Hydrodynamic Performance of Two-Dimensional Undulating Foils in Triangular Formation

ABSTRACTAs inspired by studies of fish schooling in literature, this work investigates hydrodynamic performance of a two-dimensional undulating-foil triad in viscous flows via numerical simulation. The chord of foil oscillates in the form of a streamwise traveling wave. The triad is in triangular formation, i.e., two foils followed by one. A series of triad configuration are computed assuming the same wave speed, amplitude, and frequency of chord traveling wave for each foil. The results show that, to achieve highest thrust efficiency, the two leading foils should separate from each other by 0.4 chord length, perform antiphase undulating motion, and the leading edge of the trailing foil stay 0.2 chord length in front of the trailing edges of the leading foils. An underlining mechanism, vortex pair shedding from the leading foil interacting with boundary-layer vorticity field of the trailing foil, has been identified to explain the efficiency enhancement. This optimal triad configuration is different from that obtained in a previous potential flow analysiss.

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Characteristics of Bluff Body Stabilized Turbulent Premixed Flames

Volume 2: Combustion, Fuels and Emissions, Parts A and B ◽

10.1115/gt2011-45089 ◽

2011 ◽

Cited By ~ 1

Author(s):

Alejandro M. Briones ◽

Balu Sekar ◽

Hugh Thornburg

Keyword(s):

Bluff Body ◽

Three Dimensional ◽

Leading Edge ◽

Reacting Flows ◽

Chemical Mechanism ◽

Reacting Flow ◽

Bluff Bodies ◽

Two Dimensional ◽

Aspect Ratios ◽

Trailing Edges

Non-reacting and reacting flows past typical flameholders are modeled with URANS and LES. The continuity, momentum, energy, species, and turbulence governing equations are solved using two- and three-dimensional configurations. Either 2-step global or 44-step reduced chemical mechanism for C3H8-air combustion, accounting for turbulence-chemistry interaction, and with temperature- and species-dependent thermodynamic and transport properties is utilized. For square and rectangular bluff bodies the flow separates at the leading edges, whereas for triangular bluff body separation occurs only at the trailing edges. These bluff bodies exhibit two shear layers at the trailing edges that shed asymmetric vortices. For rectangular bluff bodies with aspect ratios (AR) less than 2.3 there is backflow from the wake. With increasing AR from unity, backflow is gradually diminished, and the von Ka´rma´n Strouhal number (StvK) decreases. For 2.0<AR<2.3, StvK jumps to a higher value and separation again occurs at the trailing edges for AR = 2.3. Further increase in AR decreases StvK again. The simulations with URANS qualitatively and quantitatively match experimental results for StvK vs. AR. Quantitative discrepancies are, however, found for AR≥2.3. In addition, two-dimensional non-reacting flows with URANS are sufficient to predict StvK. Moreover, two-dimensional simulations of reacting flow indicate that the flame promotes static and dynamic stability for AR = 1.0 and 2.3. The flame is dynamically unstable for AR = 2.0, exhibiting a von Ka´rma´n flow pattern. Stable flames anchored at the most downstream separation location (e.g., the flame anchored at AR = 1.0 is attached to the leading edge, whereas that of AR = 2.3 is attached to the trailing edge). Realizable k-ε URANS and LES simulations for the triangular cylinder closely match the experimental StvK for both non-reacting and reacting flows. Nonetheless, LES predicts a smaller recirculation length than k-ε URANS. LES predicts a flow field in which Be´rnard/von Ka´rma´n (BvK) instability is suppressed, whereas URANS predicts a competition between the Kelvin-Helmholtz (KH) instability and BvK.

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Linearized planing-surface theory with surface tension. Part II: Detachment with discontinuous slope

The Journal of the Australian Mathematical Society Series B Applied Mathematics ◽

10.1017/s0334270000000229 ◽

1982 ◽

Vol 23 (3) ◽

pp. 259-277 ◽

Cited By ~ 4

Author(s):

E. O. Tuck

Keyword(s):

Surface Tension ◽

Leading Edge ◽

Correct Choice ◽

Jump Discontinuity ◽

Edge Condition ◽

Two Dimensional ◽

Trailing Edge ◽

Surface Theory ◽

Natural Choice ◽

Trailing Edges

AbstractIn Part I of this series, surface tension was included in the classical two-dimensional planing-surface problem, and the usual smooth-detachment trailing-edge condition enforced. However, the results exhibited a paradox, in that the classical results were not approached in the limit as the surface tension approached zero. This paradox is resolved here by abandoning the smooth-detachment condition, that is, by allowing a jump discontinuity in slope between the planing surface and the free surface at the trailing edge. A unique solution is obtainable for any input planing surface at fixed wetted length if one allows such jumps at both leading and trailing edges. If, as is the case in practice, the wetted length is allowed to vary, uniqueness may be restored by requiring either, but not both, of these slope discontinuities to vanish. The results of Part I correspond to the seemingly more-natural choice of making the trailing-edge detachment continuous, but it appears that the correct choice is to require the leading-edge attachment to be continuous.

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Steady longitudinal vortices in supersonic turbulent separated flows

Journal of Fluid Mechanics ◽

10.1017/s0022112010006105 ◽

2011 ◽

Vol 672 ◽

pp. 451-476 ◽

Cited By ~ 14

Author(s):

ERICH SCHÜLEIN ◽

VICTOR M. TROFIMOV

Keyword(s):

High Speed ◽

Large Scale ◽

Roughness Element ◽

Vortex Generator ◽

Vortex Pair ◽

Leading Edge ◽

Separated Flows ◽

Vortex Generators ◽

Longitudinal Vortices ◽

Oil Flow

Large-scale longitudinal vortices in high-speed turbulent separated flows caused by relatively small irregularities at the model leading edges or at the model surfaces are investigated in this paper. Oil-flow visualization and infrared thermography techniques were applied in the wind tunnel tests at Mach numbers 3 and 5 to investigate the nominally 2-D ramp flow at deflection angles of 20°, 25° and 30°. The surface contour anomalies have been artificially simulated by very thin strips (vortex generators) of different shapes and thicknesses attached to the model surface. It is shown that the introduced streamwise vortical disturbances survive over very large downstream distances of the order of 104 vortex-generator heights in turbulent supersonic flows without pressure gradients. It is demonstrated that each vortex pair induced in the reattachment region of the ramp is definitely a child of a vortex pair, which was generated originally, for instance, by the small roughness element near the leading edge. The dependence of the spacing and intensity of the observed longitudinal vortices on the introduced disturbances (thickness and spanwise size of vortex generators) and on the flow parameters (Reynolds numbers, boundary-layer thickness, compression corner angles, etc.) has been shown experimentally.

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Spatiotemporal dynamics of actin concentration during cytokinesis and locomotion in Dictyostelium

Journal of Cell Science ◽

10.1242/jcs.111.15.2097 ◽

1998 ◽

Vol 111 (15) ◽

pp. 2097-2108 ◽

Cited By ~ 3

Author(s):

S. Yumura ◽

Y. Fukui

Keyword(s):

Relative Concentration ◽

Leading Edge ◽

Live Cells ◽

Cell Behavior ◽

Cleavage Furrow ◽

Two Dimensional ◽

Dynamic Changes ◽

Temporal Regulation ◽

Ph Dependent ◽

Actin Concentration

To study the spatial and temporal regulation of the actin cytoskeleton, we have analyzed the actin concentration dynamics in live Dictyostelium. The relative actin concentration was analyzed with respect to cell behavior by fluorescence morphometry. We electroporated rhodamine-actin into Dictyostelium cells and acquired images with 200–300 millisecond temporal and approximately 250 nm spatial resolutions. To convert fluorescence intensity into actin concentration, the observation was made on nearly two-dimensional cells, and the actin signal was ratioed over a volume marker (FITC-BSA or GFP). Since the emission of FITC and GFP is pH-dependent, we first measured the cytoplasmic pH in live cells and determined that the pHi in pseudopods is same as that of general cytoplasm. During cytokinesis, the relative concentration of actin in the cleavage furrow was significantly higher than in the general cytoplasm. In migrating cells, actin was recruited surprisingly rapidly, particularly in the pseudopod. We found that the region of high actin concentration moves relative to the leading edge when a pseudopod projects or retracts. When the pseudopod retracts, the actin density dissipates within 5 seconds. We have also found that actin accumulates in developing pseudopods in an oscillatory manner, and this timing coordinates with advancement of the centroid. This is the first study to reveal the dynamic changes in relative concentration of actin in live cells and to quantitatively correlate these changes with the locomotive behavior of the amoeba.

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Minimal Wave Speed in a Predator-Prey System with Distributed Time Delay

Discrete Dynamics in Nature and Society ◽

10.1155/2018/4873803 ◽

2018 ◽

Vol 2018 ◽

pp. 1-8

Author(s):

Fuzhen Wu ◽

Dongfeng Li

Keyword(s):

Time Delay ◽

Traveling Wave ◽

Wave Speed ◽

Upper And Lower Solutions ◽

Traveling Wave Solutions ◽

Predator Prey ◽

Wave Solutions ◽

Prey System ◽

Minimal Wave Speed ◽

Distributed Time Delay

This paper is concerned with the minimal wave speed of traveling wave solutions in a predator-prey system with distributed time delay, which does not satisfy comparison principle due to delayed intraspecific terms. By constructing upper and lower solutions, we obtain the existence of traveling wave solutions when the wave speed is the minimal wave speed. Our results complete the known conclusions and show the precisely asymptotic behavior of traveling wave solutions.

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The Impact of Geometric Variation on Compressor Two-Dimensional Incidence Range

Journal of Turbomachinery ◽

10.1115/1.4028355 ◽

2014 ◽

Vol 137 (2) ◽

Cited By ~ 12

Author(s):

Martin N. Goodhand ◽

Robert J. Miller ◽

Hang W. Lung

Keyword(s):

Design Process ◽

Design Space ◽

Leading Edge ◽

Critical Value ◽

Two Dimensional ◽

Suction Surface ◽

Design Intent ◽

Manufacture Process ◽

The Impact ◽

Geometric Variation

An important question for a designer is how, in the design process, to deal with the small geometric variations which result from either the manufacture process or in-service deterioration. For some blade designs geometric variations will have little or no effect on the performance of a row of blades, while in others their effects can be significant. This paper shows that blade designs which are most sensitive are those which are susceptible to a distinct switch in the fluid mechanisms responsible for limiting blade performance. To demonstrate this principle, the sensitivity of compressor 2D incidence range to manufacture variations is considered. Only one switch in mechanisms was observed, the onset of flow separation at the leading edge. This switch is only sensitive to geometric variations around the leading edge, 0–3% of the suction surface. The consequence for these manufacture variations was a 10% reduction in the blade's positive incidence range. For this switch, the boundary in the design space is best defined in terms of the blade pressure distribution. Blade designs where the acceleration exceeds a critical value just downstream of the leading edge are shown to be robust to geometric variation. Two historic designs, supercritical blades and blades with sharp leading edges, though superior in design intent, are shown to sit outside this robust region and thus, in practice, perform worse. The improved understanding of the robust, region of the design space is then used to design a blade capable of a robust, 5% increase in operating incidence range.

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