scholarly journals Stabilizing effect of flexibility in the wake of a flapping foil

2012 ◽  
Vol 710 ◽  
pp. 659-669 ◽  
Author(s):  
C. Marais ◽  
B. Thiria ◽  
J. E. Wesfreid ◽  
R. Godoy-Diana
Keyword(s):  
Symmetry Breaking ◽  
Vortex Dynamics ◽  
Momentum Balance ◽  
Near Wake ◽  
Low Speed ◽  
Flapping Foil ◽  
Balance Calculation ◽  
Stabilizing Effect ◽  
Dynamical Features ◽  
Regime Transitions

AbstractThe wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chordwise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse Bénard–von Kármán wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analysing the vortex dynamics in the very near wake.

scholarly journals A model for the symmetry breaking of the reverse Bénard–von Kármán vortex street produced by a flapping foil

2009 ◽  
Vol 622 ◽  
pp. 23-32 ◽  
Author(s):  
RAMIRO GODOY-DIANA ◽  
CATHERINE MARAIS ◽  
JEAN-LUC AIDER ◽  
JOSÉ EDUARDO WESFREID
Keyword(s):  
Symmetry Breaking ◽  
Phase Velocity ◽  
Vortex Street ◽  
Near Wake ◽  
Flapping Foil ◽  
Von Karman ◽  
Starting Point ◽  
Vortex Streets ◽  
Von Kármán ◽  
Advection Velocity

The vortex streets produced by a flapping foil of span to chord aspect ratio of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the mechanisms giving rise to the symmetry breaking of the reverse Bénard–von Kármán (BvK) vortex street that characterizes fishlike swimming and forward flapping flight are examined. Two-dimensional particle image velocimetry (PIV) measurements in the midplane perpendicular to the span axis of the foil are used to characterize the different flow regimes. The deflection angle of the mean jet flow with respect to the horizontal observed in the average velocity field is used as a measure of the asymmetry of the vortex street. Time series of the vorticity field are used to calculate the advection velocity of the vortices with respect to the free stream, defined as the phase velocity Uphase, as well as the circulation Γ of each vortex and the spacing ξ between consecutive vortices in the near wake. The observation that the symmetry-breaking results from the formation of a dipolar structure from each couple of counter-rotating vortices shed on each flapping period serves as the starting point to build a model for the symmetry-breaking threshold. A symmetry-breaking criterion based on the relation between the phase velocity of the vortex street and an idealized self-advection velocity of two consecutive counter-rotating vortices in the near wake is established. The predicted threshold for symmetry breaking accounts well for the deflected wake regimes observed in the present experiments and may be useful to explain other experimental and numerical observations of similar deflected propulsive vortex streets reported in the literature.

scholarly journals The Effect of Boattail Angles on the Near-Wake Structure of Axisymmetric Afterbody Models at Low-Speed Condition

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
The Hung Tran
Keyword(s):  
Strouhal Number ◽  
Reynolds Shear Stress ◽  
Flow Behavior ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Turbulent Intensity ◽  
Near Wake ◽  
Wake Structure ◽  
Low Speed ◽  
Speed Condition

The effect of a boattail angle on the structure of the wake of an axisymmetric model was investigated at low-speed condition. Four conical boattail models with angles of 0° (blunt-based body), 10°, 16°, and 22° were selected for this study. The Reynolds number based on the diameter of the model was around 1.97×104. Particle image velocimetry (PIV) was used to measure the velocity of the wake flow. The time-averaged flow characteristics including the length of recirculation of the afterbody, turbulent intensity, and Reynolds shear stress were analyzed and compared among those boattail models. The experimental results showed that the length of recirculation decreases with increasing boattail angle to 16°. At a boattail angle above 16°, the flow was fully separated near the shoulder and near-wake structure was highly changed. The turbulent intensity at a boattail angle of 22° showed a similar level to that in the case of the blunt-based body. Flow behavior on boattail surface should be accounted as an important parameter affecting the wake width and drag of the model. Power spectral density and proper orthogonal decomposition (POD) analyses showed that a Strouhal number of StD=0.2 dominated for the boattail model up to 16°. The fully separated flow was dominated by a Strouhal number of StD=0.03−0.06, which was firstly presented in this study.

Near wake vortex dynamics of a hovering hawkmoth

2008 ◽  
Vol 25 (1) ◽  
pp. 23-36 ◽  
Author(s):  
Hikaru Aono ◽  
Wei Shyy ◽  
Hao Liu
Keyword(s):  
Vortex Dynamics ◽  
Wake Vortex ◽  
Near Wake

Optical-Flow Algorithm for Near-Wake Analysis of Axisymmetric Blunt-Based Body at Low-Speed Conditions

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
The Hung Tran ◽  
Lin Chen
Keyword(s):  
Optical Flow ◽  
Cross Correlation ◽  
Transient Flow ◽  
Wake Flow ◽  
Near Wake ◽  
Dominant Type ◽  
Low Speed ◽  
Flow Method ◽  
Optical Flow Method ◽  
Flow Algorithm

Abstract In this study, ability of an optical-flow algorithm in extracting wake structure of axisymmetric model was investigated. The initial data for optical-flow processing were obtained in low-speed conditions by particle image velocimetry method. The Reynolds number based on the model diameter was around ReD = 1.97 × 104 in this study. Both the time-averaged and transient flow characteristics of near-wake flow were illustrated and examined by the optical-flow analysis method proposed. The processing results of optical-flow method showed good agreement with conventional cross-correlation methods. The ability of optical-flow method to extract flow fields was, thereby, confirmed for blunt-based flow at low-speed conditions. This study showed that the antisymmetric flow behavior of the near wake is the dominant type at low-speed conditions. Differing to traditional methods and cross-correlation results, the optical-flow results showed a frequency at around StD = 0.015 of the near wake for the first time, which is connected to vortex shedding behavior of the wake flow.

A Quantitative Comparison of Delta Wing Vortices in the Near-Wake For Incompressible and Supersonic Free Streams

2005 ◽  
Vol 127 (6) ◽  
pp. 1071-1084 ◽  
Author(s):  
Frank Y. Wang ◽  
Ivana M. Milanovic ◽  
Khairul B. M. Q. Zaman ◽  
Louis A. Povinelli
Keyword(s):  
Incompressible Flow ◽  
Axial Velocity ◽  
High Speed ◽  
Delta Wing ◽  
Vortex Pair ◽  
Pressure Probe ◽  
Spanwise Direction ◽  
Near Wake ◽  
Low Speed ◽  
Axis Switching

When requiring quantitative data on delta wing vortices for design purposes, low-speed results have often been extrapolated to configurations intended for supersonic operation. This practice stems from a lack of database in high-speed flows due to measurement difficulties. In the present paper an attempt is made to examine this practice by comparing data from an incompressible flow experiment designed specifically to correspond to an earlier experiment in supersonic flows. The comparison is made for a 75° sweptback delta wing at angles of attack of 7° and 12°. For the incompressible flow, detailed flow-field properties including vorticity and turbulence characteristics are obtained by hot-wire and pressure probe surveys. The results are compared, wherever possible, with available data from the earlier Mach 2.49 experiment. The results indicate that quantitative similarities exist in the distributions of total pressure and swirl velocities. Qualitative similarities also exist in other properties, however, many differences are observed. The vortex core is smaller and rounded at low speed. At high speed, it is elongated in the spanwise direction near the trailing edge but goes through “axis switching” within a short distance downstream. The vortex is located farther outboard, i.e., the spacing between the two legs of the vortex pair is larger, at low speed. The axial velocity distribution within the core is significantly different in the two flow regimes. A “jet-like” profile, observed at low speed, either disappears or becomes “wake-like” at high speed. The axial velocity characteristics are examined in the light of an analytical model.

scholarly journals Numerical analysis on transitions and symmetry-breaking in the wake of a flapping foil

2012 ◽  
Vol 28 (6) ◽  
pp. 1551-1556 ◽  
Author(s):  
Guo-Yi He ◽  
Qi Wang ◽  
Xing Zhang ◽  
Shu-Guang Zhang
Keyword(s):  
Numerical Analysis ◽  
Symmetry Breaking ◽  
Flapping Foil
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