about 1.6 in these experiments.

Hysell, D. L.; Nossa, E.; McCarrick, M.

2011-04-01

171

Radial electric field required to suppress ion temperature gradient modes in the Electric Tokamak  

SciTech Connect

The Electric Tokamak (ET), currently under construction at the University of California{endash}Los Angeles, is designed to rotate poloidally via a radial current induced by fast wave rf heating fast enough to bifurcate the plasma into a global {open_quotes}{ital H} mode{close_quotes} ({open_quotes}high confinement mode{close_quotes}). A global gyrokinetic code is used to explore and illustrate some of the effects on ion temperature gradient turbulence. The realistic radial electric field required to completely suppress these modes for ET parameters is demonstrated to be {lt}{minus}30 V/cm at its maximum near the half radius. The effects of {ital both} a poloidally supersonic bulk rotation threshold and the shear in this rotation near that supersonic threshold were shown to be important in reducing these modes. {copyright} {ital 1999 American Institute of Physics.}

Kissick, M.W.; Leboeuf, J.; Cowley, S.C.; Dawson, J.M.; Decyk, V.K.; Gourdain, P.; Gauvreau, J.; Pribyl, P.A.; Schmitz, L.W. [University of California--Los Angeles, Los Angeles, California 90095-1542 (United States)] [University of California--Los Angeles, Los Angeles, California 90095-1542 (United States); Sydora, R.D. [University of Alberta, Edmonton, Alberta, T6G 2J1 (Canada)] [University of Alberta, Edmonton, Alberta, T6G 2J1 (Canada); Tynan, G.R. [University of California--San Diego, San Diego, California 92093-0417 (United States)] [University of California--San Diego, San Diego, California 92093-0417 (United States)

1999-12-01

172

Sleep-waking discharge patterns of neurons recorded in the rat perifornical lateral hypothalamic area  

PubMed Central

The perifornical lateral hypothalamic area (PF-LHA) has been implicated in the control of several waking behaviours, including feeding, motor activity and arousal. Several cell types are located in the PF-LHA, including projection neurons that contain the hypocretin peptides (also known as orexins). Recent findings suggest that hypocretin neurons are involved in sleep-wake regulation. Loss of hypocretin neurons in the human disorder narcolepsy is associated with excessive somnolence, cataplexy and increased propensity for rapid eye movement (REM) sleep. However, the relationship of PF-LHA neuronal activity to different arousal states is unknown. We recorded neuronal activity in the PF-LHA of rats during natural sleep and waking. Neuronal discharge rates were calculated during active waking (waking accompanied by movement), quiet waking, non-REM sleep and REM sleep. Fifty-six of 106 neurons (53 %) were classified as wake/REM-related. These neurons exhibited peak discharge rates during waking and REM sleep and reduced discharge rates during non-REM sleep. Wake-related neurons (38 %) exhibited reduced discharge rates during both non-REM and REM sleep when compared to that during waking. Wake-related neurons exhibited significantly higher discharge rates during active waking than during quiet waking. The discharge of wake-related neurons was positively correlated with muscle activity across all sleep-waking states. Recording sites were located within the hypocretin-immunoreactive neuronal field of the PF-LHA. Although the neurotransmitter phenotype of recorded cells was not determined, the prevalence of neurons with wake-related discharge patterns is consistent with the hypothesis that the PF-LHA participates in the regulation of arousal, muscle activity and sleep-waking states.

Alam, Md Noor; Gong, Hui; Alam, Tarannum; Jaganath, Rajesh; McGinty, Dennis; Szymusiak, Ronald

2002-01-01

173

Vortex wake and exhaust plume interaction, including ground effect  

Microsoft Academic Search

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

Ihab Gaber Adam

1998-01-01

174

Suppression of a charge-density-wave ground state in high magnetic fields: Spin and orbital mechanisms  

Microsoft Academic Search

The charge density wave (CDW) transition temperature in the quasi-one-dimensional (Q1D) organic material (Per)2Au(mnt)2 is relatively low (TCDW˜12 K). Hence in a mean field BCS model, the CDW state should be completely suppressed in magnetic fields of order 30 40 T. To explore this possibility, the magnetoresistance of (Per)2Au(mnt)2 was investigated in magnetic fields to 45 T for 0.5 K

D. Graf; J. S. Brooks; E. S. Choi; S. Uji; J. C. Dias; M. Almeida; M. Matos

2004-01-01

175

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

SciTech Connect

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

Bays-muchmore, B.

1991-01-01

176

Suppression of epileptiform activity by a single short-duration electric field in rat hippocampus in vitro.  

PubMed

The mechanisms behind the therapeutic effects of electrical stimulation of the brain in epilepsy and other disorders are poorly understood. Previous studies in vitro have shown that uniform electric fields can suppress epileptiform activity through a direct polarizing effect on neuronal membranes. Such an effect depends on continuous DC stimulation with unbalanced charge. Here we describe a suppressive effect of a brief (10 ms) DC field on stimulus-evoked epileptiform activity in rat hippocampal brain slices exposed to Cs(+) (3.5 mM). This effect was independent of field polarity, was uncorrelated to changes in synchronized population activity, and persisted during blockade of synaptic transmission with Cd(2+) (500 ?M). Antagonists of A(1), P(2X), or P(2Y) receptors were without effect. The suppressive effect depended on the alignment of the external field with the somato-dendritic axis of CA1 pyramidal cells; however, temporal coincidence with the epileptiform activity was not essential, as suppression was detectable for up to 1 s after the field. Pyramidal cells, recorded during epileptiform activity, showed decreased discharge duration and truncation of depolarizing plateau potentials in response to field application. In the absence of hyperactivity, the applied field was followed by slow membrane potential changes, accompanied by decreased input resistance and attenuation of the depolarizing afterpotential following action potentials. These effects recovered over a 1-s period. The study suggests that a brief electric field induces a prolonged suppression of epileptiform activity, which can be related to changes in neuronal membrane properties, including attenuation of signals depending on the persisting Na(+) current. PMID:23486200

Mikkelsen, Ronni; Andreasen, Mogens; Nedergaard, Steen

2013-06-01

177

Interaction of Aircraft Wakes From Laterally Spaced Aircraft  

NASA Technical Reports Server (NTRS)

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

Proctor, Fred H.

2009-01-01

178

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

179

Polarization wake of penetrating ions: oscillator model  

NASA Astrophysics Data System (ADS)

The wake potential induced by a swift nonrelativistic ion has been studied theoretically for a random stopping medium consisting of quantal-harmonic-oscillator atoms. The primary purpose has been to study the influence of atomic binding on the frequently-studied wake potential in a Fermi gas. Quantitative comparisons at constant plasma frequency and increasing oscillator frequency show a gradual decrease in wavelength and a slight decrease in amplitude of the oscillatory part of the wake potential, as well as a systematic decrease in screening of the near-field next to the projectile. These findings can be expected on the basis of the Drude-Lorentz formula for the effective resonance frequency. We find a distinct dependence of the induced potential on the ion charge as long as the plasma frequency exceeds the oscillator frequency. In the opposite case of a dominating oscillator frequency we find little difference between the field induced by a point charge and that by a neutral atom. As an application area we briefly discuss the proximity effect in the energy loss of molecular ions. We find that the polarization wake modifies the proximity effect, in contrast to the frequently-expressed view that it causes the proximity effect.

Schinner, A.; Sigmund, P.

2012-02-01

180

Convective instability in wake intermediate asymptotics  

NASA Astrophysics Data System (ADS)

The paper presents a multiscale analysis of the intermediate region of the two-dimensional convectively unstable wake past a bluff body. A recent asymptotic expansion solution was used as basic flow (Tordella & Belan, Phys. Fluids, vol. 15, 2003, 1897). This solution was obtained by matching an inner to an outer flow, both of which are Navier Stokes solutions. By introducing a spatio-temporal multiscaling into the instability problem, an inhomogeneous Orr Sommerfeld equation and an associated modulation equation are obtained. The streamwise variation of the instability characteristics can then be deduced from the wave modulation, by considering the system to be perturbed by waves with a complex wavenumber that corresponds of the dominant saddle point of the local dispersion relation, taken at different positions downstream of the wake, and at different Reynolds numbers. The corrections of no parallelism are remarkable in the intermediate wake. When the disturbance is related to an early intermediate station, the corrections lead to absolute instability in the upstream portion of the intermediate wake, where, in addition, the spatial growth rate decreases. When the disturbance is related to a section in the far field, conditions of minimal temporal stability are reached about 20 body scales downstream. In the far field the temporal damping increases with the Reynolds number.

Belan, M.; Tordella, D.

2006-04-01

181

Aircraft wake turbulence avoidance  

NASA Technical Reports Server (NTRS)

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

Mcgowan, W. A.

1971-01-01

182

EEG microstates of wakefulness and NREM sleep.  

PubMed

EEG-microstates exploit spatio-temporal EEG features to characterize the spontaneous EEG as a sequence of a finite number of quasi-stable scalp potential field maps. So far, EEG-microstates have been studied mainly in wakeful rest and are thought to correspond to functionally relevant brain-states. Four typical microstate maps have been identified and labeled arbitrarily with the letters A, B, C and D. We addressed the question whether EEG-microstate features are altered in different stages of NREM sleep compared to wakefulness. 32-channel EEG of 32 subjects in relaxed wakefulness and NREM sleep was analyzed using a clustering algorithm, identifying the most dominant amplitude topography maps typical of each vigilance state. Fitting back these maps into the sleep-scored EEG resulted in a temporal sequence of maps for each sleep stage. All 32 subjects reached sleep stage N2, 19 also N3, for at least 1 min and 45 s. As in wakeful rest we found four microstate maps to be optimal in all NREM sleep stages. The wake maps were highly similar to those described in the literature for wakefulness. The sleep stage specific map topographies of N1 and N3 sleep showed a variable but overall relatively high degree of spatial correlation to the wake maps (Mean: N1 92%; N3 87%). The N2 maps were the least similar to wake (mean: 83%). Mean duration, total time covered, global explained variance and transition probabilities per subject, map and sleep stage were very similar in wake and N1. In wake, N1 and N3, microstate map C was most dominant w.r.t. global explained variance and temporal presence (ratio total time), whereas in N2 microstate map B was most prominent. In N3, the mean duration of all microstate maps increased significantly, expressed also as an increase in transition probabilities of all maps to themselves in N3. This duration increase was partly--but not entirely--explained by the occurrence of slow waves in the EEG. The persistence of exactly four main microstate classes in all NREM sleep stages might speak in favor of an in principle maintained large scale spatial brain organization from wakeful rest to NREM sleep. In N1 and N3 sleep, despite spectral EEG differences, the microstate maps and characteristics were surprisingly close to wakefulness. This supports the notion that EEG microstates might reflect a large scale resting state network architecture similar to preserved fMRI resting state connectivity. We speculate that the incisive functional alterations which can be observed during the transition to deep sleep might be driven by changes in the level and timing of activity within this architecture. PMID:22658975

Brodbeck, Verena; Kuhn, Alena; von Wegner, Frederic; Morzelewski, Astrid; Tagliazucchi, Enzo; Borisov, Sergey; Michel, Christoph M; Laufs, Helmut

2012-09-01

183

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

184

Hose instability of relativistically strong femtosecond laser pulses with few field oscillations in a plasma under the excitation of a plasma wake wave  

NASA Astrophysics Data System (ADS)

A detailed theoretical analysis is carried out of the hose instability of relativistically strong laser pulses propagating in a plasma, whose duration is less than the period of a plasma wake wave. An analytic expression is obtained for the displacement of the mass center of a wave pulse, and the effect of this instability on the modification of the spectrum of laser radiation is analyzed for a wide range of initial parameters. It is shown that the development of instability is characterized by a power-law (rather than exponential) time dependence along the propagation path and does not deteriorate the self-compression of laser pulses.

Balakin, A. A.; Litvak, A. G.; Mironov, V. A.; Skobelev, S. A.

2014-04-01

185

First Lunar Wake Passage of ARTEMIS: Discrimination of Wake Effects and Solar Wind Fluctuations by 3D Hybrid Simulations  

NASA Technical Reports Server (NTRS)

The spacecraft P1 of the new ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) mission passed the lunar wake for the first time on February 13, 2010. We present magnetic field and plasma data of this event and results of 3D hybrid simulations. As the solar wind magnetic field was highly dynamic during the passage, a simulation with stationary solar wind input cannot distinguish whether distortions were caused by these solar wind variations or by the lunar wake; therefore, a dynamic real-time simulation of the flyby has been performed. The input values of this simulation are taken from NASA OMNI data and adapted to the P1 data, resulting in a good agreement between simulation and measurements. Combined with the stationary simulation showing non-transient lunar wake structures, a separation of solar wind and wake effects is achieved. An anisotropy in the magnitude of the plasma bulk flow velocity caused by a non-vanishing magnetic field component parallel to the solar wind flow and perturbations created by counterstreaming ions in the lunar wake are observed in data and simulations. The simulations help to interpret the data granting us the opportunity to examine the entire lunar plasma environment and, thus, extending the possibilities of measurements alone: A comparison of a simulation cross section to theoretical predictions of MHD wave propagation shows that all three basic MHD modes are present in the lunar wake and that their expansion governs the lunar wake refilling process.

Wiehle, S.; Plaschke, F.; Motschmann, U.; Glassmeier, K. H.; Auster, H. U.; Angelopoulos, V.; Mueller, J.; Kriegel, H.; Georgescu, E.; Halekas, J.; Sibeck, D. G.; McFadden, J. P.

2011-01-01

186

NOWVIV - Nowcasting wake vortex impact variables  

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

187

Molecular wake shield gas analyzer  

NASA Technical Reports Server (NTRS)

Techniques for measuring and characterizing the ultrahigh vacuum in the wake of an orbiting spacecraft are studied. A high sensitivity mass spectrometer that contains a double mass analyzer consisting of an open source miniature magnetic sector field neutral gas analyzer and an identical ion analyzer is proposed. These are configured to detect and identify gas and ion species of hydrogen, helium, nitrogen, oxygen, nitric oxide, and carbon dioxide and any other gas or ion species in the 1 to 46 amu mass range. This range covers the normal atmospheric constituents. The sensitivity of the instrument is sufficient to measure ambient gases and ion with a particle density of the order of one per cc. A chemical pump, or getter, is mounted near the entrance aperture of the neutral gas analyzer which integrates the absorption of ambient gases for a selectable period of time for subsequent release and analysis. The sensitivity is realizable for all but rare gases using this technique.

Hoffman, J. H.

1980-01-01

188

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

Microsoft Academic Search

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.

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

1988-01-01

189

Harmonics suppression of vacuum chamber eddy current induced fields with application to SSC Low Energy Booster Magnets  

SciTech Connect

An expression for eddy currents induced in a thin-walled conductor due to a time-dependent electromagnet field excitation is formulated, allowing subsequent analytical development of a closed form expression predicting vacuum chamber eddy current induced field harmonics in iron-core multipole electromagnets. A passive technique for harmonics suppression is presented with specific application to the design of the Superconducting Super Collider (SSC) Low Energy Booster (LEB) Magnets.

Schlueter, R.D.; Halbach, K.

1993-09-01

190

Sleep, waking and neurobehavioural performance.  

PubMed

Waking neurobehavioural or cognitive functioning is largely dependent on two mechanisms both in synchrony and in opposition to each other: the sleep homeostatic and circadian systems. The influences of these systems are particularly evident during periods of sustained wakefulness or sleep deprivation. Although the effects of these two systems on neurobehavioural functioning during periods of extended wakefulness have been demonstrated experimentally, there does not exist an adequate theory to describe the underlying brain mechanisms responsible for these neurobehavioural deficits. Much research has in fact concentrated not on understanding the nature of these deficits, but rather in counteracting them, via the use of countermeasures, such as naps and wake promoting compounds. PMID:12957855

Rogers, Naomi L; Dorrian, Jillian; Dinges, David F

2003-09-01

191

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

192

Physiological properties of raphe magnus neurons during sleep and waking.  

PubMed

Neurons in the medullary raphe magnus (RM) that are important in the descending modulation of nociceptive transmission are classified by their response to noxious tail heat as ON, OFF, or NEUTRAL cells. Experiments in anesthetized animals demonstrate that RM ON cells facilitate and OFF cells inhibit nociceptive transmission. Yet little is known of the physiology of these cells in the unanesthetized animal. The first aim of the present experiments was to determine whether cells with ON- and OFF-like responses to noxious heat exist in the unanesthetized rat. Second, to determine if RM cells have state-dependent discharge, the activity of RM neurons was recorded during waking and sleeping states. Noxious heat applied during waking and slow wave sleep excited one group of cells (ON-U) in unanesthetized rats. Other cells were inhibited by noxious heat (OFF-U) applied during waking and slow wave sleep states in unanesthetized rats. NEUTRAL-U cells did not respond to noxious thermal stimulation applied during either slow wave sleep or waking. ON-U and OFF-U cells were more likely to respond to noxious heat during slow wave sleep than during waking and were least likely to respond when the animal was eating or drinking. Although RM cells rarely respond to innocuous stimulation applied during anesthesia, ON-U and OFF-U cells were excited and inhibited, respectively, by innocuous somatosensory stimulation in the unanesthetized rat. The spontaneous activity of >90% of the RM neurons recorded in the unanesthetized rat was influenced by behavioral state. OFF-U cells discharged sporadically during waking but were continuously active during slow wave sleep. By contrast, ON-U and NEUTRAL-U cells discharged in bursts during waking and either ceased to discharge entirely or discharged at a low rate during slow wave sleep. We suggest that OFF cell discharge functions to suppress pain-evoked reactions during sleep, whereas ON cell discharge facilitates pain-evoked responses during waking. PMID:10036262

Leung, C G; Mason, P

1999-02-01

193

The stability of laminar symmetric separated wakes  

NASA Astrophysics Data System (ADS)

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

Castro, Ian P.

2005-06-01

194

Wakes in inhomogeneous plasmas  

NASA Astrophysics Data System (ADS)

The Debye shielding of a charge immersed in a flowing plasma is an old classic problem. 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 nonoscillatory and weaker.

Kompaneets, Roman; Ivlev, Alexei V.; Nosenko, Vladimir; Morfill, Gregor E.

2014-04-01

195

Wake Vortex Research in the USA (WakeNet-USA)  

NASA Technical Reports Server (NTRS)

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

Lang, Steve; Bryant, Wayne

2006-01-01

196

Wake Shield Facility  

NASA Technical Reports Server (NTRS)

The Wake Shield Facility is a free-flying research and development facility that is designed to use the pure vacuum of space to conduct scientific research in the development of new materials. The thin film materials technology developed by the WSF could some day lead to applications such as faster electronics components for computers. The WSF Free-Flyer is a 12-foot-diameter stainless steel disk that, while traveling in orbit at approximately 18,000 mph, leaves in its wake a vacuum 1,000 to 10,000 times better than the best vacuums currently achieved on Earth. While it is carried into orbit by the Space Shuttle, the WSF is a fully equipped spacecraft in its own right, with cold gas propulsion for separation from the orbiter and a momentum bias attitude control system. All WSF functions are undertaken by a spacecraft computer with the WSF remotely controlled from the ground. The ultra vacuum, nearly empty of all molecules, is then used to conduct a series of thin film growths by a process called epitaxy which produces exceptionally pure and atomically ordered thin films of semiconductor compounds such as gallium arsenide. Using this process, the WSF offers the potential of producing thin film materials, and the devices they will make possible.

2004-01-01

197

Aerodynamic flow vectoring of a wake using asymmetric synthetic jet actuation  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

198

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING EAST WITH PHOTO ...  

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

WAKE ISLAND AIRFIELD TERMINAL, BUILDING 1502 LOOKING EAST WITH PHOTO SCALE CENTERED ON BUILDING (12/30/2008) - Wake Island Airfield, Terminal Building, West Side of Wake Avenue, Wake Island, Wake Island, UM

199

Suppression of 1\\/f noise by permanent magnetic field in ion-implanted HgCdTe photodiodes  

Microsoft Academic Search

A brief report on the experimental evidence and theoretical explanation of substantial effects of a permanent magnetic field (PMF) of B=0-2 T on 1\\/f noise in HgCdTe n+-p high performance planar photodiodes (HPPD) is presented. It is shown that the PMF suppresses 1\\/f noise in the HPPD at lower frequencies fLf0. These

Yu. L. Khait; V. Garber; G. Bahir; I. Snapiro

2001-01-01

200

Vertical Fence Wake Manipulation Using Periodic Variation of Upstream Flow  

NASA Astrophysics Data System (ADS)

The effect of periodic variation of upstream flow on the separated shear flows behind the vertical fence was experimentally investigated. Upstream flow was modified using small obstacles and this device made the periodic change of streamwise velocities in front of the fence. The experiments were performed in a circulating water channel. The Reynolds number based on the height of fence and freestream velocity was varied from 2000 to 6000. The vertical fence was submerged in the turbulent boundary layer. Stereoscopic-PIV method was used to measure the instantaneous velocity fields around a vertical fence. 800 instantaneous velocity fields were acquired in each experimental condition and the mean properties were calculated using the ensemble average method. The obtained results were compared with those of uncontrolled fence flow. The results showed the vertical fence under the upstream flow change has the local downwash flow behind the fence and this flow suppressed the separation bubble and made the smaller recirculation region compared with uncontrolled fence wake.

Lee, Seung-Hwan; Tu, Xin Cheng; Kim, Hyoung-Woo; Kim, Hyoung-Bum

2011-11-01

201

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

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

202

Atmospheric-wake vortex interactions  

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

203

Gravitational wakes in Saturn's rings  

Microsoft Academic Search

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

H. Salo

1992-01-01

204

Wind Turbine Wake Experiment - Wieringermeer (WINTWEX-W)  

NASA Astrophysics Data System (ADS)

The Wind Turbine Wake Experiment - Wieringermeer (WINTWEX-W) is a cooperative wake measurement campaign conducted by the Norwegian Centre of Offshore Wind Energy (Norcowe) and the Energy Research Centre of the Netherlands (ECN). A scanning, four static Windcubes as well as a downstream looking nacelle LiDAR are placed for half a year downstream of one of five research wind turbines in ECNs' wind turbine test farm Wieringermeer. In order to capture wake characteristics under different weather conditions a 60° sector for three different elevations and two vertical cross-sections are scanned every minute with additional wind profile information every second at 2, 5 and 12 rotor diameter distances. Another static Windcube, a forward-looking nacelle LiDAR and three Sonics are placed upstream to measure the undisturbed approaching flow field. During the campaign several scanning algorithms are tested to capture most wake features. The aim of the campaign is a qualitative and quantitative description of single wind turbine wake evolution, propagation and persistency, as well as to improve CFD wake models by delivering a detailed data set of several real atmospheric conditions.

Kumer, Valerie; Reuder, Joachim; Svardal, Benny; Eecen, Peter

2014-05-01

205

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

206

On the expansion of ionospheric plasma into the near-wake of the Space Shuttle Orbiter  

NASA Technical Reports Server (NTRS)

During the Spacelab 2 mission, while the Plasma Diagnostics Package was attached to the Remote Manipulator System, differential ion vector measurements were obtained in the near wake at a distance of 4-5 Shuttle radii. The Orbiter's wake was found to fill in at a much faster rate than can be explained by simple thermal motion. The measurements strongly suggest that filling of the Orbiter's wake is produced by the process of 'collisionless plasma expansion into a vacuum' and that, for oblique angles of the magnetic field and velocity vectors, the near wake plasma depletion a few radii downstream is not sensitive to the body scale size.

Stone, N. H.; Wright, K. H., Jr.; Samir, U.; Hwang, K. S.

1988-01-01

207

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.

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

2013-01-01

208

Chitin amendment increases soil suppressiveness toward plant pathogens and modulates the actinobacterial and oxalobacteraceal communities in an experimental agricultural field.  

PubMed

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; van Elsas, Jan Dirk

2013-09-01

209

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

PubMed

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. PMID:24710825

Winfield, J M; Douglas, N H M; Desouza, N M; Collins, D J

2014-05-01

210

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

211

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

212

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

213

Suppression of 1/f noise by permanent magnetic field in ion-implanted HgCdTe photodiodes  

NASA Astrophysics Data System (ADS)

A brief report on the experimental evidence and theoretical explanation of substantial effects of a permanent magnetic field (PMF) of B=0-2 T on 1/f noise in HgCdTe n+-p high performance planar photodiodes (HPPD) is presented. It is shown that the PMF suppresses 1/f noise in the HPPD at lower frequencies fLf0. These PMF effects follow from the earlier proposed nanoscopic stochastic theory of 1/f noise in solids. The proposed theoretical model is in good agreement with observations.

Khait, Yu. L.; Garber, V.; Bahir, G.; Snapiro, I.

2001-10-01

214

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

215

Self-similar potential in the near wake. [of plasma  

NASA Technical Reports Server (NTRS)

The plasma potential is measured near the edge of an electrically floating obstacle placed in a steady-state, supersonic, unmagnetized, neutral plasma flow. Equipotential contours show the sheath of the upstream side of the obstacle wrapping around the edge of the obstacle and fanning out into the near wake. Both fluid theory and the data find the near-wake plasma potential to be self-similar when ionization, charge exchange, and magnetic field can be neglected. The theory also finds that fluid velocity is self-similar, the near wake is nonneutral, and plasma density is not self-similar. Strong electric fields are found near the obstacle and equipotential contours are found to conform to all boundaries.

Diebold, D.; Hershkowitz, N.; Intrator, T.; Bailey, A.

1987-01-01

216

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

Microsoft Academic Search

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

Sandeep Gupta

2006-01-01

217

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

Microsoft Academic Search

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

S Wharton; J K Lundquist; N Marjanovic

2012-01-01

218

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

219

Development and Testing of Laser Doppler System Components for Wake Vortex Monitoring. Volume 1: Scanner Development, Laboratory and Field Testing and System Modeling.  

National Technical Information Service (NTIS)

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 prov...

D. J. Wilson M. C. Krause E. W. Coffey C. C. Huang B. B. Edwards

1974-01-01

220

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

221

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.

Wang, Chun; Dreher, Bogdan

2014-01-01

222

Suppression of bunch transverse instabilities by the chamber asymmetry.  

National Technical Information Service (NTIS)

Axial asymmetry of a vacuum chamber gives rise to wake forces producing betatron tune shifts for tail particles. In the result, the bunch transverse instabilities could be suppressed or even eliminated.

A. Burov V. Danilov

1998-01-01

223

Suppression of bunch transverse instabilities by the chamber asymmetry  

SciTech Connect

Axial asymmetry of a vacuum chamber gives rise to wake forces producing betatron tune shifts for tail particles. In the result, the bunch transverse instabilities could be suppressed or even eliminated.

Burov, A.; Danilov, V.

1998-07-01

224

Drastic high magnetic field effect on suppression of Escherichia coli death  

Microsoft Academic Search

When Escherichia coli B was aerobically grown in a medium containing one-fourth the concentration of the LB medium supplemented with glutamic acid at 43°C under an inhomogeneous 5.2–6.1 T magnetic field, the number of cells in the stationary phase under the high magnetic field was 100,000 times higher than that under a geomagnetic field. The finding that the amount of

Shin-ichiro Horiuchi; Yoshimasa Ishizaki; Kazumasa Okuno; Takashi Ano; Makoto Shoda

2001-01-01

225

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

226

Suppression of the Shastry-Sutherland phase in Ce2Pd2Sn at a field-induced critical point  

NASA Astrophysics Data System (ADS)

The magnetic phase diagram of Ce2Pd2Sn is investigated through the field dependence of thermal, transport, and magnetic properties at low temperature. The upper transition, TM=4.8K is slightly affected by magnetic field up to B=1T whereas the lower one TC(B) rapidly increases from 2.1 K joining TM in a critical point at Tcr=(4.1±0.2)K for Bcr=(0.11±0.01)T . At that point, the intermediate phase, previously described as an unstable Shastry-Sutherland lattice, is suppressed. A detailed analysis around the critical point reveals a structure in the maximum of the ?M/?B(B) derivative, which may be related to a step in magnetization predicted by theory for the mentioned lattice.

Sereni, J. G.; Gómez Berisso, M.; Schmerber, G.; Kappler, J. P.

2010-05-01

227

Intergrain forces in low-Mach-number plasma wakes  

NASA Astrophysics Data System (ADS)

Large-scale particle-in-cell calculations of the plasma wake interactions of two negatively charged grains smaller than the Debye length are carried out using the coptic code over a wide range of subsonic plasma flow velocities. In plasmas with the temperature ratio Te/Ti=100, it is found that a single grain's oscillatory wake disappears for flow Mach numbers M less than approximately 0.3, which is the parameter regime where Landau damping is expected to be strong. Neutral collisions suppress potential oscillations above M=0.3, but not the trailing attractive potential peak caused by ion focusing. The transverse (grain-aligning) force on a downstream particle in the wake of another is obtained rigorously from the code in three-dimensional simulations. It shows general agreement with the force that would be deduced from the single-grain wake potential gradient. Except for relatively large grains in the nonlinear collisional regime, the grain-aligning force is very small for slow flow.

Hutchinson, I. H.

2012-06-01

228

Intergrain forces in low-Mach-number plasma wakes.  

PubMed

Large-scale particle-in-cell calculations of the plasma wake interactions of two negatively charged grains smaller than the Debye length are carried out using the coptic code over a wide range of subsonic plasma flow velocities. In plasmas with the temperature ratio T(e)/T(i)=100, it is found that a single grain's oscillatory wake disappears for flow Mach numbers M less than approximately 0.3, which is the parameter regime where Landau damping is expected to be strong. Neutral collisions suppress potential oscillations above M=0.3, but not the trailing attractive potential peak caused by ion focusing. The transverse (grain-aligning) force on a downstream particle in the wake of another is obtained rigorously from the code in three-dimensional simulations. It shows general agreement with the force that would be deduced from the single-grain wake potential gradient. Except for relatively large grains in the nonlinear collisional regime, the grain-aligning force is very small for slow flow. PMID:23005230

Hutchinson, I H

2012-06-01

229

Comparing offshore wind farm wake observed from satellite SAR and wake model results  

NASA Astrophysics Data System (ADS)

Offshore winds can be observed from satellite synthetic aperture radar (SAR). In the FP7 EERA DTOC project, the European Energy Research Alliance project on Design Tools for Offshore Wind Farm Clusters, there is focus on mid- to far-field wind farm wakes. The more wind farms are constructed nearby other wind farms, the more is the potential loss in annual energy production in all neighboring wind farms due to wind farm cluster effects. It is of course dependent upon the prevailing wind directions and wind speed levels, the distance between the wind farms, the wind turbine sizes and spacing. Some knowledge is available within wind farm arrays and in the near-field from various investigations. There are 58 offshore wind farms in the Northern European seas grid connected and in operation. Several of those are spaced near each other. There are several twin wind farms in operation including Nysted-1 and Rødsand-2 in the Baltic Sea, and Horns Rev 1 and Horns Rev 2, Egmond aan Zee and Prinses Amalia, and Thompton 1 and Thompton 2 all in the North Sea. There are ambitious plans of constructing numerous wind farms - great clusters of offshore wind farms. Current investigation of offshore wind farms includes mapping from high-resolution satellite SAR of several of the offshore wind farms in operation in the North Sea. Around 20 images with wind farm wake cases have been retrieved and processed. The data are from the Canadian RADARSAT-1/-2 satellites. These observe in microwave C-band and have been used for ocean surface wind retrieval during several years. The satellite wind maps are valid at 10 m above sea level. The wakes are identified in the raw images as darker areas downwind of the wind farms. In the SAR-based wind maps the wake deficit is found as areas of lower winds downwind of the wind farms compared to parallel undisturbed flow in the flow direction. The wind direction is clearly visible from lee effects and wind streaks in the images. The wind farm wake cases are modeled by various types of wake models. In the EERA DTOC project the model suite consists of engineering models (Ainslie, DWM, GLC, PARK, WASP/NOJ), simplified CFD models (FUGA, FarmFlow), full CFD models (CRES-flowNS, RANS), mesoscale model (SKIRON, WRF) and coupled meso-scale and microscale models. The comparison analysis between the satellite wind wake and model results will be presented and discussed. It is first time a comprehensive analysis is performed on this subject. The topic gains increasing importance because there is a growing need to precisely model also mid- and far-field wind farms wakes for development and planning of offshore wind farm clusters.

Bay Hasager, Charlotte

2014-05-01

230

Suppression of Secondary Emission in a Magnetic Field Using a Sawtooth and Isosceles Triangle Surface  

SciTech Connect

The effect of surface roughness on the secondary electron emission from a sawtooth and isosceles triangle surface in a magnetic field under electron bombardment is investigated using a Monte-Carlo method. Some of the secondary electrons emitted from the surface return to the surface within their first few gyrations, resulting in a low effective secondary electron yield. Both sawtooth and isosceles triangle surface in magnetic field can significantly reduce the secondary emission yield below the multipacting threshold with weak dependence on the size of surface and magnetic field.

Wang, L.; Raubenheimer, T.O.; Stupakov, G.; /SLAC

2006-09-26

231

The Role of Mesopontine NGF in Sleep and Wakefulness  

PubMed Central

The microinjection of nerve growth factor (NGF) into the cat pontine tegmentum rapidly induces rapid eye movement (REM) sleep. To determine if NGF is involved in naturally-occurring REM sleep, we examined whether it is present in mesopontine cholinergic structures that promote the initiation of REM sleep, and whether the blockade of NGF production in these structures suppresses REM sleep. We found that cholinergic neurons in the cat dorsolateral mesopontine tegmentum exhibited NGF-like immunoreactivity. In addition, the microinjection of an oligodeoxyribonucleotide (OD) directed against cat NGF mRNA into this region resulted in a reduction in the time spent in REM sleep in conjunction with an increase in the time spent in wakefulness. Sleep and wakefulness returned to baseline conditions 2 to 5 days after antisense OD administration. The preceding antisense OD-induced effects occurred in conjunction with the suppression of NGF-like immunoreactivity within the site of antisense OD injection. These data support the hypothesis that NGF is involved in the modulation of naturally-occurring sleep and wakefulness.

Ramos, Oscar V.; Torterolo, Pablo; Lim, Vincent; Chase, Michael H.; Sampogna, Sharon; Yamuy, Jack

2011-01-01

232

Numerical simulation of supersonic wake flow with parallel computers  

SciTech Connect

Simulating a supersonic wake flow field behind a conical body is a computing intensive task. It requires a large number of computational cells to capture the dominant flow physics and a robust numerical algorithm to obtain a reliable solution. High performance parallel computers with unique distributed processing and data storage capability can provide this need. They have larger computational memory and faster computing time than conventional vector computers. We apply the PINCA Navier-Stokes code to simulate a wind-tunnel supersonic wake experiment on Intel Gamma, Intel Paragon, and IBM SP2 parallel computers. These simulations are performed to study the mean flow in the near wake region of a sharp, 7-degree half-angle, adiabatic cone at Mach number 4.3 and freestream Reynolds number of 40,600. Overall the numerical solutions capture the general features of the hypersonic laminar wake flow and compare favorably with the wind tunnel data. With a refined and clustering grid distribution in the recirculation zone, the calculated location of the rear stagnation point is consistent with the 2D axisymmetric and 3D experiments. In this study, we also demonstrate the importance of having a large local memory capacity within a computer node and the effective utilization of the number of computer nodes to achieve good parallel performance when simulating a complex, large-scale wake flow problem.

Wong, C.C. [Sandia National Labs., Albuquerque, NM (United States); Soetrisno, M. [Amtec Engineering, Inc., Bellevue, WA (United States)

1995-07-01

233

Contrail Formation in Aircraft Wakes Using Large-Eddy Simulations  

NASA Technical Reports Server (NTRS)

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

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

2002-01-01

234

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

NASA Technical Reports Server (NTRS)

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

VanZante, Dale E.

1998-01-01

235

Detection of wind wakes offshore from satellite SAR  

Microsoft Academic Search

A study is presented on the mapping of ocean wind fields for detection of wind wakes downstream of an offshore wind farm. The study is based on ERS-2 Synthetic Aperture Radar (SAR) scenes obtained in 2003 over Horns Reef in the North Sea. A large offshore wind farm (80 wind turbines) is located 14-20 km offshore of Denmark on this

M. B. Christiansen; C. B. Hasager

2004-01-01

236

A Method for Calculating Helicopter Vortex Paths and Wake Velocities.  

National Technical Information Service (NTIS)

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

E. S. Levinsky T. Strand

1970-01-01

237

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

238

Electron temperatures in the wake of an ionospheric satellite  

NASA Technical Reports Server (NTRS)

We present measurements of electron temperature (Te) made by a retarding potential analyzer and a Langmuir probe (both flush-mounted on the spin-stabilized satellite Explorer 31) to investigate the variation of Te around the satellite. Most of the time there is a Te variation, which repeats for given ionospheric conditions. The variation is strongly controlled by the angle between the velocity vector and the probe normal, Te usually being enhanced in the near wake of the satellite. Magnetic field control of Te, if it is present, is hidden by the stronger velocity vector control. Our results indicate that the magnitude of the Te enhancement in the wake does not depend on the average ion mass (M), although the electron density depletion in the wake is strongly correlated with M.

Troy, B. E., Jr.; Maier, E. J.; Samir, U.

1975-01-01

239

An Improved Wake Vortex Tracking Algorithm for Multiple Aircraft  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

240

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

241

T-italic/sub c-italic/ suppression and critical fields in thin superconducting Nb films  

SciTech Connect

Thin Nb films from 30 to 150 A-circle thick have been electron-beam evaporated onto sapphire substrates with aluminum overlayers, then deposited to protect the surface. T-italic/sub c-italic/'s and magnetoresistance were found to be consistent with localization models and measured parallel critical fields fit a generalized Ginzburg-Landau theory. Perpendicular critical fields, however, were found to have an anomalous curvature near T-italic/sub c-italic/. These results, we believe, are due to the disorder of the film and have also been seen in other systems.

Quateman, J.H.

1986-08-01

242

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

243

Spontaneous Generation of the Magnetic Field and Suppression of the Heat Conduction in Cold Fronts  

Microsoft Academic Search

We have determined the physical mechanism responsible for plasma instabilities, which were first found by Ramani & Laval in 1978, associated with anisotropic velocity distributions induced by a temperature gradient in which there are growing low-frequency transverse magnetic waves, even in the absence of background magnetic fields. We have shown that the physical mechanism responsible for the growth of one

Nobuhiro Okabe; Makoto Hattori

2003-01-01

244

ASRS Reports on Wake Vortex Encounters  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

245

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

246

Effects of atmospheric stability on the evolution of wind turbine wakes: Volumetric LiDAR scans  

NASA Astrophysics Data System (ADS)

Aerodynamic optimization of wind farm layout is a fundamental task to reduce wake effects on downstream wind turbines, thus to maximize wind power harvesting. However, downstream evolution and recovery of wind turbine wakes are strongly affected by the characteristics of the incoming atmospheric boundary layer (ABL) flow, like the vertical profiles of the mean wind velocity and the turbulence intensity, which are in turn affected by the ABL stability regime. Therefore, the characterization of the variability of wind turbine wakes under different ABL stability regimes becomes fundamental to better predict wind power harvesting and improve wind farm efficiency. To this aim, wind velocity measurements of the wake produced by a 2 MW Enercon E-70 wind turbine were performed with three scanning Doppler wind Light Detection and Ranging (LiDAR) instruments. One LiDAR was typically devoted to the characterization of the incoming wind, in particular wind velocity, shear and turbulence intensity at the height of the rotor disc. The other two LiDARs performed scans in order to characterize the wake velocity field produced by the tested wind turbine. The main challenge in performing field measurements of wind turbine wakes is represented by the varying wind conditions, and by the consequent adjustments of the turbine yaw angle needed to maximize power production. Consequently, taking into account possible variations of the relative position between LiDAR measurement volume and wake location, different LiDAR measurement procedures were carried out in order to perform 2-D and 3-D characterizations of the mean wake velocity field. However, larger measurement volumes and higher spatial resolution require longer sampling periods; thus, to investigate wake turbulence tests were also performed by staring the LiDAR laser beam over fixed directions and with the maximum sampling frequency. Furthermore, volumetric scans of the wind turbine wake were performed under different wind conditions via two simultaneous LiDARs. Through the evaluation of the minimum wake velocity deficit as a function of the downstream distance, it is shown that the stability regime of the ABL has a significant effect on the wake evolution; specifically the wake recovers faster under convective conditions. This result suggests that atmospheric inflow conditions, and particularly thermal stability, should be considered for improved wake models and predictions of wind power harvesting.

Valerio Iungo, Giacomo; Porté-Agel, Fernando

2014-05-01

247

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

248

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

249

Vortex shedding in compressor blade wakes  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

250

Vortex wake and exhaust plume interaction, including ground effect  

NASA Astrophysics Data System (ADS)

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

Adam, Ihab Gaber

251

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

252

Dynamics of wake structure in clapping propulsion  

NASA Astrophysics Data System (ADS)

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

Kim, Daegyoum; Gharib, Morteza

2009-11-01

253

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

254

Prediction and Control of Vortex Dominated and Vortex-wake Flows  

NASA Technical Reports Server (NTRS)

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

Kandil, Osama

1996-01-01

255

Measurement procedures for characterization of wind turbine wakes with scanning Doppler wind LiDARs  

NASA Astrophysics Data System (ADS)

The wake flow produced from an Enercon E-70 wind turbine is investigated through three scanning Doppler wind LiDARs. One LiDAR is deployed upwind to characterize the incoming wind, while the other two LiDARs are located downstream to carry out wake measurements. The main challenge in performing measurements of wind turbine wakes is represented by the varying wind conditions, and by the consequent adjustments of the turbine yaw angle needed to maximize power production. Consequently, taking into account possible variations of the relative position between the LiDAR measurement volume and wake location, different measuring techniques were carried out in order to perform 2-D and 3-D characterizations of the mean wake velocity field. However, larger measurement volumes and higher spatial resolution require longer sampling periods; thus, to investigate wake turbulence tests were also performed by staring the LiDAR laser beam over fixed directions and with the maximum sampling frequency. The characterization of the wake recovery along the downwind direction is performed. Moreover, wake turbulence peaks are detected at turbine top-tip height, which can represent increased fatigue loads for downstream wind turbines within a wind farm.

Iungo, G. V.; Porté-Agel, F.

2013-05-01

256

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

257

Recent results about fan noise: Its generation, radiation and suppression  

NASA Technical Reports Server (NTRS)

Fan noise including its generation, radiation characteristics, and suppression by acoustic treatment is studied. In fan noise generation, results from engine and fan experiments, using inflow control measures to suppress noise sources related to inflow distortion and turbulence, are described. The suppression of sources related to inflow allows the experiments to focus on the fan or engine internal sources. Some of the experiments incorporated pressure sensors on the fan blades to sample the flow disturbances encountered by the blades. From these data some inferences can be drawn about the origins of the disturbances. Also, hot wire measurements of a fan rotor wake field are presented and related to the fan's noise signature. The radiation and the suppression of fan noise are dependent on the acoustic modes generated by the fan. Fan noise suppression and radiation is described by relating these phenomena to the mode cutoff ratio parameter. In addition to its utility in acoustic treatment design and performance prediction, cutoff ratio was useful in developing a simple description of the radiation pattern for broadband fan noise. Some of the findings using the cutoff ratio parameter are presented.

Feiler, C. E.

1982-01-01

258

Oscillating airfoils and their wake  

NASA Technical Reports Server (NTRS)

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

Send, W.

1985-01-01

259

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

260

Suppressing feedback in a distributed video coding system by employing real field codes  

NASA Astrophysics Data System (ADS)

Single-view distributed video coding (DVC) is a video compression method that allows for the computational complexity of the system to be shifted from the encoder to the decoder. The reduced encoding complexity makes DVC attractive for use in systems where processing power or energy use at the encoder is constrained, for example, in wireless devices and surveillance systems. One of the biggest challenges in implementing DVC systems is that the required rate must be known at the encoder. The conventional approach is to use a feedback channel from the decoder to control the rate. Feedback channels introduce their own difficulties such as increased latency and buffering requirements, which makes the resultant system unsuitable for some applications. Alternative approaches, which do not employ feedback, suffer from either increased encoder complexity due to performing motion estimation at the encoder, or an inaccurate rate estimate. Inaccurate rate estimates can result in a reduced average rate-distortion performance, as well as unpleasant visual artifacts. In this paper, the authors propose a single-view DVC system that does not require a feedback channel. The consequences of inaccuracies in the rate estimate are addressed by using codes defined over the real field and a decoder employing successive refinement. The result is a codec with performance that is comparable to that of a feedback-based system at low rates without the use of motion estimation at the encoder or a feedback path. The disadvantage of the approach is a reduction in average rate-distortion performance in the high-rate regime for sequences with significant motion.

Louw, Daniel J.; Kaneko, Haruhiko

2013-12-01

261

DPIV study of near-stall wake-rotor interactions in a transonic compressor  

NASA Astrophysics Data System (ADS)

Wake-rotor interactions at near-stall conditions are studied using digital particle image velocimetry (DPIV) in a transonic compressor at the Compressor Aero Research Lab (CARL) at Wright-Patterson Air Force Base. The wake generators (WGs) are designed to simulate wakes of highly loaded stators without producing flow swirl. Flow visualization, instantaneous, and average velocity field data are presented for near-stall compressor operating conditions. These results describe the interaction of the wake shed from the upstream WG with the downstream potential field of the rotor leading edge for two axial spacings at 75 and 90% span. The advantage of ensemble averaging is demonstrated when operating at challenging conditions such as near-stall, which poses special difficulties in obtaining DPIV data. Ensemble averaging, which is possible because of a natural phase locking provided by the blade potential field, allows robust statistics.

Copenhaver, W.; Estevadeordal, J.; Gogineni, S.; Gorrell, S.; Goss, L.

2002-08-01

262

Wake Characteristics of a Single Turbine During the CWEX-10/11 Crop Wind-Energy EXperiments  

NASA Astrophysics Data System (ADS)

In the summer of 2010 and 2011 for the Crop Wind-energy EXperiment (CWEX), flux stations measured differences in micrometeorology upstream and downstream of a single turbine within a large wind farm in Iowa. Profiling LiDARs were positioned upwind and downwind of a single turbine for two months in 2011 to document the wake profiles of mean wind speed and turbulent kinetic energy (TKE). Nacelle-based measurements of wind speed, wind direction, and power produced verified the likely presence of a wake above the downwind flux station. As described in the CWEX overview paper (Rajewski et al. 2013) the flux stations detected (1) turbine-wake events for wakes overhead but not intersecting the surface, (2) wakes with a direct surface influence, and (3) flow perturbations caused by the static pressure field around a line of turbines. We refine our conceptual model of wind turbine flow by comparing downwind-upwind flux and profile station differences for categories of waked and non-waked flow according to turbine hub-height speed and direction, ambient thermal stratification, and the operating status of the turbines. For nighttime stable conditions (some for which a low level jet is present) we measured both within the rotor depth and at the surface higher turbulence and stronger intermittency of the flow on the wake edges as compared to the wake core. We additionally observe frequent periods with 20-30° of directional shear from the surface to the top of the rotor as evidenced by a downwind flux station in non-waked flow with concurrent LiDAR measurement of a wake in the rotor layer. Momentum power spectra and co-spectra of 20-Hz surface data corroborate with previous wind tunnel and numerical simulations of wake turbulence with higher energy intensity but at reduced scales than for non-waked conditions. The spectra demonstrate a return to ambient flow when the wind farm is brought offline.

Rajewski, D. A.; Takle, E. S.; Lundquist, J. K.; Rhodes, M. E.; Prueger, J. H.; Oncley, S. O.; Horst, T. W.; Pfeiffer, R.; Hatfield, J.; Spoth, K. K.; Doorenbos, R. K.

2013-12-01

263

Multiple-LiDAR measurements of wind turbine wakes: effect of the atmospheric stability  

NASA Astrophysics Data System (ADS)

Aerodynamic design and optimization of a wind farm layout are mainly based on the evaluation of wind turbine wake recovery by moving downstream, and on the characterization of wake interactions within a wind farm. Indeed, the power production of downstream wind turbine rows is strictly affected by the cumulative wake produced by the turbines deployed upstream. Wind turbine wakes are dependent on their aerodynamic features, and being immersed in the atmospheric boundary layer (ABL), they are also affected by surface heterogeneity, e.g. site topography and surface coverage, and atmospheric stability. The ABL stability is typically classified as neutral, convective or stable. In a neutral ABL the mechanical turbulent production is the dominating phenomenon. Conversely, for a convective ABL the turbulent kinetic energy and vertical transport phenomena are enhanced by positive buoyancy. Finally, for a stable ABL, a lower turbulence level is typically observed with an increased wind shear. For the present campaign convective ABL was typically observed during day-time, and neutral ABL for early morning and sunset periods. The aim of the present work is the evaluation of the influence of the ABL stability on downstream evolution of wind turbine wakes, which is mainly controlled by different ABL turbulence characteristics. Field measurements of the wake produced from a 2 MW Enercon E-70 wind turbine were performed with three scanning Doppler wind LiDARs. The wind and atmospheric conditions were characterized through a sonic anemometer deployed in proximity of the wind turbine. One LiDAR was placed at a distance about 12 rotor diameters upstream of the turbine in order to characterize the incoming wind. Two additional LiDARs were typically used to perform wake measurements. Tests were performed over the wake vertical symmetry plane in order to characterize wake recovery. Measurements were also carried out over conical surfaces in order to investigate the wind turbine wake with varying wind direction, thus different turbine yaw angles. Moreover, a 3D characterization of the wind turbine wake was performed by scanning the LiDAR over a 3D measurement volume. However, the large sampling period required for the 3D scans does not allow the investigation of wake dynamics. The LiDAR measurements show that wake evolution is significantly affected by the stability conditions of the ABL, thus by the different turbulence characteristics of the incoming wind. In particular, a faster wake recovery is observed in the presence of an increased turbulence of the incoming wind and for more convective atmospheric flows.

Valerio Iungo, Giacomo; Porté-Agel, Fernando

2013-04-01

264

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

265

The entrainment eddies in a wake flow  

NASA Astrophysics Data System (ADS)

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

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

266

Functional anatomy of the sleep-wakefulness cycle: wakefulness.  

PubMed

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

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

2011-01-01

267

Flow visualization of vortex interactions in multiple vortex wakes behind aircraft  

NASA Technical Reports Server (NTRS)

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

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

1975-01-01

268

Verification of the SLC wake potentials  

SciTech Connect

The accurate knowledge of the monopole, dipole, and quadrupole wake potentials is essential for SLC. These wake potentials were previously computed by the modal method. The time domain code TBCI allows independent verification of these results. This comparison shows that the two methods agree to within 10% for bunch lengths down to 1 mm. TBCI results also indicate that rounding the irises gives at least a 10% reduction in the wake potentials.

Bane, K.; Weiland, T.

1983-01-01

269

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

270

Detection of wind wakes offshore from satellite SAR  

NASA Astrophysics Data System (ADS)

A study is presented on the mapping of ocean wind fields for detection of wind wakes downstream of an offshore wind farm. The study is based on ERS-2 Synthetic Aperture Radar (SAR) scenes obtained in 2003 over Horns Reef in the North Sea. A large offshore wind farm (80 wind turbines) is located 14-20 km offshore of Denmark on this submerged reef. Meteorological observations are available from an offshore mast; wind speed is measured at four heights up to 62 m and wind direction is measured at 60 m. Maps of wind speed are generated from geophysical model functions (CMOD-4, CMOD-IFR2) with a resolution of 400 m by 400 m using wind direction obtained from in-situ measurements as model input. The wind maps display zones of reduced mean wind speed downstream of the wind farm compared to upwind conditions. The reduction is approximately 10 % immediately behind the wind farm and the wake effect is vanishing over distances in the order of 10 km downstream. This is consistent with wake model predictions. Satellite SAR provides a good estimate of the propagation of wind wakes. Information on how structures affect the local wind climate is useful for wind energy purposes, particularly for siting of future offshore wind farms.

Christiansen, M. B.; Hasager, C. B.

271

Preliminary rotor wake measurements with a laser velocimeter  

NASA Technical Reports Server (NTRS)

A laser velocimeter (LV) was used to determine rotor wake characteristics. The effect of various fuselage widths and rotor-fuselage spacings on time averaged and detailed time dependent rotor wake velocity characteristics was defined. Definition of time dependent velocity characteristics was attempted with the LV by associating a rotor azimuth position with each velocity measurement. Results were discouraging in that no apparent time dependent velocity characteristics could be discerned from the LV measurements. Since the LV is a relatively new instrument in the rotor wake measurement field, the cause of this lack of periodicity is as important as the basic research objectives. An attempt was made to identify the problem by simulated acquisition of LV-type data for a predicted rotor wake velocity time history. Power spectral density and autocorrelation function estimation techniques were used to substantiate the conclusion that the primary cause of the lack of time dependent velocity characteristics was the nonstationary flow condition generated by the periodic turbulence level that currently exists in the open throat configuration of the wind tunnel.

Hoad, D. R.; Rhodes, D. B.; Meyers, J. F.

1983-01-01

272

Harmonics suppression of vacuum chamber eddy current induced fields with application to the Superconducting Super Collider (SSC) Low Energy Booster (LEB) Magnets  

SciTech Connect

This memo presents the formulation of an expression for eddy currents induced in a thin-walled conductor due to a time-dependent electromagnet field excitation. Then follows an analytical development for prediction of vacuum chamber eddy current induced field harmonics in iron-core electromagnets. A passive technique for harmonics suppression is presented with specific application to the design of the Superconducting Super Collider (SSC) Low Energy B (LEB) Magnets.

Schlueter, R.D.; Halbach, K.

1991-12-04

273

Harmonics suppression of vacuum chamber eddy current-induced fields with application to the Superconducting Super Collider Low Energy Booster Magnets  

SciTech Connect

This report presents the formulation of an expression for eddy currents induced in a thin-walled conductor due to a time-dependent electromagnet field excitation. Then follows an analytical development for prediction of vacuum chamber eddy current-induced field harmonics in iron-core electromagnets. A passive technique for harmonics suppression is presented with specific application to the design of the Superconducting Super Collider Low Energy Booster Magnets.

Schlueter, R.; Halbach, K. [Lawrence Berkeley Lab., CA (United States)

1992-01-01

274

Wake measurements around operating wind turbines  

SciTech Connect

Researchers at Oregon State University have conducted wind measurement programs to describe the wake behind large horizontal axis turbines at Goodnoe Hills, Washington, (MOD-2), and behind the FloWind vertical axis wind turbine near Ellenburg, Washington. Wake measurements were taken using portable kite anemometers as well as fixed place anemometers under several atmospheric stability conditions and turbine operating conditions. Centerline hub height (midrotor) measurements were taken downwind and crosswind from 3-9 diameters. These wake programs are discussed and the velocity deficits measured are compared to the estimated deficits calculated from wake models.

Baker, R.W.; Katen, P.C.; Walker, S.N.

1985-05-01

275

Intrusion of the solar wind protons into the near-Moon wake revealed by SELENE (KAGUYA) (Invited)  

NASA Astrophysics Data System (ADS)

We study solar wind (SW) intrusion into the near-Moon wake using plasma data obtained by the SELENE (KAGUYA) spacecraft. It has been known that SW protons flowing around the Moon access the central region of the distant lunar wake, while SW intrusion deep into the near-Moon wake has never been expected. Here we report two mechanisms (which we call Type-I and Type-II, respectively) of SW proton entry deep into the near-Moon wake, using plasma and magnetic field data obtained by the SELENE spacecraft at 100 km height from the lunar surface. The key process of the Type-I entry is acceleration of the SW protons by the bipolar electric field around the wake boundary. This entry mechanism lets the SW protons come fairly deep into the wake (solar zenith angle (SZA) 150 degrees) at 100 km height. On the other hand, the Type-II entry is based on the SW proton scattering on the dayside; SW protons are once scattered at the lunar dayside surface, picked-up by the SW motional electric field, and finally access the deepest wake (SZA>150 degrees). Both entry mechanisms occur in the direction perpendicular to the interplanetary magnetic field, but only Type-II mechanism lets the SW protons come into the deepest wake that the SW protons are not anticipated to access. The Type-II entry forms the proton-governed region (PGR) in one hemisphere of the near-Moon wake, giving rise to a strong asymmetry of the near-Moon wake environment. Our results mean that not only SW electrons but also protons are crucial for plasma environment in the wake.

Nishino, M. N.; Maezawa, K.; Fujimoto, M.; Saito, Y.; Yokota, S.; Asamura, K.; Tanaka, T.; Tsunakawa, H.; Shibuya, H.; Matsushima, M.; Shimizu, H.; Takahashi, F.; Terasawa, T.

2009-12-01

276

Wake structure of a finite circular cylinder of small aspect ratio  

NASA Astrophysics Data System (ADS)

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

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

2004-11-01

277

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

NASA Technical Reports Server (NTRS)

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

Hannon, Stephen M.; Thomson, J. Alex

1994-01-01

278

Characterization of stress-induced suppression of long-term potentiation in the hippocampal CA1 field of freely moving rats.  

PubMed

Several lines of evidence have shown that exposure to stress impairs long-term potentiation (LTP) in the CA1 field of the hippocampus, but the detailed mechanisms for this effect remain to be clarified. The present study elucidated the synaptic mechanism of stress-induced LTP suppression in conscious, freely moving rats using electrophysiological approaches. Open field stress (i.e., novel environment stress) and elevated platform stress (i.e., uncontrollable stress) were employed. Basal synaptic transmission was significantly reduced during exposure to elevated platform stress but not during exposure to open field stress. LTP induction was blocked by elevated platform stress but not influenced by open field stress. Significant increases in serum corticosterone levels were observed in the elevated platform stress group compared with the open field stress group. Furthermore, LTP suppression induced by elevated platform stress was prevented by pretreatment with an anxiolytic drug diazepam (1 mg/kg, i.p.). These results suggest that stress-induced LTP suppression depends on the relative intensity of the stressor. The inhibitory synaptic response induced by an intense psychological stress, such as elevated platform stress, may be attributable to LTP impairment in the CA1 field of the hippocampus. PMID:18582439

Hirata, Riki; Togashi, Hiroko; Matsumoto, Machiko; Yamaguchi, Taku; Izumi, Takeshi; Yoshioka, Mitsuhiro

2008-08-21

279

Suppression of contour perception by band-limited noise and its relation to nonclassical receptive field inhibition  

Microsoft Academic Search

Band-spectrum noise has been shown to suppress the visual perception of printed letters. The suppression exhibits a specific dependence on the spatial frequency of the noise, and the frequency domain of most effective inhibition has been related to the size of the letters. In this paper, we address two important questions that were left open by previous studies: (1) Is

Nicolai Petkov; Michel A. Westenberg

2003-01-01

280

Effect of wind turbine wakes on cropland surface fluxes in the US Great Plains during a Nocturnal Low Level Jet  

Microsoft Academic Search

Installation of large scale wind farms is becoming a common operation in the Midwest, and wind farms frequently are situated among fields of agricultural crops. Each wind turbine is known to alter the behavior of the air mass downwind of the rotor; consequently, the rotor wakes alter the local microclimate. Quantification of the effects of wind turbine wakes on local

M. E. Rhodes; M. Aitken; J. K. Lundquist; E. S. Takle; J. H. Prueger

2010-01-01

281

Nonlinear interaction between the sinuous and varicose instability modes in a plane wake  

NASA Astrophysics Data System (ADS)

The nonlinear interaction between sinuous and varicose instability modes in a plane wake is examined in the nonlinear-nonequilibrium critical layer regime. Equations governing the evolution of the instability wave amplitudes and critical layer vorticity distributions are derived. Numerical solutions for these equations are obtained for a number of wake defects and initial amplitude ratios. The results show that the primary effects of the nonlinear interaction are the suppression of the varicose mode and the downstream shift of the peak of the sinuous mode.

Leib, S. J.; Goldstein, M. E.

1989-03-01

282

Collisionless Damping of Laser Wakes in Plasma Channels  

SciTech Connect

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

Li, X.; Shvets, G.

1998-08-01

283

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

284

32 CFR 935.60 - Wake Island Judicial Authority.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Wake Island Judicial Authority. 935.60 Section 935... TERRITORIAL AND INSULAR REGULATIONS WAKE ISLAND CODE Judiciary § 935.60 Wake Island Judicial Authority. (a) The...

2013-07-01

285

32 CFR 935.61 - Wake Island Court.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Wake Island Court. 935.61 Section 935.61 National... TERRITORIAL AND INSULAR REGULATIONS WAKE ISLAND CODE Judiciary § 935.61 Wake Island Court. (a) The trial judicial...

2013-07-01

286

Pluto's Plasma Wake Oriented Away from the Ecliptic Plane  

NASA Astrophysics Data System (ADS)

Conditions similar to those observed in the solar wind interaction with Venus and Mars with a planetary atmosphere and in the absence of an intrinsic magnetic field should also be applicable to Pluto. With up to 24 ?bars inferred for the Pluto atmosphere it is possible that the feeble solar photon radiation flux that reaches by its orbit, equivalent to ~10-3 of that at earth, is sufficient to produce an ionization component that can be eroded by the solar wind. In view of the reduced solar wind density (~ 10-3 with respect to that by 1 AU) that should be available by Pluto its kinetic energy will be significantly smaller than that by earth. However, the parameter values that are implied for the interaction process between the solar wind and the local upper ionosphere are sufficient to produce a plasma wake that should extend downstream from Pluto. In view of its low gravity force the plasma wake should have a wider cross-section than that in the Venus and Mars plasma environment. Since Pluto rotates with its rotational axis tilted close to its orbital plane the plasma wake will be influenced by a Magnus force that is nearly north-south oriented. That force will be responsible for propelling the plasma wake with a component that can be directed away from the ecliptic plane. It is estimated that transport of solar wind momentum to the upper Pluto's ionosphere implies rotation periods smaller than that of the solid body, and thus larger values of the Magnus force that can increase the orientation of the plasma wake away from the ecliptic plane.

Perez De Tejada, H. A.; Durand-Manterola, H.; Lundin, R. N.; Reyes-Ruiz, M.

2013-12-01

287

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

288

Wake Vortex Detection: Phased Microphone vs. Linear Infrasonic Array  

NASA Technical Reports Server (NTRS)

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

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

2014-01-01

289

PIV analysis of near-wake behind a sphere at a subcritical Reynolds number  

Microsoft Academic Search

The vortical structure of near-wake behind a sphere is investigated using a PIV technique in a circulating water channel at Re = 11,000. The measured velocity fields show a detailed vortical structure in the recirculation region such as recirculation vortices, reversed velocity zone, and out-of-plane vorticity distribution. The vorticity distribution of the sphere wake shows waviness in cross-sectional planes. The

Young Il Jang; Sang Joon Lee

2008-01-01

290

A WIND TUNNEL EXPERIMENT ON WAKE STRUCTURE OF A WIND TURBINE  

Microsoft Academic Search

The flow structure in the near-wake region has been investigated with a wind-tunnel experiment, especially paying attention to the scale effects of a wind turbine model and freestream turbulence (FST) on the wake behavior. For a 1\\/50 scale-model of a 600kW wind turbine model, wind load measurement with a KISTLER was carried out, and a velocity field behind a model

Yasuo HATTORI; Manabu YAMAMOTO; Yuzuru EGUCHI; Koji KONDO; Hitoshi SUTO

291

Effects of Solar Wind Conditions on the Plasma Wake Within a Polar Crater: Preliminary Results  

NASA Technical Reports Server (NTRS)

As the solar wind sweeps horizontally past a shadowed lunar crater it simultaneously diffuses toward the surface through an ambipolar process, forming a plasma wake (e.g., Figure 1). Importantly, the resulting electric field structure diverts solar wind protons toward the cold crater floor where they may represent a source of surficial hydrogen. We present a handful of two-dimensional kinetic simulations exploring the range of wake structures and surface particle fluxes possible under various background plasma conditions.

Zimmerman, M. I.; Farrell, W. M.; Stubbs, T. J.

2011-01-01

292

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

293

A Statistical Study of the Lunar Plasma Wake using ARTEMIS Measurements  

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 on the day side and the generation of a plasma wake on the night side. 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, starting on February 13, 2010. The wake fly-bys have occurred in a variety of orbit geometries and distances from the planet, ranging up to many lunar radii downstream. They have also occurred for a variety of external conditions. In this presentation, we will share a statistical study of the numerous wake-crossing events of the ARTEMIS probes, using data primarily from the ARTEMIS fluxgate magnetometer (FGM) and electrostatic analyzers (ESAs) to identify when the spacecraft entered and exited the wake. Using the morphology of the wake boundary, we will determine the spatial extent of the lunar wake as a function of distance behind the Moon and its response to external conditions.

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

2011-12-01

294

Effect of the solar-wind proton entry into the deepest lunar wake  

NASA Astrophysics Data System (ADS)

We study effect of the solar wind (SW) proton entry deep into the near-Moon wake that was recently discovered by the SELENE mission. Because previous lunar-wake models are based on electron domination, no effect of SW proton entry on the near-Moon wake environment has been taken into account so far. Recent SELENE observations revealed that a part of the SW protons are reflected at the lunar dayside surface and picked-up by the SW electric field (Saito et al., GRL, 2008), and some of them access the deepest lunar wake; this process is called type-II entry (Nishino et al., GRL, 2009). Here we show that the type-II entry of SW protons forms proton-governed region (PGR) to drastically change the electromagnetic environment of the lunar wake. Broadband electrostatic noise found in the PGR is manifestation of electron two-stream instability, which is attributed to the counter-streaming electrons absorbed from the ambient SW to maintain the quasi-neutrality. Acceleration of the absorbed electrons up to ~1 keV means a superabundance of positive charges of 10-4-10-7 cm-3 in the near-Moon wake, which should be immediately canceled out by the incoming high-speed electrons. This is a general phenomenon in the lunar wake, because PGR does not necessarily require peculiar SW condition for its formation.

Nishino, Masaki N.; Fujimoto, Masaki; Saito, Yoshifumi; Yokota, Shoichiro; Kasahara, Yoshiya; Tsunakawa, Hideo; Terasawa, Toshio

2010-05-01

295

Thrust Production and Wake Structure of an Actuated Lamprey Model  

NASA Astrophysics Data System (ADS)

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

Buchholz, James; Smits, Alexander

2004-11-01

296

Continuity between waking activities and dream activities  

Microsoft Academic Search

Empirical studies largely support the continuity hypothesis of dreaming. Despite of previous research efforts, the exact formulation of the continuity hypothesis remains vague. The present paper focuses on two aspects: (1) the differential incorporation rate of different waking-life activities and (2) the magnitude of which interindividual differences in waking-life activities are reflected in corresponding differences in dream content. Using a

Michael Schredl; Friedrich Hofmann

2003-01-01

297

Downstream Development of the Wakes behind Cylinders  

Microsoft Academic Search

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

Sadatoshi Taneda

1959-01-01

298

Neuropeptidergic control of sleep and wakefulness.  

PubMed

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

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

2014-07-01

299

The plasma wake of the Shuttle Orbiter  

NASA Technical Reports Server (NTRS)

One of the objectives of the Plasma Diagnostics Package (PDP) instrumentation on Spacelab 2 was to obtain information about the plasma wake of the Shuttle Orbiter. Plasma density and electron temperature data are presented, which were obtained while the PDP was attached to the Shuttle remote manipulator system, and while the PDP was a free-flying satellite. Wake crossings by the PDP from about 40 m to about 240 m behind the Orbiter provide information about the structure of the mid and far wake of the Orbiter. As expected, the wake is characterized by density depressions, relative to the ambient ionospheric plasma, and by enhancements of the electron temperature, particularly in the near and mid wake. The observed electron temperature enhancements appear to be in line with previous spacecraft observations.

Murphy, G. B.; Reasoner, D. L.; Tribble, A.; D'Angelo, N.; Pickett, J. S.

1989-01-01

300

Monitoring Wake Vortices for More Efficient Airports  

NASA Technical Reports Server (NTRS)

Wake vortices are generated by all aircraft during flight. The larger the aircraft, the stronger the wake, so the Federal Aviation Administration (FAA) separates aircraft to ensure wake turbulence has no effect on approaching aircraft. Currently, though, the time between planes is often larger than it needs to be for the wake to dissipate. This unnecessary gap translates into arrival and departure delays, but since the wakes are invisible, the delays are nearly inevitable. If, however, the separation between aircraft can be reduced safely, then airport capacity can be increased without the high cost of additional runways. Scientists are currently studying these patterns to identify and introduce new procedures and technologies that safely increase airport capacity. NASA, always on the cutting edge of aerospace research, has been contributing knowledge and testing to these endeavors.

2005-01-01

301

Stability Properties of Self-Propelled Wakes  

NASA Astrophysics Data System (ADS)

This thesis focuses on the stability characteristics of a momentum-less wake. Momentum-less wakes are wakes produced by self-propelled bodies and can be found in any numerous fluid flows, such as the flow behind a submarine, or fish. In this thesis we try to relate the momentum-less wake described to a wake behind a fish through specific choices in non-dimensional terms. We then look at both the temporal and spatial stability characteristics. While the temporal stability characteristics appeared to give results of interest, it appeared that the spatial stability characteristics were not as interesting. This thesis also goes into detail about two popular numerical methods of solving the Orr-Sommerfeld equation, and how to use these methods on the less common unbounded domain type flows.

Patel, Tej

302

Application of Three-Component PIV to a Hovering Rotor Wake  

NASA Technical Reports Server (NTRS)

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

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

2000-01-01

303

Modification of the near wake behind a finite-span cylinder by a single synthetic jet  

NASA Astrophysics Data System (ADS)

Modification to the flow field about a finite-span cylinder of low-aspect ratio (AR = 3) by a single synthetic jet, mounted normal to the cylinder axis, was studied experimentally using surface-mounted pressure taps, stereoscopic particle image velocimetry (SPIV), and constant-temperature anemometry. The synthetic jet altered the circulation about the cylinder and created a large spanwise change to the surface pressure, much greater than the dimensions of its orifice. SPIV measurements in the near wake showed that the synthetic jet enhances mixing of the downwash from the cylinder free end with the wake deficit, vectoring and narrowing the wake. The synthetic jet penetrates through the streamwise vorticity, enhancing mixing within the wake and reducing the power associated with the shedding frequency, St = 0.155, except below the vortex dislocation, where the shedding frequency was increased to that corresponding to a quasi-two-dimensional cylinder, St = 0.22.

DeMauro, E. P.; Leong, C. M.; Amitay, M.

2012-12-01

304

Study for prediction of rotor/wake/fuselage interference, part 1  

NASA Technical Reports Server (NTRS)

A method was developed which allows the fully coupled calculation of fuselage and rotor airloads for typical helicopter configurations in forward flight. To do this, an iterative solution is carried out based on a conventional panel representation of the fuselage and a blade element representation of the rotor where fuselage and rotor singularity strengths are determined simultaneously at each step and the rotor wake is allowed to relax (deform) in response to changes in rotor wake loading and fuselage presence. On completion of the iteration, rotor loading and inflow, fuselage singularity strength (and, hence, pressure and velocity distributions) and rotor wake are all consistent. The results of a fully coupled calculation of the flow around representative helicopter configurations are presented. The effect of fuselage components on the rotor flow field and the overall wake structure is detailed and the aerodynamic interference between the different parts of the aircraft is discussed.

Clark, D. R.; Maskew, B.

1985-01-01

305

Study for prediction of rotor/wake/fuselage interference. Part 2: Program users guide  

NASA Technical Reports Server (NTRS)

A method was developed which permits the fully coupled calculation of fuselage and rotor airloads for typical helicopter configurations in forward flight. To do this, an iterative solution is carried out based on a conventional panel representation of the fuselage and a blade element representation of the rotor where fuselage and rotor singularity strengths are determined simultaneously at each step and the rotor wake is allowed to relax (deform) in response to changes in rotor wake loading and fuselage presence. On completion of the iteration, rotor loading and inflow, fuselage singularity strength (and, hence, pressure and velocity distributions) and rotor wake are all consistent. The results of a fully coupled calculation of the flow around representative helicopter configurations are presented. The effect of fuselage components on the rotor flow field and the overall wake structure is discussed as well as the aerodynamic interference between the different parts of the aircraft. Details of the computer program are given.

Clark, D. R.; Maskew, B.

1985-01-01

306

Suppression of the internal electric field effects in ZnO/Zn(0.7)Mg(0.3)O quantum wells by ion-implantation induced intermixing.  

PubMed

Strong suppression of the effects caused by the internal electric field in ZnO/ZnMgO quantum wells following ion-implantation and rapid thermal annealing, is revealed by photoluminescence, time-resolved photoluminescence, and band structure calculations. The implantation and annealing induces Zn/Mg intermixing, resulting in graded quantum well interfaces. This reduces the quantum-confined Stark shift and increases electron-hole wavefunction overlap, which significantly reduces the exciton lifetime and increases the oscillator strength. PMID:21817603

Davis, J A; Dao, L V; Wen, X; Ticknor, C; Hannaford, P; Coleman, V A; Tan, H H; Jagadish, C; Koike, K; Sasa, S; Inoue, M; Yano, M

2008-02-01

307

Computational Simulation of a Heavy Vehicle Trailer Wake  

SciTech Connect

To better understand the flow mechanisms that contribute to the aerodynamic drag of heavy vehicles, unsteady large-eddy simulations are performed to model the wake of a truncated trailer geometry above a no-slip surface. The truncation of the heavy vehicle trailer is done to reduce the computational time needed to perform the simulations. Both unsteady and time-averaged results are presented from these simulations for two grids. A comparison of velocity fields with those obtained from a wind tunnel study demonstrate that there is a distinct di.erence in the separated wake of the experimental and computational results, perhaps indicating the influence of the geometry simplification, turbulence model, boundary conditions, or other aspects of the chosen numerical approach.

Ortega, J M; Dunn, T; McCallen, R; Salari, K

2002-12-04

308

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

309

Experimental Measurements of a Model Submarine Wake  

NASA Astrophysics Data System (ADS)

High resolution stereo-PIV measurements were made over ten body lengths downstream of a 1/18^th scale submarine model in the Deep Water Tow Basin at NSWCCD. The submarine model is an unclassified generic submarine shape (ONR Body-1) composed of an axisymmetric body, four stern appendages (control surfaces) and a propeller. This body is 5.8 m long, 0.49 m in diameter. Block gages on the struts measured streamwise force on the body and provided loading details for setting propeller speed. The model was towed through a stationary laser sheet oriented perpendicular to the tow direction to obtain three-dimensional velocity fields. The objective of the study was to quantify the submarine wake and rate of decay of the coherent vortices. These data will be used in conjunction with measurements obtained on a model towed array to validate computational models for array shape and dynamics. Results with and without the propeller will be presented. Approximately 40 instantaneous vector fields were obtained for each location. Mean and fluctuating streamwise and cross-stream velocities and vorticity were computed.

Bretall, Damien; Furey, Deborah; Cipolla, Kimberly

2005-11-01

310

Wake evolution and trailing vortex instabilities  

NASA Astrophysics Data System (ADS)

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

Odemark, Ylva; Fransson, Jens H. M.

2011-11-01

311

Aircraft control in wake vortex wind shear  

NASA Technical Reports Server (NTRS)

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

Wold, Gregory R.

1995-01-01

312

Exploring a flight deck based wake turbulence situational awareness tool  

Microsoft Academic Search

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

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

2012-01-01

313

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

National Technical Information Service (NTIS)

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 num...

R. Sondergaard B. Cantwell N. Mansour

1997-01-01

314

Secure Wake-Up Scheme for WBANs  

NASA Astrophysics Data System (ADS)

Network life time and hence device life time is one of the fundamental metrics in wireless body area networks (WBAN). To prolong it, especially those of implanted sensors, each node must conserve its energy as much as possible. While a variety of wake-up/sleep mechanisms have been proposed, the wake-up radio potentially serves as a vehicle to introduce vulnerabilities and attacks to WBAN, eventually resulting in its malfunctions. In this paper, we propose a novel secure wake-up scheme, in which a wake-up authentication code (WAC) is employed to ensure that a BAN Node (BN) is woken up by the correct BAN Network Controller (BNC) rather than unintended users or malicious attackers. The scheme is thus particularly implemented by a two-radio architecture. We show that our scheme provides higher security while consuming less energy than the existing schemes.

Liu, Jing-Wei; Ameen, Moshaddique Al; Kwak, Kyung-Sup

315

Body Shape Effects on Axisymmetric Wakes.  

National Technical Information Service (NTIS)

Experimental measurements were made of the distance of transition to turbulence of the wake behind hypersonic spherical and blunted-cone models at hypersonic speeds in a ballistic range. Transition distances behind blunt bodies were found to be independen...

L. N. Wilson

1964-01-01

316

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

317

Three-Phased Wake Vortex Decay  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

318

On the wake of a Darrieus turbine  

NASA Astrophysics Data System (ADS)

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

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

1981-05-01

319

On the wake of a Darrieus turbine  

NASA Technical Reports Server (NTRS)

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

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

1981-01-01

320

Use of Individual Flight Corridors to Avoid Vortex Wakes  

NASA Technical Reports Server (NTRS)

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

Rossow, Vernon J.

2001-01-01

321

Moonlet Wakes in Saturn's Cassini Division  

NASA Astrophysics Data System (ADS)

We have detected several features with wavelike characteristics in the Voyager Radio Science (RSS) earth occultation data and Voyager photopolarimeter (PPS) stellar occultation data of Saturn's Cassini Division. We identified these structures using a non-linear autoregressive power spectral algorithm called Burg. This method is powerful for detecting short sections of quasiperiodic structure. We successfully used this same technique to identify six previously unseen Pan wakes in the Voyager PPS and Voyager RSS occultation data (Horn, Showalter and Russell, 1996, {/it Icarus} {/bf 124}, 663). Applying the Burg technique to the RSS data we find a number of wavelike structures in the Cassini Division. We see three distinct features that look like moonlet wakes. Two are Cassini Division features detected by Marouf and Tyler (1986, {/it Nature} {/bf 323}, 120) in the Voyager RSS data. Flynn and Cuzzi (1989, {/it Icarus} {/bf 82}, 180) determined that these features were azimuthally symmetric in the Voyager images and were most likely not moonlet wakes. The third wavelike structure resembles an outer moonlet wake. If it is a wake it may correspond to a previously undetected moonlet located in a Cassini Division gap between 118,929 km and 118,966 km. We see at least one wavelike feature in the PPS data. This feature falls close to the outer edge of the Huygens gap in the Cassini Division. It is consistent with an outer moonlet wake. If it is a wake it may correspond to a previously undetected moonlet inside the Huygens gap. Several other wavelike features in the Cassini Division resemble moonlet wakes. We plan to pursue these structures further in the future. This work was performed at JPL/Caltech under contract with NASA.

Spilker, L. J.; Showalter, M. R.

1997-07-01

322

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

NASA Astrophysics Data System (ADS)

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

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

323

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

NASA Astrophysics Data System (ADS)

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

Medici, D.; Alfredsson, P. H.

2006-05-01

324

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

NASA Astrophysics Data System (ADS)

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

Shen, Xiang; Wang, Tongguang; Zhong, Wei

325

Suppression of Divergence of Low Energy Ion Beams by Space Charge Neutralization with Low Energy Electrons Emitted from Field Emitter Arrays  

SciTech Connect

Suppression of divergence of low energy neon ion beam was experimentally demonstrated by neutralizing the space charge of ion beam with low energy electrons emitted from silicon field emitter arrays (Si-FEAs). Treatment of the FEAs with trifluoromethane plasma realized surface carbonization which was efficient to elongate the lifetime of the FEA and to improve the electron energy distribution. Together with the improvement of the performance of Si-FEA, we have developed a novel electron deceleration system to produce low energy electrons. A low energy neon ion beam was produced and the beam property was investigated with and without the electron supply from surface carbonized Si-FEA (Si:C-FEA). As a result, the divergence of the neon ion beam was largely suppressed with presence of the electrons.

Ishikawa, Junzo [Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, 487-8501 (Japan); Gotoh, Yasuhito; Taguchi, Shuhei; Nicolaescu, Dan; Tsuji, Hiroshi; Kimoto, Tsunenobu [Department of Electronic Science and Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan); Takeuchi, Mitsuaki [JST Innovation Plaza Kyoto, Goryo-ohara, Nishiky-ku, Kyoto 615-, Japan and Photonics and Electronics Research Center, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan); Sakai, Shigeki [Nissin Ion Equpment Co., Ltd., 575 Kuze-Tonoshiro-cho, Minami-ku, Kyoto 601-8205 (Japan)

2011-01-07

326

Ion current collection in spacecraft wakes  

SciTech Connect

This work investigates the current--voltage characteristics of a highly negatively biased, isolated probe in the plasma wake of a much larger, unbiased object. The system is investigated for both flowing and stationary plasmas. For the stationary plasma case, spherical probe theory and a simple correction factor to account for the shadow of the plate adequately explain the results. The introduction of a flowing plasma leads to a much more complicated situation. In this case, since the potential field is asymmetric and the ions no longer conserve angular momentum, spherical probe theory is inadequate. The current--voltage characteristics take a much more complicated form than in the stationary case. Indeed, the simulations show that the current--voltage characteristics of varying axial probe positions cross each other. The high complexity of the ion orbits makes theoretical predictions of where the crossover occurs difficult. However, the potential structure around the probe and the ion trajectories are analyzed to explain the current--voltage characteristics. These results show regions can exist in energy--angular momentum space which cause incoming ions to orbit the probe and then escape the simulation. The fraction of particles which do orbit and then escape as a function of probe potential and position determines the shape of the current--voltage characteristic. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.

Biasca, R. [Phillips Laboratory, Hanscom Air Force Base, Massachusetts 01731 (United States)] [Phillips Laboratory, Hanscom Air Force Base, Massachusetts 01731 (United States); Wang, J. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (United States)] [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (United States)

1995-01-01

327

Wind tunnel simulations of wind turbine wake interactions in neutral and stratified wind flow.  

NASA Astrophysics Data System (ADS)

A second programme of work is about to commence as part of a further four years of funding for the UK-EPSRC SUPERGEN-Wind large-wind-farm consortium. The first part of the initial programme at Surrey was to establish and set up appropriate techniques for both on- and off-shore boundary layers (though with an emphasis on the latter) at a suitable scale, and to build suitable rotating model wind turbines. The EnFlo wind tunnel, a UK-NCAS special facility, is capable of creating scaled neutral, stable and unstable boundary layers in its 20m long working section. The model turbines are 1/300-scale of 5MW-size, speed controlled with phase-lock measurement capability, and the blade design takes into account low Reynolds-number effects. Velocity measurements are primarily made using two-component LDA, combined with a ‘cold-wire' probe in order to measure the local turbulent heat flux. Simulation of off-shore wakes is particularly constrained because i) at wind tunnel scale the inherently low surface roughness can be below that for fully rough conditions, ii) the power required to stratify the flow varies as the square of the flow speed, and could easily be impractically large, iii) low blade Reynolds number. The boundary layer simulations, set up to give near-equilibrium conditions in terms of streamwise development, and the model turbines have been designed against these constraints, but not all constraints can be always met simultaneously in practice. Most measurements so far have been made behind just one or two turbines in neutral off- and on-shore boundary layers, at stations up to 12 disk diameters downstream. These show how, for example, the wake of a turbine affects the development of the wake of a downwind turbine that is laterally off-set by say half or one diameter, and how the unaffected part from the first turbine merges with the affected wake of the second. As expected a lower level of atmospheric turbulence causes the wakes to develop and fill-in more slowly compared with the on-shore case. A turbine can also suppress the level of atmospheric turbulence below hub height. In neutral flow, the wakes grow in width and height. However, even in mild stable stratification the vertical development of the wake deficit can be completely inhibited; at least some reduction would be expected arising from the stabilizing influence on vertical fluctuations. The width in contrast develops at about the same rate. As anticipated, the wake development is slower still in the stable case because of the lower level ambient turbulence. The maximum deficit is at a lower height than it is for neutral flow. Various aspects of the turbulence in the wake have been investigated. Second-phase work will examine a larger number of wake-turbine and wake-wake interactions, make a more detailed study of how turbines alter the atmospheric turbulence, and examine more cases of stratification. Work is also in hand related to turbines in or near forested regions, and it is expected that aspects of the physics will have links with the effect a large wind farm will have on the ABL and on the wind resource for a downwind farm. The work will produce a series of test cases to assist in the development of better wake and wind resource prediction models as well as a better understanding of wake physics.

Hancock, P. E.; Pascheke, F.

2010-09-01

328

Experiment S026: Ion wake measurement  

NASA Technical Reports Server (NTRS)

Measurements of the ion-wake and electron-wake structure and other perturbations of the ambient medium produced by the Gemini spacecraft in orbit are reported. Thruster firings in the TDA-south configuration produced a decrease in the observed ion flux to the outboard ion sensor, an apparent increase in the ion flux to the inboard ion sensor, and an enhanced electron concentration to the outboard electron sensor. Visual inspection of strip-chart data was indicative that definitive wake-cone angles could be determined. It also was apparent for many cases that the electron distribution followed the ion depletion effects; this was indicative that the wake was a plasma, not an ion wake. Analysis of the data from the GATV orbits was indicative that electron and ion temperatures were higher than was estimated. The high electron temperature accounted for the sensor saturation observed during the mission. This temperature measurement was 0.3 electron volt at a 200-n. mi. altitude during daytime conditions. Spacecraft wake structure has been observed as far as 100 feet from the vehicle.

Medved, D. B.; Troy, B. E., Jr.

1971-01-01

329

Wakes in viscous quark-gluon plasma  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

330

Inferior colliculus unitary activity in wakefulness, sleep and under barbiturates.  

PubMed

The spontaneous unitary activity and the response to contralateral tone-burst were analyzed in the inferior colliculus (IC) of guinea pigs during the sleep-waking cycle and under the effects of pentobarbital anesthesia. Minor changes were observed in both spontaneous and evoked activity between wakefulness (W) and slow wave sleep (SWS). On the other hand, a consistent increase in the mean spontaneous firing rate and a significant decrement in the signal-to-noise ratio (S/N ratio) was observed during paradoxical sleep (PS). Pentobarbital anesthesia reduced the spontaneous and evoked firing rate, the duration of the excitatory response and increased the duration of the post-excitatory suppression. We conclude, that the processing of auditory information in the IC change markedly during PS. Because the IC is a compulsory station for almost all the ascending auditory pathways, the observed decrease in the S/N ratio may deeply affect the auditory perception during this behavioral state. Finally, the alteration of the neuronal activity induced by pentobarbital differs not only with the activity observed during W, but also with the activity observed during both SWS and PS. PMID:12062467

Torterolo, Pablo; Falconi, Atilio; Morales-Cobas, Gabriela; Velluti, Ricardo A

2002-05-10

331

The fate and efficacy of benomyl applied to field soils to suppress activity of arbuscular mycorrhizal fungi.  

PubMed

A systematic application of the fungicide benomyl was used to follow up the suppression of arbuscular mycorrhizal (AM) colonization and to determine its fungitoxic activity and persistence at different depths. Repeated applications of benomyl reduced AM colonization mainly in the upper 0-4 cm layer of the treated soils. Furthermore, AM colonization decreased with soil depth. The activity and persistence of this fungicide was reduced over small changes in depth in the first 10 cm of the soil profile beneath a semiarid herbland at Brookfield Conservation Park (South Australia). Repeated applications of the fungicide only slightly increased the levels of toxicity in the soils, probably because of biodegradation of the fungicide in soils with a recent history of exposure to the fungicide. The decline in fungicide activity at depth was correlated with a decline in the suppressive effect of the fungicide on the activity of AM fungi. PMID:19767864

O'Connor, Patrick; Manjarrez, Maria; Smith, Sally E

2009-07-01

332

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

333

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

334

Deep Water Oceanic Wakes: a simple case study  

NASA Astrophysics Data System (ADS)

In present study we investigate the formation and evolution of oceanic wakes generated by obstacle of a real island shape (in our case - Madeira island) but without consideration of bathymetry around it. Numerical simulations using Regional Ocean Modeling System (ROMS) are presented. ROMS is a free-surface, terrain-following, primitive equations ocean model (Shchepetkin and McWilliams, 2005). Numerical models are often used to study the formation and evolution of the leeward, mesoscale and sub-mesoscale flows around of islands (Dietrich et al., 1996; Dong et al., 2007; Heywood et al., 1996). Madeira archipelago is a group of deep-sea islands located at Northeast Atlantic at about 33°N, 17°W. The biggest island of the archipelago is Madeira Island with about 50 km in east-west and 20 km in north-south direction. Its obstruction to the incoming oceanic and atmospheric flows induces leeward wake instabilities. This phenomena is observed using remote sensing and field data (Caldeira et al., 2002). We use the similar methodology to study Madeira island wakes problem as it was presented at Dong et al., 2007. The main difference between their study and ours is that they carried out experiences with an idealized cylindrical obstacle and we are using an island with its real shape at the surface and with vertical sides. The island was centered in a geostrophic channel like configuration with a prescribed surface intensified meridional (southward) inflow at the upstream boundary (i.e., our study is dedicated to the wakes, generated at the eastern and western part of Madeira Island). Eastern and Western channel boundaries were set to slippery-tangential and zero normal conditions, whereas boundaries around the island were set to zero-normal and no-slip flow. A clamped condition with a sponge layer was applied at the southern outflow boundary for outgoing current and density profile. The initial conditions for the entire domain were set equal to the upstream boundary condition expect at the island points. Our numerical simulations were devoted to study of various dynamical flow regimes. Obtained results showed that oceanic wakes formations were sensitive to three dimensionless parameters that representing a ratio between inertial and frictional forces - Reynolds number (Re), rotational effects - Rossby number (Ro) and stratification effects - Burger number (Bu). Wake asymmetries induce different behaviour for cyclonic and anti-cyclonic eddies than that showed by Dong et al., 2007. References: Caldeira, R.M.A., S. Groom, P. Miller, D. Pilgrim and N. Nezlin, 2002: Sea-surface signatures of the island mass effect phenomena around Madeira Island, Northeast Atlantic, Remote Sensing of the Environment, 80, 336-360. Dietrich, D.E., M.J. Bowman, C.A. Lin and A. Mestas-Nunez, 1996: Numerical studies of small island wakes, Geophysics, Astrophysics and Fluid Dynamics, 83, 195-231. Dong, C., J.C. McWilliams and A. Shchepetkin, 2007: Island Wakes in Deep Water, Journal of Physical Oceanography, 37, 962-981. Heywood, K.J., D.P Stevens, G.R. Bigg, 1996: Eddy formation behind the tropical island of Aldabra, Deep-Sea Research I, 43, 555-578. Shchepetkin, A.F., and J.C. McWilliams, 2005: The Regional Ocean Modeling System: A split-explicit, free-surface, topography-following-coordinate oceanic model, Ocean Modelling, 9, 347-404.

Luis, E. A.; Boutov, D.

2009-04-01

335

Response of a circular cylinder wake to a symmetric actuation by non-thermal plasma discharges  

NASA Astrophysics Data System (ADS)

In this study, the flow past a circular cylinder is manipulated by two plasma discharges placed on both sides of the model (at ±50°). A parametric investigation by force balance is conducted to define the sensitivity of the flow field to unsteady perturbations imparted by plasma actuators (dielectric barrier discharge) at 15.6 m/s ( Re D = 40,000). Effects of simple sinusoidal waveform, burst modulation and amplitude modulation are compared for low-frequency excitations. Regardless of the excitation mode, the cylinder experiences a large increase in the drag coefficient. The larger drag increase is observed for excitation related to the lock-on regime. Fast PIV measurements and triple decomposition by proper orthogonal decomposition are performed to extract the dynamical changes in the cylinder wake and to discriminate the control effects on the coherent and fluctuating turbulence. As expected, the control principally acts on the coherent flow structures. When forced, the vortices form closer to the base of the cylinder regardless of the actuation mode. This results in the drag increase observed by force measurements. The effectiveness of burst modulation is also due to the suppression of irregular shedding that is observed in the natural flow sequence and to a high level of correlation between the upper and lower vortex shedding. Finally, flow visualizations indicate that similar results can be obtained at higher Reynolds number ( Re D = 128,000, 50 m/s).

Benard, N.; Moreau, E.

2013-02-01

336

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.

337

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

338

User's guide for a flat wake rotor inflow/wake velocity prediction code, DOWN  

NASA Technical Reports Server (NTRS)

A computer code named DOWN was created to implement a flat wake theory for the calculation of rotor inflow and wake velocities. A brief description of the code methodology and instructions for its use are given. The code will be available from NASA's Computer Software Management and Information Center (COSMIC).

Wilson, John C.

1991-01-01

339

Momentum and heat transport in a finite-length cylinder wake  

NASA Astrophysics Data System (ADS)

This paper reports an experimental study of turbulent momentum and heat transport in the wake of a wall-mounted finite-length square cylinder, with its length-to-width ratio L/ d = 3-7. The cylinder was slightly heated so that heat produced could be considered as a passive scalar. A moveable three-wire probe (a combination of an X-wire and a cold wire) was used to measure velocity and temperature fluctuations at a Reynolds number of 7,300 based on d and the free-stream velocity. Measurements were performed at 10 and 20 d downstream of the cylinder at various spanwise locations. Results indicate that L/ d has a pronounced effect on Reynolds stresses, temperature variance and heat fluxes. The downwash flow from the free end of the cylinder acts to suppress spanwise vortices and, along with the upwash flow from the cylinder base, makes the finite-length cylinder wake highly three-dimensional. Reynolds stresses, especially the lateral normal stress, are significantly reduced as a result of suppressed spanwise vortices at a small L/ d. The downwash flow acts to separate the two rows of spanwise vortices further apart from the wake centerline, resulting in a twin-peak distribution in temperature variance. While the downwash flow entrains high-speed fluid into the wake, responsible for a small deficit in the time-averaged streamwise velocity near the free end, it does not alter appreciably the distribution of time-averaged temperature. It has been found that the downwash flow gives rise to a counter-gradient transport of momentum about the central region of the wake near the free end of the cylinder, though such a counter-gradient transport does not occur for heat transport.

Wang, H.; Zhou, Y.; Chan, C.; Zhou, T.

2009-06-01

340

Wake deficit measurements on the Jess and Souza Ranches, Altamont Pass  

SciTech Connect

This report is ninth in a series of documents presenting the findings of field test under DOE's Cooperative Field Test Program (CFTP) with the wind industry. This report provides results of a project conducted by Altamont Energy Corp. (AEC) to measure wake deficits on the Jess and Sousa Ranches in Altamont Pass, CA. This research enhances and complements other DOE-funded projects to refine estimates of wind turbine array effects. This project will help explain turbine performance variability caused by wake effects. 4 refs., 28 figs., 106 tabs.

Nierenburg, R. (Altamont Energy Corp., San Rafael, CA (USA))

1990-04-01

341

Wakefulness and loss of awareness  

PubMed Central

Objective: The ascending reticular activating system (ARAS) modulates circadian wakefulness, which is preserved in a persistent vegetative state (PVS). Its metabolism is preserved. Impairment of metabolism in the polymodal associative cortices (i.e., precuneus) is characteristic of PVS where awareness is abolished. Because the interaction of these 2 structures allows conscious sensory perception, our hypothesis was that an impaired functional connectivity between them participates in the loss of conscious perception. Methods: 15O-radiolabeled water PET measurement of regional cerebral blood flow (rCBF) was performed at rest and during a proprioceptive stimulation. Ten patients in PVS and 10 controls were compared in a cross-sectional study. The functional connectivity from the primary sensorimotor cortex (S1M1) and the ARAS in both groups was also investigated. Results: Compared with controls, patients showed significantly less rCBF in posterior medial cortices (precuneus) and higher rCBF in ARAS at rest. During stimulation, bilateral Brodmann area 40 was less activated and not functionally correlated to S1M1 in PVS as it was in controls. Precuneus showed a lesser degree of deactivation in patients. Finally, ARAS whose activity was functionally correlated to that of the precuneus in controls was not in PVS. Conclusions: Global neuronal workspace theory predicts that damage to long-distance white matter tracts should impair access to conscious perception. During persistent vegetative state, we identified a hypermetabolism in the ascending reticular activating system (ARAS) and impaired functional connectivity between the ARAS and the precuneus. This result emphasizes the functional link between cortices and brainstem in the genesis of perceptual awareness and strengthens the hypothesis that consciousness is based on a widespread neural network.

Silva, S.; Alacoque, X.; Fourcade, O.; Samii, K.; Marque, P.; Woods, R.; Mazziotta, J.; Chollet, F.; Loubinoux, I.

2010-01-01

342

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

NASA Astrophysics Data System (ADS)

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

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

2010-01-01

343

Wake studies at the Goodnoe Hills MOD-2 site  

SciTech Connect

Wake measurements were performed using kite anemometers at the Goodnoe Hills MOD-2 site. The objectives of the work were to take measurements to define the wake and to verify the numerical wake model that treats wake similarly to jet-like flow. It is found that essentially two principal parameters govern wind turbine wakes - the thrust force that the turbine exerts on the flow, which determines the initial velocity or momentum deficit, and the ambient (free stream) and mechanical (wake) turbulence which control wake expansion and therefore wake velocity deficit decay. The basic wake theory equations for application to a wind turbine are developed and discussed, and measurement data and associated error are calculated and compared to the model. (LEW)

Baker, R.W.; Walker, S.N.

1982-10-01

344

Suppression of vortex-induced vibration using moving surface boundary-layer control  

NASA Astrophysics Data System (ADS)

Experimental results of flow around a circular cylinder with moving surface boundary-layer control (MSBC) are presented. Two small rotating cylinders strategically located inject momentum in the boundary layer of the cylinder, which delays the separation of the boundary layer. As a consequence, the wake becomes narrower and the fluctuating transverse velocity is reduced, resulting in a recirculation free region that prevents the vortex formation. The control parameter is the ratio between the tangential velocity of the moving surface and the flow velocity (Uc/U). The main advantage of the MSBC is the possibility of combining the suppression of vortex-induced vibration (VIV) and drag reduction. The experimental tests are preformed at a circulating water channel facility and the circular cylinders are mounted on a low-damping air bearing base with one degree-of-freedom in the transverse direction of the channel flow. The mass ratio is 1.8. The Reynolds number ranges from 1600 to 7500, the reduced velocity varies up to 17, and the control parameter interval is Uc/U=5-10. A significant decreasing in the maximum amplitude of oscillation for the cylinder with MSBC is observed. Drag measurements are obtained for statically mounted cylinders with and without MSBC. The use of the flow control results in a mean drag reduction at Uc/U=5 of almost 60% compared to the plain cylinder. PIV velocity fields of the wake of static cylinders are measured at Re=3000. The results show that the wake is highly organized and narrower compared to the one observed in cylinders without control. The calculation of the total variance of the fluctuating transverse velocity in the wake region allows the introduction of an active closed-loop control. The experimental results are in good agreement with the numerical simulation studies conducted by other researchers for cylinders with MSBC.

Korkischko, I.; Meneghini, J. R.

2012-10-01

345

Effects of chronic desipramine pretreatment on open field-induced suppression of blood natural killer cell activity and cytokine response depend on the rat's behavioral characteristics.  

PubMed

Effects of 14 consecutive day exposure to desipramine (10mg/kg i.p.), by itself or following chronic open field (OF) exposure, on subsequent neuroimmunological effects of acute (30 min) OF stress and the involvement of individual differences in response to novelty or social position in the anti-depressive responsiveness were investigated. Chronic desipramine pretreatment did not protect against OF stress-induced suppression of blood anti-tumor natural killer cell activity but increased plasma interleukin-10 and decreased interferon-? and corticosterone concentration. These effects were particularly dangerous for the animals with increased responsivity to stress (desipramine alone) or with low behavioral activity (desipramine after chronic stress). PMID:24461377

Wrona, Danuta; Listowska, Magdalena; Kubera, Marta; Glac, Wojciech; Grembecka, Beata; Pluci?ska, Karolina; Majkutewicz, Irena; Podlacha, Magdalena

2014-03-15

346

Wake-shock interaction at a Mach number of 6  

NASA Technical Reports Server (NTRS)

Measurements of mean pitot pressure, static pressure, and total temperature were made in the two dimensional turbulent mixing region of a wake downstream of an interaction with a shock-expansion wave system. The results indicated that: (1) the shock increased the mixing, and (2) the expansion field that followed the shock decreased the turbulent mixing. The overall effect of the shock-expansion wave interaction was dependent on the orientation of the expansion wave with respect to the intersecting shock wave. These data could be used to validate nonequilibrium turbulence modeling and numerical solution of the time averaged Navier-Stokes equations.

Walsh, M. J.

1978-01-01

347

Sleep-to-wake transition movement disorders.  

PubMed

Consciousness and vigilance level are important factors for the manifestation and variability of many disorders, including movement disorders. Usually lumped together into unspecified "Wakefulness," the transition between wakefulness and sleep--the pre-dormitum, and between sleep and wakefulness--the post-dormitum, possess intrinsic cerebral metabolic patterns and mental, behavioural, and neurophysiological characteristics which make these peculiar states of vigilance independent. Moreover, the pre- and post-dormitum, with the relative state-dependent changes in firing patterns of many neuronal supra-pinal populations, act to release pacemakers responsible for different sleep-related motor phenomena. The relevance of pre-dormitum and post-dormitum as states different from full wakefulness and full sleep is, indeed, indicated by disorders which appear exclusively during either state, including motor disorders such as propriospinal myoclonus and awakening epilepsy. We will focus on three paradigmatic physiological/pathological motor phenomena (rhythmic movement disorder, hypnic jerks, and propriospinal myoclonus) strictly linked to the sleep-wake transition periods. Thereafter we will briefly discuss how the process of pre-dormitum and post-dormitum can lead to such disruption of motor control. PMID:22136891

Vetrugno, Roberto; Montagna, Pasquale

2011-12-01

348

Characteristics of Low-Frequency Waves at the Lunar Wake Boundary  

NASA Astrophysics Data System (ADS)

The Moon has generally been considered to be a simple absorbing body that does not have a complex interaction with the solar wind. Recent studies using Kaguya and Chandrayaan, however, how demonstrated that this is not the case. The ARTEMIS spacecraft (formerly THEMIS-B and -C) entered lunar orbit in July 2011 and now provide an opportunity to make robust, long-term observations of this plasma interaction. During a November 2012 wake crossing, when the IMF was steady and nearly radial, Halekas et al. [2013] documented a previously unseen feature of the Moon environment. As ARTEMIS P2 approached the wake, it observed low-amplitude fast magnetonic waves that were convected from upstream; inside the rarefaction region, the compressional strength of these waves intensified; and through the wake boundary, the waves changed from correlated to anti-correlated density and field fluctuations. Halekas et al. explained this structure as the superposition of the magnetosonic waves and lateral wake motion driven by the same. In this study, we use wake observations through the ARTEMIS mission to characterize the presence and behavior of these waves as a function of the solar wind and IMF conditions and of spacecraft location relative to the Moon. With this survey, we test the Halekas et al. predictions that these phenomena will be most common during radial IMF conditions, but will still be observable in oblique fields. Finally, we discuss what implications these results have for the more common situation where a bow shock is present.

Leisner, J. S.; Glassmeier, K.; Constantinescu, D. O.; Halekas, J. S.; Fornacon, K.

2013-12-01

349

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

350

Wake flow measurements with a mobile laser Doppler velocimeter system  

NASA Technical Reports Server (NTRS)

Wake flow measurements were conducted with a mobile laser Doppler velocimeter (LDV). The measurements included surveys of aircraft wake vortices behind a B-747 aircraft, aircraft carrier wake measurements from aboard the U.S.S. Nimitz, and tower wake measurements for a 100 kW wind turbine. Results of these tests demonstrated that a mobile ground-based LDV is a versatile and useful tool for the measurement of full-scale three-dimensional wake flows. The potential is demonstrated for utilization of this system to study complex wakes for a variety of applications.

Wilson, D. J.; Zalay, A. D.; Brashears, M. R.; Craven, C. E.; Shrider, K. R.; Jordan, A. J.

1978-01-01

351

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

352

Delayed Orexin Signaling Consolidates Wakefulness and Sleep: Physiology and Modeling  

PubMed Central

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

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

2011-01-01

353

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

354

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

355

Ship wake detection by Raman lidar.  

PubMed

We carried out a remote study of ship wakes by optical methods. Both Mie and Raman scattering signals and their evolution were simultaneously recorded by gated detector (intensified CCD). The Mie scattering signal was detectable within 1?min after water disturbance by a high-speed boat. According to an approximation of experimental data, Raman signal fluctuations can be detected for a much longer time under the same conditions. We have demonstrated that Raman spectroscopy is substantially more sensitive to water perturbation compared to conventional acoustic (sonar) technique and can be used for ship wake detection and monitoring. PMID:21283224

Bunkin, Alexey F; Klinkov, Vladimir K; Lukyanchenko, Vladislav A; Pershin, Sergey M

2011-02-01

356

Eye movements accompanying daydreaming, visual imagery, and thought suppression  

Microsoft Academic Search

Relationship between oculometer activity during the waking state and internally produced cognitive processes such as daydreaming and visual imagery were studied by means of continuous electro-oculograms. 24 female college students served as Ss. Eye movements and blinks were more frequent following instructions to engage in active rather than passive thinking; more frequent following instructions to suppress than to generate a

John S. Antrobus; Judith S. Antrobus; Jerome L. Singer

1964-01-01

357

Wind turbine wake properties from Doppler lidar measurements  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

358

Hypothalamic Arousal Regions Are Activated during Modafinil- Induced Wakefulness  

Microsoft Academic Search

Modafinil is an increasingly popular wake-promoting drug used for the treatment of narcolepsy, but its precise mechanism of action is unknown. To determine potential pathways via which modafinil acts, we administered a range of doses of modafinil to rats, recorded sleep\\/wake activity, and studied the pattern of neuronal activation using Fos immunohistochemistry. To contrast modafinil-induced wakefulness with spontaneous wakefulness, we

Thomas E. Scammell; Ivy V. Estabrooke; Marie T. McCarthy; Richard M. Chemelli; Masashi Yanagisawa; Matthew S. Miller; Clifford B. Saper

2000-01-01

359

Suppression of experimental allergic encephalomyelitis is specific to the frequency and intensity of nocturnally applied, intermittent magnetic fields in rats.  

PubMed

Female Lewis rats (n=72) were inoculated with an emulsion of spinal cord and complete Freund's adjuvant. They were then exposed for approximately 6 min every hour between midnight and 08:00 h for 2 weeks to either 7 or 40 Hz amplitude-modulated magnetic fields whose temporal pattern was designed to simulate a (geomagnetic) storm sudden commencement. The peak strengths of the fields averaged between either 30-50 nT (low intensity) or 500 nT (high intensity). Rats exposed to the 7 Hz, low intensity magnetic fields displayed significantly less severe overt signs of experimental allergic encephalomyelitis than rats exposed to either of the two intensities of the 40 Hz fields, the high intensity 7 Hz field, or the reference (<10 nT) condition. The latter groups did not differ significantly from each other. Predicted severity based upon the numbers of foci of infiltrations of lymphocytes within the brains of the rats also demonstrated the ameliorating effects of the low intensity, 7 Hz exposures. These results suggest very specific characteristics of complex, weak magnetic fields within the sleeping environment could affect the symptoms of autoimmunity. PMID:11018304

Cook, L L; Persinger, M A

2000-10-13

360

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

361

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

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

362

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

PubMed

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

Cheng, J Y; Chahine, G L

2001-12-01

363

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

NASA Astrophysics Data System (ADS)

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

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

2003-09-01

364

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

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

365

PIV analysis of near-wake behind a sphere at a subcritical Reynolds number  

Microsoft Academic Search

The vortical structure of near-wake behind a sphere is investigated using a PIV technique in a circulating water channel at\\u000a Re = 11,000. The measured velocity fields show a detailed vortical structure in the recirculation region such as recirculation\\u000a vortices, reversed velocity zone, and out-of-plane vorticity distribution. The vorticity distribution of the sphere wake shows\\u000a waviness in cross-sectional planes. The time-averaged turbulent

Young Il Jang; Sang Joon Lee

2008-01-01

366

Wake potential in a nonuniform self-gravitating dusty magnetoplasma in the presence of ion streaming  

SciTech Connect

A detailed investigation of the electrostatic asymmetric shielding potential and consequent generation of the dynamical oscillatory wake potential has been examined analytically in an inhomogeneous self-gravitating dusty magnetoplasma in the presence of uniform ion streaming. It is found that the wake potential depends significantly on the test particle speed, ambient magnetic field, ion streaming velocity, and the plasma inhomogeneity. The periodic oscillatory potential might lead to an alternative approach to the Jeans instability for the formation of dust agglomeration leading to gravitational collapse of the self-gravitating systems.

Salimullah, M.; Ehsan, Z.; Zubia, K.; Shah, H. A.; Murtaza, G. [Salam Chair in Physics and Department of Physics, Government College University, Lahore 54000 (Pakistan); Department of Physics, Government College University, Lahore 54000 (Pakistan); Salam Chair in Physics, Government College University, Lahore 54000 (Pakistan)

2007-10-15

367

Transverse wake effects in an optical waveguide IFEL accelerator  

SciTech Connect

A recent proposal describes a project to build a short demonstration stage IFEL accelerator using as input beam the output of the Accelerator Test Facility high brightness linac (ATF) currently under construction at BNL. A second stage of the proposal involves the design and construction of a GeV IFEL accelerator. Both IFEL accelerators could be built using standard focussing as discussed in the proposal. While this is fairly natural in the short (60cm) demonstration accelerator, it requires multiple refocussing for the 1 GeV machine. It would therefore be valuable if an optical waveguide could be used to contain the laser beam over the full length of the accelerator. However, it is clear that if the use of a waveguide is to compete with the direct focussing method, the aperture of the waveguide must be comparable to the laser beam waist for a Rayleigh length of approx. = 30cm. In other words, the waveguide aperture must be at most a few mm. A natural concern for such close fitting structures is the effect of the wake fields produced in the waveguide by the beam. In particular, the wake fields produced by the front of the beam pulse may lead to Transverse oscillations of the rear of the beam pulse. Wilson's two particle model of this head-tail effect is used to estimate the magnitude of the emittance growth in an optical waveguide IFEL accelerator. 6 refs., 2 tabs.

Sandweiss, J.

1987-08-13

368

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

Microsoft Academic Search

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

D. Medici; P. H. Alfredsson

2006-01-01

369

Status of wind-turbine wakes research in the Federal Wind-Energy Program  

Microsoft Academic Search

The MOD-2 cluster at Goodnoe Hills, Washington, provides a unique test facility for conducting research on the performance of operating large multi-megawatt-size wind turbines. During the summer of 1982 a series of wake studies were conducted at Goodnoe Hills, sponsored by the Department of Energy's Wing Energy Technology Division. These studies represent the first in a series of field experiments

D. L. Hadley; D. S. Renne

1983-01-01

370

Wave–mean flow interaction in an MHD wake behind bluff body  

Microsoft Academic Search

Experimental studies of the effect of constant magnetic field on the process of mean velocity profile stabilization in a wake behind a bluff body are described. To interpret the obtained results, a theoretical model is proposed explaining the scheme of wave-mean flow interaction. We assume that the stabilization process is based on the injection of energy of respective turbulent modes

H. Branover; A. Eidelman; E. Golbraikh; A. Kapusta; B. Mikhailovich

2004-01-01

371

Look at the Influence of Building Orientation on Plume Dispersion in the Wake of a Building.  

National Technical Information Service (NTIS)

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 Flu...

A. H. Huber

1986-01-01

372

Injection of Tunnel Ionized Electrons into Laser-Produced Wakes  

NASA Astrophysics Data System (ADS)

The injection of electrons via tunneling ionization into a laser driven wakefield accelerator has been studied with experiments, theory and simulations. In this work the large difference in ionization potentials between successive ionization states of trace atoms, is used as the mechanism for injecting electrons into a laser driven wakefield. Here a mixture of helium and trace amounts of either nitrogen or methane (CH4) gas was used. Electrons from the K shell of either nitrogen or carbon were tunnel ionized near the peak of the laser pulse and injected directly into the wake. The wake was created by electrons from majority helium atoms with contributions, depending on which gas mixture was used, from the either the L shell of nitrogen or the hydrogen and the L shell of carbon. Using the helium nitrogen gas mixture, the threshold intensity at which trapping was observed to occur corresponded to the intensity required to ionize the 6th electron from nitrogen (i.e. the first electron within the K shell). This indicates ionization of the K-shell of nitrogen triggers injection, and the subsequent trapping and acceleration of electrons. The electron energy spectrum was observed to be quasi-continuous for a laser ao ? 2. This spectral shape is a result of the continuous ionization and injection which occurs as long as the magnitude of the laser field remains above that required to ionize the K-shell of the nitrogen. The relative amount of charge accelerated, the intensity threshold, and spectral shape of accelerated electrons were found to be in good agreement with 3-D particle in cell code simulations which modeled the interaction. The trapping of electrons into a wakefield was studied theoretically using a 3-D Hamiltonian description of an electron within a electromagnetic field. From this description, a constant of motion can be found which relates the scalar and vector potentials of the wake and laser to the momentum of an electron. In this manner the required potential difference that an electron must experience to become trapped can be solved for. This estimate of the required potential difference for trapping to occur was found to be in good agreement with that observed in 2-D particle in cell code simulations. Using the 3-D scaling laws for the laser wakefield accelerator in the blowout regime, it was found injecting electrons directly into the wakefield, as they are when injected via ionization, significantly increases the potential difference, or the amount of energy available to the electron, to become trapped. This increase in available potential can in turn be used to lower the absolute wake amplitude necessary for trapping to occur. This reduction in wake amplitude means that electrons can be trapped into wakes with lower amplitudes driven by lasers with lower peak powers. The acceleration of electrons at reduced laser powers using ionization injection has been confirmed experimentally. Using the helium nitrogen or helium methane gas mixtures, accelerated electrons have been routinely observed using laser powers 2--4 times lower than what has been required to accelerate electrons from plasmas created from pure helium gas at similar densities.

Pak, Arthur

373

The neural circuit of orexin (hypocretin): maintaining sleep and wakefulness  

Microsoft Academic Search

Sleep and wakefulness are regulated to occur at appropriate times that are in accordance with our internal and external environments. Avoiding danger and finding food, which are life-essential activities that are regulated by emotion, reward and energy balance, require vigilance and therefore, by definition, wakefulness. The orexin (hypocretin) system regulates sleep and wakefulness through interactions with systems that regulate emotion,

Takeshi Sakurai

2007-01-01

374

Dynamics of tethered versus free-swimming animals: A wake structure comparison in jellyfish  

NASA Astrophysics Data System (ADS)

Previous research has shown that jellyfish utilize the formation and shedding of vortices to help feed and move the animal. Laboratory experiments often require restricting the motion of an animal by tethering/fluming to allow for repeatable results. However, past research has not addressed the differences that arise when the motion of an animal is restricted/confined. This presentation will attend to this issue by comparing the wake structure of a tethered and free-swimming Aurelia aurita. Digital Particle Image Velocimetry is used to collect measurements of the velocity field surrounding an animal that is either tethered or swimming freely. Dynamical systems methods are used to compute Lagrangian coherent structures (LCS), which is used to identify the geometries of structures in the wake of the animal. Using LCS, a comparison between the wake of a tethered and free-swimming animal can be made. This research provides a quantitative measure of the differences between a tethered and freely moving jellyfish.

Katija, Kakani; Dabiri, John O.

2006-11-01

375

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

376

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

2010-10-01

377

Brainstem Neural Mechanisms of Sleep and Wakefulness  

Microsoft Academic Search

The three diffuse projection systems arising in the brainstem, that is, noradrenergic projection originating in the locus coeruleus, serotonergic projection from the dorsal raphe nucleus, and cholinergic projection from neurons gathering in the laterodorsal tegmental nucleus and scattering in the pedunculopontine tegmental nucleus, may function as controllers of sleep and wakefulness. We have investigated the functional roles of these projections

Yukihiko Kayama; Yoshimasa Koyama

1998-01-01

378

Rarefaction shock in the near wake  

NASA Technical Reports Server (NTRS)

Laboratory experiments and fluid theory find a stationary rarefaction shock in the near wake of an electrically grounded obstacle placed in a steady state, supersonic plasma flow. The shock is only found when two electron temperatures, differing by at least an order of magnitude, are present. These shocks are analogous to rarefaction shocks in plasma free expansions.

Diebold, D.; Hershkowitz, N.; Eliezer, S.

1987-01-01

379

Wake Vortex Study at Wallops Island  

NSDL National Science Digital Library

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

2007-07-30

380

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

381

Emotions in Dream and Waking Event Reports  

Microsoft Academic Search

Twenty participants hand-wrote reports of their dreams and reports of waking life events, and used an extensive lexicon of emotion words and types to rate the emotions experienced in each scene of each report. From these ratings, the incidence and intensity of 22 different emotion categories specified by a cognitive model of emotions was assessed. Emotions were found to be

Tore A. Nielsen; Daniel Deslauriers; George W. Baylor

1991-01-01

382

Island wakes in shallow coastal waters  

Microsoft Academic Search

Rattray Island, northeast Australia, is 1.5 km long, 300 m wide, and lies in well-mixed water approximately 25 m deep. Its long axis is inclined at about 60° into the direction of the dominant semidiurnal tidal current. The length of the wake in the lee of the island, as documented by aerial photographs and satellite imagery, appears to equal that

E. Wolanski; J. Imberger; M. L. Heron

1984-01-01

383

Polarimetric detection and estimation of ship wakes  

Microsoft Academic Search

Classification of Earth natural components within a full polarimetric SAR image is one of the most important applications of radar polarimetry in remote sensing. An unsupervised classification procedure, based on neural networks with competitive architectures, is applied to the full polarimetric SAR images of sea surface and ship wakes for segmentation and clustering of different ocean components. The linear feature

E. Pottier; W. M. Boerner; D. L. Schuler

1999-01-01

384

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

385

Neurovascular saturation thresholds under high intensity auditory stimulation during wake.  

PubMed

Coupling between neural activity and hemodynamic responses is important in understanding brain function, interpreting brain-imaging signals, and assessing pathological conditions. Tissue state is a major factor in neurovascular coupling and may alter the relationship between neural and hemodynamic activity. However, most neurovascular-coupling studies are performed under anesthetized or sedated states which may have severe consequences on coupling mechanisms. Our previous studies showed that following prolonged periods of sleep deprivation, evoked hemodynamic responses were muted despite consistent electrical responses, suggesting that sustained neural activity may decrease vascular compliance and limit blood perfusion. To investigate potential perfusion limitations during natural waking conditions, we simultaneously measured evoked response potentials (ERPs) and evoked hemodynamic responses using optical-imaging techniques to increase intensity auditory stimulation. The relationship between evoked hemodynamic responses and integrated ERPs followed a sigmoid relationship where the hemodynamic response approached saturation at lower stimulus intensities than the ERP. If limits in blood perfusion are caused by stretching of the vessel wall, then these results suggest there may be decreased vascular compliance due to sustained neural activity during wake, which could limit vascular responsiveness and local blood perfusion. Conditions that stress cerebral vasculature, such as sleep deprivation and some pathologies (e.g., epilepsy), may further decrease vascular compliance, limit metabolic delivery, and cause tissue trauma. While ERPs and evoked hemodynamic responses provide an indication of the correlated neural activity and metabolic demand, the relationship between these two responses is complex and the different measurement techniques are not directly correlated. Future studies are required to verify these findings and further explore neurovascular coupling during wake by assessing local field potentials, vascular expansion, hemodynamic response localization. PMID:23041761

Schei, J L; Van Nortwick, A S; Meighan, P C; Rector, D M

2012-12-27

386

Suppressive effect of magnesium oxide materials on cadmium accumulation in winter wheat grain cultivated in a cadmium-contaminated paddy field under annual rice-wheat rotational cultivation.  

PubMed

The effectiveness of two kinds of magnesium oxide (MgO) materials, commercial MgO (2250 kg ha(-1)) and a material derived from MgO and magnesium silicate minerals named 'MgO-SH-A' (2250 and 4500 kg ha(-1)1), in suppression of uptake and accumulation of cadmium (Cd) into grain of winter wheat (Triticum aestivum L. cv. Ayahikari) was examined in a Cd-contaminated alluvial paddy field under annual rice-wheat rotational system. The MgO materials were mixed into the plough-layer soil only once prior to the preceding rice cultivation. Cadmium concentration in wheat grain produced from the non-amendment control exceeded the maximum limit of Cd in wheat grain adopted by FAO/WHO (0.2 mg kg(-1)). All of the treatments with the MgO materials significantly lowered plant available Cd fraction in the plough-layer soil. However, only the treatment with the commercial MgO at 2250 kg ha(-1) produced wheat grain whose Cd concentration was not only significantly lower than that from the control but also less than 0.2 mg kg(-1). It is suggested that the significant suppressive effect of the commercial MgO on Cd accumulation in wheat grain would be mainly attributed to its high soil neutralizing capacity as compared to that of MgO-SH-A. PMID:19304384

Kikuchi, Tetsuro; Okazaki, Masanori; Motobayashi, Takashi

2009-08-30

387

Fire Suppression  

Microsoft Academic Search

\\u000a Water sprinkler sprays (with relatively large droplet sizes) in residential and commercial structures are probably the most\\u000a well-known application of sprays in fire suppression. In more recent years, water mists (characterized by reduced droplet\\u000a sizes, which may contain additives) have been considered as a replacement for Halon 1301, the most common fire suppressant\\u000a chemical aboard aircraft and ships, but banned

C. Presser; J. C. Yang

388

In situ and remotely-sensed mean and turbulence characteristics of wind turbine wakes  

NASA Astrophysics Data System (ADS)

With increasing deployment of wind turbines around the world, understanding how turbines extract energy from the wind is critical for estimating any limits to wind energy production and in understanding downstream impacts. High-resolution measurements of wind speed, temperature, and turbulence across the entire turbine rotor disk are essential to evaluate the effects of shear, stratification, and turbulence on turbine power performance. Energy dissipation rate measurements are also required for insight into wake dissipation processes. These data are lacking from current measurement practices, thereby limiting our understanding of atmosphere-turbine interactions and wake dynamics. The University of Colorado at Boulder's Tethered Lifting System (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. In Fall 2012, two TLS will be deployed at NREL's National Wind Technology Center to collect simultaneous measurements of the inflow and outflow characteristics around the DOE 1.5 MW turbine. These measurements will enable us to study the effects of shear, stratification, and turbulence on turbine power production and wake dynamics, and compare our results with other measurement systems and wake models. In addition to these in situ measurements, the experimental design requires accurate and timely information on the location of the turbine wake, which will be provided by a Leosphere 200S scanning lidar deployed nearby. This presentation will highlight some of the observations from this field campaign on wake wind speed deficits (observed with both in situ and remotely sensed instrumentation), turbulence enhancement (observed with the remote sensing instruments), and turbulent kinetic energy dissipation rate (observed with the in situ instruments).

Lundquist, J. K.; Bariteau, L.; Boquet, M.; Clifton, A.; Rhodes, M. E.

2012-12-01

389

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

390

Activation of 5-HT6 receptors modulates sleep-wake activity and hippocampal theta oscillation.  

PubMed

The modulatory role of 5-HT neurons and a number of different 5-HT receptor subtypes has been well documented in the regulation of sleep-wake cycles and hippocampal activity. A high level of 5-HT(6) receptor expression is present in the rat hippocampus. Further, hippocampal function has been shown to be modulated by both 5-HT(6) agonists and antagonists. In the current study, the potential involvement of 5-HT(6) receptors in the control of hippocampal theta rhythms and sleep-wake cycles has been investigated. Hippocampal activity was recorded by intracranial hippocampal electrodes both in anesthetized (n = 22) and in freely moving rats (n = 9). Theta rhythm was monitored in different sleep-wake states in freely moving rats and was elicited by stimulation of the brainstem reticular formation under anesthesia. Changes in theta frequency and power were analyzed before and after injection of the 5-HT(6) antagonist (SAM-531) and the 5-HT(6) agonist (EMD386088). In freely moving rats, EMD386088 suppressed sleep for several hours and significantly decreased theta peak frequency, while, in anesthetized rats, EMD386088 had no effect on theta power but significantly decreased theta frequency, which could be blocked by coadministration of SAM-531. SAM-531 alone did not change sleep-wake patterns and had no effect on theta parameters in both unanesthetized and anesthetized rats. Decreases in theta frequency induced by the 5-HT(6) receptor agonist correspond to previously described electrophysiological patterns shared by all anxiolytic drugs, and it is in line with its behavioral anxiolytic profile. The 5-HT(6) antagonist, however, failed to potentiate theta power, which is characteristic of many pro-cognitive substances, indicating that 5-HT(6) receptors might not tonically modulate hippocampal oscillations and sleep-wake patterns. PMID:23336058

Ly, Susanna; Pishdari, Bano; Lok, Ling Ling; Hajos, Mihaly; Kocsis, Bernat

2013-01-16

391

Activation of 5-HT6 Receptors Modulates Sleep-Wake Activity and Hippocampal Theta Oscillation  

PubMed Central

The modulatory role of 5-HT neurons and a number of different 5-HT receptor subtypes has been well documented in the regulation of sleep–wake cycles and hippocampal activity. A high level of 5-HT6 receptor expression is present in the rat hippocampus. Further, hippocampal function has been shown to be modulated by both 5-HT6 agonists and antagonists. In the current study, the potential involvement of 5-HT6 receptors in the control of hippocampal theta rhythms and sleep–wake cycles has been investigated. Hippocampal activity was recorded by intracranial hippocampal electrodes both in anesthetized (n = 22) and in freely moving rats (n = 9). Theta rhythm was monitored in different sleep–wake states in freely moving rats and was elicited by stimulation of the brainstem reticular formation under anesthesia. Changes in theta frequency and power were analyzed before and after injection of the 5-HT6 antagonist (SAM-531) and the 5-HT6 agonist (EMD386088). In freely moving rats, EMD386088 suppressed sleep for several hours and significantly decreased theta peak frequency, while, in anesthetized rats, EMD386088 had no effect on theta power but significantly decreased theta frequency, which could be blocked by coadministration of SAM-531. SAM-531 alone did not change sleep–wake patterns and had no effect on theta parameters in both unanesthetized and anesthetized rats. Decreases in theta frequency induced by the 5-HT6 receptor agonist correspond to previously described electrophysiological patterns shared by all anxiolytic drugs, and it is in line with its behavioral anxiolytic profile. The 5-HT6 antagonist, however, failed to potentiate theta power, which is characteristic of many pro-cognitive substances, indicating that 5-HT6 receptors might not tonically modulate hippocampal oscillations and sleep–wake patterns.

2012-01-01

392

Shielded coil structure suppressing leakage magnetic field from 100W-class wireless power transfer system with higher efficiency  

Microsoft Academic Search

In this paper, the shielded coil structure using the ferrites and the metallic shielding is proposed. It is compared with the unshielded coil structure (i.e. a pair of circular loop coils only) to demonstrate the differences in the magnetic field distributions and system performance. The simulation results using the 3D Finite Element Analysis (FEA) tool show that it can considerably

Hongseok Kim; Chiuk Song; Jonghoon Kim; Jiseong Kim; Joungho Kim

2012-01-01

393

Discharge modulation of rat dorsal raphe neurons during sleep and waking: effects of preoptic/basal forebrain warming.  

PubMed

In cats, putative serotonergic neurons (PSNs) recorded from the dorsal raphe nucleus (DRN) across the sleep-wake cycle exhibit the so-called rapid eye movement sleep-off (REM-off) discharge pattern. Since, the sleep-wake discharge patterns of DRN neurons in behaving rats is poorly known, the present study examined this neuronal populations. The PSNs recorded in this study exhibited: (1) progressive decrease in discharge rate from waking to NREM to REM sleep; (2) long action potential duration, and (3) reduction of discharge rate after systemic administration of a selective 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). Evidence supports the hypothesis that NREM sleep is modulated by thermoregulatory mechanisms localized in the preoptic area and adjacent basal forebrain (POA/BF). We previously reported that POA/BF warming suppresses the discharge of wake-promoting neurons in the posterior hypothalamus and the basal forebrain. Since the DRN is one component of the brainstem arousal system and receives projections from POA/BF, we examined the effects of local POA/BF warming by 1.5-2.0 degrees C during waking on the discharge of DRN neurons. POA/BF warming reduced the discharge in 14 of 19 PSNs and in 12 of 17 other wake-related neurons in the DRN. DRN neuronal discharge reduction occurred without accompanying EEG frequency or behavioral changes. These results suggest that PSNs recorded in DRN in unrestrained and unanesthetized rats exhibit a "wake-active REM-off" discharge pattern and further support the hypothesis that the POA/BF warm-sensitive hypnogenic system induces sleep by a coordinated inhibition of multiple arousal systems including that modulated by the DRN. PMID:10967295

Guzmán-Marín, R; Alam, M N; Szymusiak, R; Drucker-Colín, R; Gong, H; McGinty, D

2000-09-01

394

Dopamine agonist suppression of rapid-eye-movement sleep is secondary to sleep suppression mediated via limbic structures  

SciTech Connect

The effects of pergolide, a direct dopamine receptor agonist, on sleep and wakefulness, motor behavior and /sup 3/H-spiperone specific binding in limbic structures and striatum in rats was studied. The results show that pergolide induced a biphasic dose effect, with high doses increasing wakefulness and suppressing sleep while low dose decreased wakefulness, but increased sleep. It was shown that pergolide-induced sleep suppression was blocked by ..cap alpha..-glupenthixol and pimozide, two dopamine receptor antagonists. It was further shown that pergolide merely delayed the rebound resulting from rapid-eye-movement (REM) sleep deprivation, that dopamine receptors stimulation had no direct effect on the period, phase or amplitude of the circadian rhythm of REM sleep propensity and that there was no alteration in the coupling of REM sleep episodes with S/sub 2/ episodes. Rapid-eye-movement sleep deprivation resulted in increased sensitivity to the pergolide-induced wakefulness stimulation and sleep suppression and pergolide-induced motor behaviors of locomotion and head bobbing. /sup 3/H-spiperone specific binding to dopamine receptors was shown to be altered by REM sleep deprivation in the subcortical limbic structures. It is concluded that the REM sleep suppressing action of dopamine receptor stimulation is secondary to sleep suppression per se and not secondary to a unique effect on the REM sleep. Further, it is suggested that the wakefulness stimulating action of dopamine receptor agonists is mediated by activation of the dopamine receptors in the terminal areas of the mesolimbocortical dopamine projection system.

Miletich, R.S.

1985-01-01

395

Field-suppressed interfacial spin scattering in YBa2Cu3O7-?/La0.67Sr0.33MnO3 heterostructures  

NASA Astrophysics Data System (ADS)

The superconductivity and magnetoresistance (MR) of epitaxial bilayers consisting of YBa2Cu3O7-? (YBCO) and La0.67Sr0.33MnO3 (LSMO) are investigated with respect to the proximity and spin polarization effects. In the region of the mixed vortex state near the onset temperature of the superconducting transition, the diffusion of the spin-polarized quasiparticles from the LSMO to the YBCO is evidenced by their distinct magnetotransport behaviors, which are characterized by two superconducting transitions. It is revealed that the second transition is strongly dependent on the applied magnetic field, demonstrating an abnormal pair-breaking effect. This effect, together with the reduced MR, is believed to arise from an increase in the spin injection efficiency induced by the suppressed interfacial spin scattering, disclosing a new perspective for understanding the magnetotransport phenomena in proximity-coupled superconductor/ferromagnet (S/F) hybrids.

Chen, Changzhao; Li, Yang; Cai, Chuanbing

2012-07-01

396

Three-dimensional nature of vortices in the near wake of a wavy cylinder  

NASA Astrophysics Data System (ADS)

The near wake of a wavy cylinder has been experimentally investigated using various techniques, including Laser Doppler Anemometer, Laser-Induced Fluorescence Visualization and Digital Particle Image Velocimetry. The work aims to provide understanding to the mechanism of the cross-flow around a wavy cylinder as well as to comprehend why the introduction of relatively small degrees of spanwise waviness could have a significant effect on drag reduction and a corresponding suppression of cylinder vibration. Distributions of the mean and fluctuating velocity components along the streamwise, spanwise and transverse direction are presented. The development of the flow patterns and the corresponding vortex interactions are obtained by flow visualization. The experimental results indicated that the average vortex formation length behind the wavy cylinder is longer than that behind the circular cylinder. It appears that there is a direct link between the long formation length and drag reduction and vibration suppression. For the wavy cylinder, the wake on the saddle plane has a longer vortex formation region and a more rapid reverse flow, as well as being wider than that on the nodal plane. Furthermore, the spanwise flow is from the saddle plane towards the nodal plane on both sides of the wavy cylinder. It was deduced that the free shear layers shed from the points near the saddles extend along the spanwise direction, while the shear layers near the nodes contract. The turbulence correlations show that the vortex streets in the circular cylinder wake are more regular, while the wavy cylinder wake shows more incoherent turbulence due to enhanced turbulent mixing; a consequence of three-dimensional (3-D) effects. The mean velocity and turbulence data also provide an important database for the validation of turbulence modelling and 3-D numerical simulations.

Lam, K.; Wang, F. H.; So, R. M. C.

2004-07-01

397

Axisymmetric Turbulent Wakes with New Nonequilibrium Similarity Scalings  

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

398

Storm-centric view of Tropical Cyclone oceanic wakes  

NASA Astrophysics Data System (ADS)

Tropical cyclones (TCs) have a dramatic impact on the upper ocean. Storm-generated oceanic mixing, high amplitude near-inertial currents, upwelling, and heat fluxes often warm or cool the surface ocean temperatures over large regions near tropical cyclones. These SST anomalies occur to the right (Northern Hemisphere) or left (Southern Hemisphere) of the storm track, varying along and across the storm track. These wide swaths of temperature change have been previously documented by in situ field programs as well as IR and visible satellite data. The amplitude, temporal and spatial variability of these surface temperature anomalies depend primarily upon the storm size, storm intensity, translational velocity, and the underlying ocean conditions. Tropical cyclone 'cold wakes' are usually 2 - 5 °C cooler than pre-storm SSTs, and persist for days to weeks. Since storms that occur in rapid succession typically follow similar paths, the cold wake from one storm can affect development of subsequent storms. Recent studies, on both warm and cold wakes, have mostly focused on small subsets of global storms because of the amount of work it takes to co-locate different data sources to a storm's location. While a number of hurricane/typhoon websites exist that co-locate various datasets to TC locations, none provide 3-dimensional temporal and spatial structure of the ocean-atmosphere necessary to study cold/warm wake development and impact. We are developing a global 3-dimensional storm centric database for TC research. The database we propose will include in situ data, satellite data, and model analyses. Remote Sensing Systems (RSS) has a widely-used storm watch archive which provides the user an interface for visually analyzing collocated NASA Quick Scatterometer (QuikSCAT) winds with GHRSST microwave SSTs and SSM/I, TMI or AMSR-E rain rates for all global tropical cyclones 1999-2009. We will build on this concept of bringing together different data near storm locations when developing the storm-centric database. This database will be made available to researchers via the web display tools previously developed for RSS web pages. The database will provide scientists with a single data format collection of various atmospheric and oceanographic data, and will include all tropical storms since 1998, when the passive MW SSTs from the TMI instrument first became available. Initial results showing an analysis of Typhoon Man-Yi will be presented.

Gentemann, C. L.; Scott, J. P.; Smith, D.

2012-12-01

399

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.

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

2013-01-01

400

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

401

Cosmic string wakes and large-scale structure  

NASA Technical Reports Server (NTRS)

The formation of structure from infinite cosmic string wakes is modeled for a universe dominated by cold dark matter (CDM). Cross-sectional slices through the wake distribution tend to outline empty regions with diameters which are not inconsistent with the range of sizes of the voids in the CfA slice of the universe. The topology of the wake distribution is found to be spongy rather than cell-like. Correlations between CDM wakes do not extend much beyond a horizon length, so it is unlikely that CDM wakes are responsible for the correlations between clusters of galaxies. An estimate of the fraction of matter to accrete onto CDM wakes indicates that wakes could be more important in galaxy formation than previously anticipated.

Charlton, Jane C.

1988-01-01

402

The influence of wing-wake interactions on the production of aerodynamic forces in flapping flight.  

PubMed

We used two-dimensional digital particle image velocimetry (DPIV) to visualize flow patterns around the flapping wing of a dynamically scaled robot for a series of reciprocating strokes starting from rest. The base of the wing was equipped with strain gauges so that the pattern of fluid motion could be directly compared with the time history of force production. The results show that the development and shedding of vortices throughout each stroke are highly stereotyped and influence force generation in subsequent strokes. When a wing starts from rest, it generates a transient force as the leading edge vortex (LEV) grows. This early peak, previously attributed to added-mass acceleration, is not amenable to quasi-steady models but corresponds well to calculations based on the time derivative of the first moment of vorticity within a sectional slice of fluid. Forces decay to a stable level as the LEV reaches a constant size and remains attached throughout most of the stroke. The LEV grows as the wing supinates prior to stroke reversal, accompanied by an increase in total force. At stroke reversal, both the LEV and a rotational starting vortex (RSV) are shed into the wake, forming a counter-rotating pair that directs a jet of fluid towards the underside of the wing at the start of the next stroke. We isolated the aerodynamic influence of the wake by subtracting forces and flow fields generated in the first stroke, when the wake is just developing, from those produced during the fourth stroke, when the pattern of both the forces and wake dynamics has reached a limit cycle. This technique identified two effects of the wake on force production by the wing: an early augmentation followed by a small attenuation. The later decrease in force is consistent with the influence of a decreased aerodynamic angle of attack on translational forces caused by downwash within the wake and is well explained by a quasi-steady model. The early effect of the wake is not well approximated by a quasi-steady model, even when the magnitude and orientation of the instantaneous velocity field are taken into account. Thus, the wake capture force represents a truly unsteady phenomenon dependent on temporal changes in the distribution and magnitude of vorticity during stroke reversal. PMID:12771174

Birch, James M; Dickinson, Michael H

2003-07-01

403

Practical Use and Risk of Modafinil, a Novel Waking Drug  

PubMed Central

Objectives Modafinil is a waking drug prescribed to narcolepsy patients, but its usage among healthy individuals is increasing to enhance their alertness or to mitigate fatigue. This study was conducted to investigate practical use and toxic effects on neuro-immune interaction of modafinil. Methods This study reviewed the significance of psychoactive drugs, and discussed the benefits and risks of the application of modafinil, which seems to be ideal as an anti-psychotic or anti-fatigue agent. Results Modafinil is known to have less or no adverse effects than those found in traditional psychostimulants such as amphetamine, methylphenidate or cocaine. It can be applied as an anti-psychotic or anti-fatigue agent. However, the waking mechanism of modafinil is yet to be fully revealed. Recent studies reported that modafinil may be subject to abuse and addiction. In addition prolonged sleeplessness induces stress responses and impairs immune function. Conclusions Modafinil can be used by anyone, who wishes to work late, stay awake, enhance their cognitive reactions, or brighten their moods. Users may already be under a great level of stress, i.e. cancer patients or soldiers in a battle field. A psychoneuroimmunological approach is thus needed to investigate the multi-functional effects of modafinil.

2012-01-01

404

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

405

Electrically-activated source extension graphene nanoribbon field effect transistor: Novel attributes and design considerations for suppressing short channel effects  

NASA Astrophysics Data System (ADS)

In this paper a double gate graphene nanoribbon field effect transistor with electrically-activated source extension is proposed. Source region of the proposed structure includes two sections, an electrically-activated extension and a doped section. The electrically extension, which is located between doped source section and gate region, is biased independent of the gate to form a virtual extension for source. The electrically-activated extension creates a step in potential profile which increases the horizontal distance between conduction and valance bands at channel to source junction. This step reduces the probability of band to band tunneling, lowers the leakage current and improves drain induced barrier lowering. The devices have been simulated based on self consistent solution of Poisson and Schrodinger equations within non-equilibrium Green’s function formalism. In addition, the effects of the edge and third nearest neighbor are included for more accurate outcomes. Simulations show that the proposed structure is a more reliable device because of its higher ON/Off current ratio, shorter delay time, and smaller power delay product beside lower subthreshold swing than conventional graphene nanoribbon field effect transistor.

Naderi, Ali; Keshavarzi, Parviz

2014-08-01

406

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

407

The three-dimensional wake of a cylinder undergoing a combination of translational and rotational oscillation in a quiescent fluid  

NASA Astrophysics Data System (ADS)

Previous two-dimensional numerical studies have shown that a circular cylinder undergoing both oscillatory rotational and translational motions can generate thrust so that it will actually self-propel through a stationary fluid. Although a cylinder undergoing a single oscillation has been thoroughly studied, the combination of the two oscillations has not received much attention until now. The current research reported here extends the numerical study of Blackburn et al. [Phys. Fluids 11, L4 (1999)] both experimentally and numerically, recording detailed vorticity fields in the wake and using these to elucidate the underlying physics, examining the three-dimensional wake development experimentally, and determining the three-dimensional stability of the wake through Floquet stability analysis. Experiments conducted in the laboratory are presented for a given parameter range, confirming the early results from Blackburn et al. [Phys. Fluids 11, L4 (1999)]. In particular, we confirm the thrust generation ability of a circular cylinder undergoing combined oscillatory motions. Importantly, we also find that the wake undergoes three-dimensional transition at low Reynolds numbers (Re~=100) to an instability mode with a wavelength of about two cylinder diameters. The stability analysis indicates that the base flow is also unstable to another mode at slightly higher Reynolds numbers, broadly analogous to the three-dimensional wake transition mode for a circular cylinder, despite the distinct differences in wake/mode topology. The stability of these flows was confirmed by experimental measurements.

Nazarinia, M.; Lo Jacono, D.; Thompson, M. C.; Sheridan, J.

2009-06-01

408

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

PubMed Central

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