The Experts below are selected from a list of 39231 Experts worldwide ranked by ideXlab platform

George V Lauder - One of the best experts on this subject based on the ideXlab platform.

  • hydrodynamics of surface swimming in leopard frogs rana pipiens
    The Journal of Experimental Biology, 2004
    Co-Authors: Christoffer L Johansson, George V Lauder
    Abstract:

    SUMMARY The kinematics of swimming frogs have been studied extensively in the past and, based on these results, hypotheses regarding the hydrodynamics of frog swimming can be generated. To test these hypotheses we used digital particle image velocimetry (DPIV) to quantify the flow structure of the wake produced by the feet during the propulsion phase of the kick of surface swimming frogs ( Rana pipiens ). These frogs use two different gaits, asynchronous and synchronous kicking, and the magnitude of the thrust produced by the feet differs between asynchronous (34±5.4 mN foot –1 ) and synchronous kicking (71±13.3 mN foot –1 ), as does maximum swimming speed, with higher swimming speed and forces produced during the synchronous kicks. Previous studies have suggested that an interaction between the feet, resulting in a single posteriorly directed fluid Jet, as the feet come together at the end of synchronous kicks, may augment force production. Our results show, however, that each foot produces its own distinct vortex ring, in both asynchronous and synchronous kicking of the feet. There is no evidence of a Central Jet being produced even during powerful synchronous kicks (maximum thrust calculated was 264 mN foot –1 ). An alternative mechanism of force production could be the lift-based paddling recently suggested for delta-shaped feet of swimming birds. However, the orientation of the vortex rings generated by the feet is almost perpendicular to the swimming direction for both gaits and there is only a slight asynchrony of the shedding of the distal (start) and proximal (stop) vortex rings, which is different from what would be expected by a dominantly lift-based mechanism. Thus, our results do not support lift as a major mechanism contributing to thrust. Instead, our data support the hypothesis that propulsion is based on drag and acceleration reaction forces where the thrust is generated by separated, but attached, vortex rings on the suction side of the feet, resulting in vortices that are shed behind the frogs during both asynchronous and synchronous kicking.

  • hydrodynamics of caudal fin locomotion by chub mackerel scomber japonicus scombridae
    The Journal of Experimental Biology, 2002
    Co-Authors: Jennifer C Nauen, George V Lauder
    Abstract:

    As members of the derived teleost fish clade Scombridae, mackerel exhibit high-performance aquatic locomotion via oscillation of the homocercal forked caudal fin. We present the first quantitative flow visualization of the wake of a scombrid fish, chub mackerel Scomber japonicus (20‐26 cm fork length, FL), swimming steadily in a recirculating flow tank at cruising speeds of 1.2 and 2.2 FL s ‐1 . Thrust was calculated from wake measurements made separately in the horizontal (frontal) plane and vertical (parasagittal) planes using digital particle image velocimetry (DPIV) and compared with drag measurements obtained by towing the same specimens of S. japonicus post mortem. Patterns of flow indicated that the wake consisted of a series of linked elliptical vortex rings, each with Central Jet flow. The length of the minor axis (height) of the vortex rings was approximately equal to caudal fin span; the length of the major ring axis was dependent on swimming speed and was up to twice the magnitude of ring height. Profiles of wake velocity components were similar to theoretical profiles of vortex rings. Lift, thrust and lateral forces were calculated from DPIV measurements. At 1.2 FL s ‐1 , lift forces measured relative to the X axis were low in magnitude (‐1±1 mN, mean ± S.D., N=20) but oriented at a mean angle of 6 ° to the body axis. Reaction forces tend to rotate the fish about its center of mass, tipping the head down. Thus, the homocercal caudal fin of S. japonicus functions asymmetrically in the vertical plane. Pitching moments may be balanced anteriorly via lift generation by the pectoral fins. Thrust estimates for the two smallest fish based on DPIV analysis were not significantly different from drag measurements made by towing those same animals. At a speed of 1.2 FL s ‐1 , thrust magnitude was 11±6 mN (mean ± S.D, N=40). Lateral force magnitudes were approximately double thrust magnitudes (22±6 mN, mean ± S.D., N=20), resulting in a mean mechanical performance ratio (thrust/total force) of 0.32 at 1.2 FL s ‐1 . An increase in speed by a factor of 1.8 resulted in a mean increase in thrust by a factor of 4.4, a mean increase in lateral forces by a factor of 3, no change in the magnitude of lift produced and an increase in mean mechanical performance to 0.42. The relatively high lateral forces generated during swimming may be a necessary consequence of force production via propagated waves of bending. Movies available on-line. Summary

  • wake dynamics and fluid forces of turning maneuvers in sunfish
    The Journal of Experimental Biology, 2001
    Co-Authors: Eliot G Drucker, George V Lauder
    Abstract:

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

  • locomotor forces on a swimming fish three dimensional vortex wake dynamics quantified using digital particle image velocimetry
    The Journal of Experimental Biology, 1999
    Co-Authors: Eliot G Drucker, George V Lauder
    Abstract:

    Quantifying the locomotor forces experienced by swimming fishes represents a significant challenge because direct measurements of force applied to the aquatic medium are not feasible. However, using the technique of digital particle image velocimetry (DPIV), it is possible to quantify the effect of fish fins on water movement and hence to estimate momentum transfer from the animal to the fluid. We used DPIV to visualize water flow in the wake of the pectoral fins of bluegill sunfish (Lepomis macrochirus) swimming at speeds of 0.5-1.5 L s(−)(1), where L is total body length. Velocity fields quantified in three perpendicular planes in the wake of the fins allowed three-dimensional reconstruction of downstream vortex structures. At low swimming speed (0.5 L s(−)(1)), vorticity is shed by each fin during the downstroke and stroke reversal to generate discrete, roughly symmetrical, vortex rings of near-uniform circulation with a Central Jet of high-velocity flow. At and above the maximum sustainable labriform swimming speed of 1.0 L s(−)(1), additional vorticity appears on the upstroke, indicating the production of linked pairs of rings by each fin. Fluid velocity measured in the vicinity of the fin indicates that substantial spanwise flow during the downstroke may occur as vortex rings are formed. The forces exerted by the fins on the water in three dimensions were calculated from vortex ring orientation and momentum. Mean wake-derived thrust (11.1 mN) and lift (3.2 mN) forces produced by both fins per stride at 0.5 L s(−)(1) were found to match closely empirically determined counter-forces of body drag and weight. Medially directed reaction forces were unexpectedly large, averaging 125 % of the thrust force for each fin. Such large inward forces and a deep body that isolates left- and right-side vortex rings are predicted to aid maneuverability. The observed force balance indicates that DPIV can be used to measure accurately large-scale vorticity in the wake of swimming fishes and is therefore a valuable means of studying unsteady flows produced by animals moving through fluids.

Victor Francia - One of the best experts on this subject based on the ideXlab platform.

  • an experimental investigation of the swirling flow in a tall form counter current spray dryer
    Experimental Thermal and Fluid Science, 2015
    Co-Authors: Victor Francia, Luis Martin, Andrew E Bayly, Mark J H Simmons
    Abstract:

    This work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a Central Jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity Ω (i.e. 1<Ω<2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity Ω decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 105-2.2{dot operator}105. Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30-40%.

E Sissa - One of the best experts on this subject based on the ideXlab platform.

  • sphere zimpol observations of the symbiotic system r aquarii i imaging of the stellar binary and the innermost Jet clouds
    Astronomy and Astrophysics, 2017
    Co-Authors: H M Schmid, A Bazzon, Julien Milli, R Roelfsema, N Engler, D Mouillet, E Lagadec, E Sissa
    Abstract:

    Context. R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular Jet outflow, and an extended emission line nebula. Because of its proximity to the Sun, this object has been studied in much detail with many types of high resolution imaging and interferometric techniques. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT). Aims. We describe SPHERE /ZIMPOL test observations of the R Aqr system taken in H alpha and other filters in order to demonstrate the exceptional performance of this high resolution instrument. We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We use our data for a detailed characterization of the inner Jet region of R Aqr. Methods. We analyze the high resolution approximate to 25 mas images from SPHERE /ZIMPOL and determine from the H alpha emission the position, size, geometric structure, and line fluxes of the Jet source and the clouds in the innermost region <2 `' (< 400 AU) of R Aqr. The data are compared to simultaneous HST line filter observations. The H alpha fluxes and the measured sizes of the clouds yield H alpha emissivities for many clouds from which one can derive the mean density, mass, recombination time scale, and other cloud parameters. Results. Our H alpha data resolve for the first time the R Aqr binary and we measure for the Jet source a relative position 45 mas West (position angle ‐89.5 degrees) of the mira. The Central Jet source is the strongest H alpha component with a flux of about 2.5 x 10(‐12) erg cm(‐2) s(‐1). North east and south west from the Central source there are many clouds with very diverse structures. Within 0.5 `' (100 AU) we see in the SW a string of bright clouds arranged in a zig‐zag pattern and, further out, at 1 `'‐2 `', fainter and more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure between 0.2 `'‐0.7 `' and faint perpendicular `` wisps'' further out. Some Jet clouds are also detected in the ZIMPOL [O I] and He I filters, as well as in the HST‐WFC3 line filters for H alpha, [O III], [N II], and [O I]. We determine Jet cloud parameters and find a very well defined correlation N‐e proportional to r(‐1.3) between cloud density and distance to the Central binary. Densities are very high with typical values of N‐e approximate to 3 x 10(5) cm(‐3) for the ``outer'' clouds around 300 AU, N‐e approximate to 3 x 10(6) cm(‐3) for the ``inner'' clouds around 50 AU, and even higher for the Central Jet source. The high Ne of the clouds implies short recombination or variability timescales of a year or shorter. Conclusions. H alpha high resolution data provide a lot of diagnostic information for the ionized Jet gas in R Aqr. Future H alpha observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this Jet outflow and its interaction with the wind of the red giant companion.

  • sphere zimpol observations of the symbiotic system r aqr i imaging of the stellar binary and the innermost Jet clouds
    arXiv: Solar and Stellar Astrophysics, 2017
    Co-Authors: H M Schmid, A Bazzon, Julien Milli, R Roelfsema, N Engler, D Mouillet, E Lagadec, E Sissa, J F Sauvage, C Ginski
    Abstract:

    R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular Jet outflow, and an extended emission line nebula. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT). We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We determine from the Halpha emission the position, size, geometric structure, and line fluxes of the Jet source and the clouds in the innermost region (<2") of R Aqr and determine Halpha emissivities mean density, mass, recombination time scale, and other cloud parameters. Our data resolve for the first time the R Aqr binary and we measure for the Jet source a relative position 46+/-1 mas West of the mira. The Central Jet source is the strongest Halpha component. North east and south west from the Central source there are many clouds with very diverse structures. We see in the SW a string of bright clouds arranged in a zig-zag pattern and, further out, more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure and faint perpendicular "wisps" further out. Some Jet clouds are also detected in the ZIMPOL [OI] and He I filters, as well as in the HST line filters for Halpha, [OIII], [NII], and [OI]. We determine Jet cloud parameters and find a very well defined anti-correlation between cloud density and distance to the Central binary. Future Halpha observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this Jet outflow and its interaction with the wind of the red giant companion.

Wouter J Waalewijn - One of the best experts on this subject based on the ideXlab platform.

  • beam thrust cross section for drell yan production at next to next to leading logarithmic order
    Physical Review Letters, 2011
    Co-Authors: Iain W Stewart, Frank J Tackmann, Wouter J Waalewijn
    Abstract:

    At the LHC and Tevatron strong initial-state radiation (ISR) plays an important role. It can significantly affect the partonic luminosity available to the hard interaction or contaminate a signal with additional Jets and soft radiation. An ideal process to study ISR is isolated Drell-Yan production, pp{yields}Xl{sup +}l{sup -} without Central Jets, where the Jet veto is provided by the hadronic event shape beam thrust {tau}{sub B}. Most hadron collider event shapes are designed to study Central Jets. In contrast, requiring {tau}{sub B}<<1 provides an inclusive veto of Central Jets and measures the spectrum of ISR. For {tau}{sub B}<<1 we carry out a resummation of {alpha}{sub s}{sup n}ln{sup m{tau}}{sub B} corrections at next-to-next-to-leading-logarithmic order. This is the first resummation at this order for a hadron-hadron collider event shape. Measurements of {tau}{sub B} at the Tevatron and LHC can provide crucial tests of our understanding of ISR and of {tau}{sub B}'s utility as a Central Jet veto.

  • beam thrust cross section for drell yan production at next to next to leading logarithmic order
    APS, 2011
    Co-Authors: Iain W Stewart, Frank J Tackmann, Wouter J Waalewijn
    Abstract:

    At the LHC and Tevatron strong initial-state radiation (ISR) plays an important role. It can significantly affect the partonic luminosity available to the hard interaction or contaminate a signal with additional Jets and soft radiation. An ideal process to study ISR is isolated Drell-Yan production, pp → Xl+ l- without Central Jets, where the Jet veto is provided by the hadronic event shape beam thrust τB. Most hadron collider event shapes are designed to study Central Jets. In contrast, requiring τ B << 1 provides an inclusive veto of Central Jets and measures the spectrum of ISR. For τ B << 1 we carry out a resummation of α s(n)ln(m)τ B corrections at next-to-next-to-leading-logarithmic order. This is the first resummation at this order for a hadron-hadron collider event shape. Measurements of τ B at the Tevatron and LHC can provide crucial tests of our understanding of ISR and of τ B's utility as a Central Jet veto.

  • beam thrust cross section for drell yan production at next to next to leading logarithmic order
    APS, 2011
    Co-Authors: Iain W Stewart, Frank J Tackmann, Wouter J Waalewijn
    Abstract:

    At the LHC and Tevatron strong initial-state radiation (ISR) plays an important role. It can significantly affect the partonic luminosity available to the hard interaction or contaminate a signal with additional Jets and soft radiation. An ideal process to study ISR is isolated Drell-Yan production, $pp\ensuremath{\rightarrow}X{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ without Central Jets, where the Jet veto is provided by the hadronic event shape beam thrust ${\ensuremath{\tau}}_{B}$. Most hadron collider event shapes are designed to study Central Jets. In contrast, requiring ${\ensuremath{\tau}}_{B}\ensuremath{\ll}1$ provides an inclusive veto of Central Jets and measures the spectrum of ISR. For ${\ensuremath{\tau}}_{B}\ensuremath{\ll}1$ we carry out a resummation of ${\ensuremath{\alpha}}_{s}^{n}{ln}^{m}{\ensuremath{\tau}}_{B}$ corrections at next-to-next-to-leading-logarithmic order. This is the first resummation at this order for a hadron-hadron collider event shape. Measurements of ${\ensuremath{\tau}}_{B}$ at the Tevatron and LHC can provide crucial tests of our understanding of ISR and of ${\ensuremath{\tau}}_{B}$'s utility as a Central Jet veto.

Mark J H Simmons - One of the best experts on this subject based on the ideXlab platform.

  • an experimental investigation of the swirling flow in a tall form counter current spray dryer
    Experimental Thermal and Fluid Science, 2015
    Co-Authors: Victor Francia, Luis Martin, Andrew E Bayly, Mark J H Simmons
    Abstract:

    This work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a Central Jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity Ω (i.e. 1<Ω<2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity Ω decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 105-2.2{dot operator}105. Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30-40%.