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  • 1
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    Unsteady propulsion near a solid boundary (2014)
    Cambridge University Press
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    Publikationsdatum: 2014-02-21
    Beschreibung: Experimental and computational results are presented on an aerofoil undergoing pitch oscillations in ground effect, that is, close to a solid boundary. The time-averaged thrust is found to increase monotonically as the mean position of the aerofoil approaches the boundary while the propulsive efficiency stays relatively constant, showing that ground effect can enhance thrust at little extra cost for a pitching aerofoil. Vortices shed into the wake form pairs rather than vortex streets, so that in the mean a momentum jet is formed that angles away from the boundary. The time-averaged lift production is found to have two distinct regimes. When the pitching aerofoil is between 0.4 and 1 chord lengths from the ground, the lift force pulls the aerofoil towards the ground. In contrast, for wall proximities between 0.25 and 0.4 chord lengths, the lift force pushes the aerofoil away from the ground. Between these two regimes there is a stable equilibrium point where the time-averaged lift is zero and thrust is enhanced by approximately 40Â %. © 2014 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 2
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    Turbulent pipe flow downstream of a bend (2013)
    Cambridge University Press
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    Publikationsdatum: 2013-10-29
    Beschreibung: Time-resolved stereoscopic PIV was used to investigate the curvature-induced structures downstream of a 90° bend at Reynolds numbers between 20× 103 and 115× 103. Data were taken at three downstream locations to investigate the evolution of the structures. Snapshot proper orthogonal decomposition (POD) analysis shows that the most energetic structure is not the well-known Dean motion but a bimodal single cell structure with alternating direction of rotation, called the 'swirl switching' mode. The strengths of the Dean motion and the swirl-switching structures are similar, indicating that the difference in energy is related to their duration of occurrence, where the Dean motion is associated with a comparatively rapid transition between the two states in the swirl switching mode. © 2013 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 3
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    On the relationship between efficiency and wake structure of a batoid-inspired oscillating fin (2011)
    Cambridge University Press
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    Publikationsdatum: 2011-12-05
    Beschreibung: A mechanical representation of batoid-like propulsion using a flexible fin with an elliptical planform shape is used to study the hydrodynamics of undulatory propulsion. The wake is found to consist of a series of interconnected vortex rings, whereby leading and trailing edge vortices of subsequent cycles become entangled with one another. Efficient propulsion is achieved when leading and trailing edge vortices coalesce at the spanwise location where most of the streamwise fluid momentum is concentrated in the wake of the fin. Both the Strouhal number and the wavelength are found to have a significant effect on the wake structure. In general, a decrease in wavelength promotes a wake transition from shedding a single vortex per half-oscillation period to shedding a pair of vortices per half-oscillation period. An increase in Strouhal number causes the wake to bifurcate a finite distance downstream of the trailing edge of the fin into a pair of jets oriented at an acute angle to the line of symmetry. The bifurcation distance decreases with increasing Strouhal number and wavelength, and it is shown to obey a simple scaling law. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 4
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    Scaling laws for the thrust production of flexible pitching panels (2013)
    Cambridge University Press
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    Publikationsdatum: 2013-08-30
    Beschreibung: We present experimental results on the role of flexibility and aspect ratio in bio-inspired aquatic propulsion. Direct thrust and power measurements are used to determine the propulsive efficiency of flexible panels undergoing a leading-edge pitching motion. We find that flexible panels can give a significant amplification of thrust production of O (100-200%) and propulsive efficiency of O (100%) when compared to rigid panels. The data highlight that the global maximum in propulsive efficiency across a range of panel flexibilities is achieved when two conditions are simultaneously satisfied: (i) the oscillation of the panel yields a Strouhal number in the optimal range (0. 25 〈 St 〈 0. 35) predicted by Triantafyllou, Triantafyllou & Grosenbaugh (J. Fluid Struct., vol. 7, 1993, pp. 205-224); and (ii) this frequency of motion is tuned to the structural resonant frequency of the panel. In addition, new scaling laws for the thrust production and power input to the fluid are derived for the rigid and flexible panels. It is found that the dominant forces are the characteristic elastic force and the characteristic fluid force. In the flexible regime the data scale using the characteristic elastic force and in the rigid limit the data scale using the characteristic fluid force. © 2013 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 5
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    Scaling the circulation shed by a pitching panel (2011)
    Cambridge University Press
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    Publikationsdatum: 2011-10-31
    Beschreibung: A new scaling parameter is developed for the circulation shed by a rigid, rectangular panel pitching periodically about its leading edge. This parameter is the product of a kinematic and a geometric component. The kinematic component describes the relationship between the mean vorticity flux from the panel surface and the panel motion. The geometric component depends on the ratio of pitching amplitude to the span of the panel. The kinematic component is developed based on the connection between the surface pressure distribution and the resulting surface vorticity flux, which are supported in a stroke-averaged sense by pressure measurements on the surface of the panel. The parameter gives a robust scaling for the total spanwise circulation shed in a half-cycle by the panel. It provides a useful predictive tool, in that it can be either complementary to the formation number or provide an alternative scaling parameter when vortex saturation and pinch-off do not occur. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 6
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    Bounded dam-break flows with tailwaters (2011)
    Cambridge University Press
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    Publikationsdatum: 2011-09-27
    Beschreibung: The gravitationally driven collapse of a reservoir into an initially stationary layer of fluid, termed the tailwater, is studied using the nonlinear shallow water equations. The motion is tackled using the hodograph transformation of the governing equation which allows the solutions for velocity and depth of the shallow flowing layer to be constructed by analytical techniques. The front of the flow emerges as a bore across which the depth of the fluid jumps discontinuously to the tailwater depth. The speed of the front is initially constant, but progressively slows once the finite extent of the reservoir begins to influence the motion. There then emerges a variety of phenomena depending upon the depth of the tailwater relative to the initial depth of the reservoir. Provided that the tailwater is sufficiently deep, a region of quiescent fluid emerges adjacent to the rear wall of the reservoir, followed by a region within which the velocity is negative. Also it is shown that for non-vanishing tailwater depths, continuous solutions for the velocity and height of the flowing layer breakdown after a sufficient period and develop an interior bore, the location and time of inception of which are calculated directly from quasi-analytical solutions. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 7
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    The unsteady three-dimensional wake produced by a trapezoidal pitching panel (2011)
    Cambridge University Press
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    Publikationsdatum: 2011-09-23
    Beschreibung: Particle image velocimetry (PIV) is used to investigate the three-dimensional wakes of rigid pitching panels with a trapezoidal geometry, chosen to model idealized fish caudal fins. Experiments are performed for Strouhal numbers from 0.17 to 0.56 for two different trailing edge pitching amplitudes. A Lagrangian coherent structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake. A classic reverse von Kármán vortex street pattern is observed along the mid-span of the near wake, but the vortices realign and exhibit strong interactions near the spanwise edges of the wake. At higher Strouhal numbers, the complexity of the wake increases downstream of the trailing edge as the spanwise vortices spread transversely and lose coherence as the wake splits. This wake transition is shown to correspond to a qualitative change in the LCS pattern surrounding each vortex core, and can be identified as a quantitative event that is not dependent on arbitrary threshold levels. The location of this transition is observed to depend on both the pitching amplitude and free stream velocity, but is not constant for a fixed Strouhal number. On the panel surface, the trapezoidal planform geometry is observed to create additional vortices along the swept edges that retain coherence for low Strouhal numbers or high sweep angles. These additional swept-edge structures are conjectured to add to the complex three-dimensional flow near the tips of the panel. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 8
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    Vortex suppression and drag reduction in the wake of counter-rotating cylinders (2011)
    Cambridge University Press
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    Publikationsdatum: 2011-05-12
    Beschreibung: The flow over a pair of counter-rotating cylinders is investigated numerically and experimentally. It is demonstrated that it is possible to suppress unsteady vortex shedding for gap sizes from one to five cylinder diameters, at Reynolds numbers from 100 to 200, expanding on the more limited work by Chan & Jameson (Intl J. Numer. Meth. Fluids, vol. 63, 2010, p. 22). The degree of unsteady wake suppression is proportional to the speed and the direction of rotation, and there is a critical rotation rate where a complete suppression of flow unsteadiness can be achieved. In the doublet-like configuration at higher rotational speeds, a virtual elliptic body that resembles a potential doublet is formed, and the drag is reduced to zero. The shape of the elliptic body primarily depends on the gap between the two cylinders and the speed of rotation. Prior to the formation of the elliptic body, a second instability region is observed, similar to that seen in studies of single rotating cylinders. It is also shown that the unsteady wake suppression can be achieved by rotating each cylinder in the opposite direction, that is, in a reverse doublet-like configuration. This tends to minimize the wake interaction of the cylinder pair and the second instability does not make an appearance over the range of speeds investigated here. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 9
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    Scaling the propulsive performance of heaving flexible panels (2013)
    Cambridge University Press
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    Publikationsdatum: 2013-12-05
    Beschreibung: We present an experimental investigation of flexible panels actuated with heave oscillations at their leading edge. Results are presented from kinematic video analysis, particle image velocimetry, and direct force measurements. Both the trailing edge amplitude and the mode shapes of the panel are found to scale with dimensionless parameters originating from the Euler-Bernoulli beam equation. The time-averaged net thrust increases with heaving frequency, but experiences localized boosts near resonant frequencies where the trailing edge amplitude is maximized. These boosts correspond to local maxima in the propulsive efficiency. For a constant heave amplitude, the time-averaged net thrust coefficient is shown to be a function of Strouhal number over a wide range of conditions. It appears, therefore, that self-propelled swimming (zero net thrust) only occurs over a small range of Strouhal numbers. Under these near-constant Strouhal number conditions, the propulsive economy increases with higher flexibilities and slower swimming speeds. © 2013 Cambridge University Press.
    Print ISSN: 0022-1120
    Digitale ISSN: 1469-7645
    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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  • 10
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    On the logarithmic region in wall turbulence (2013)
    Cambridge University Press
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    Publikationsdatum: 2013-01-28
    Beschreibung: Considerable discussion over the past few years has been devoted to the question of whether the logarithmic region in wall turbulence is indeed universal. Here, we analyse recent experimental data in the Reynolds number range of nominally$2imes 1{0}^{4} lt {mathit{Re}}_{au } lt 6imes 1{0}^{5} $for boundary layers, pipe flow and the atmospheric surface layer, and show that, within experimental uncertainty, the data support the existence of a universal logarithmic region. The results support the theory of Townsend (The Structure of Turbulent Shear Flow, Vol. 2, 1976) where, in the interior part of the inertial region, both the mean velocities and streamwise turbulence intensities follow logarithmic functions of distance from the wall.
    Print ISSN: 0022-1120
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    Thema: Maschinenbau , Physik
    Publiziert von Cambridge University Press
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