scholarly journals On the generation of a reverse von Kármán street for the controlled cylinder wake in the laminar regime

2006 ◽  
Vol 18 (2) ◽  
pp. 028101 ◽  
Author(s):  
Michel Bergmann ◽  
Laurent Cordier ◽  
Jean-Pierre Brancher
Keyword(s):  
Cylinder Wake ◽  
Laminar Regime ◽  
Von Karman ◽  
Karman Street ◽  
Von Kármán

Structure and stability of the finite-area von Kármán street

2012 ◽  
Vol 24 (6) ◽  
pp. 066602 ◽  
Author(s):  
Paolo Luzzatto-Fegiz ◽  
Charles H. K. Williamson
Keyword(s):  
Finite Area ◽  
Structure And Stability ◽  
Von Karman ◽  
Karman Street ◽  
Von Kármán

Velocity spikes in separated flows

1966 ◽  
Vol 25 (1) ◽  
pp. 43-50 ◽  
Author(s):  
F. B. Hanson ◽  
S. H. Kozak ◽  
P. D. Richardson
Keyword(s):  
Large Scale ◽  
Separated Flow ◽  
Circular Cylinders ◽  
Hot Wire ◽  
Separated Shear Layer ◽  
Von Karman ◽  
Air Stream ◽  
Close Proximity ◽  
Karman Street ◽  
Von Kármán

In a recent study related to transition in the wake flows behind circular cylinders held transversely to an air stream, Bloor (1964) has reported the observation of velocity ‘spikes’ and attributed these to the close proximity to the hot wire of vortex centres on the opposite side of the von Kármán vortex street. Further observations of spikes are reported here, and the characteristics of their distribution indicate that other explanations of their form must be found. Some idealized flows are considered, and it is concluded that observations of spikiness within the hot-wire output may be accountable in terms of large-scale distributions of vorticity within the flow convected past the wire, the distributions being reasonable representations of a separated flow. The observations also provide some evidence that small vortices of Strouhal frequency exist on the inside of the coherent separated shear layer, and this may assist in the understanding of the feed-back mechanism where by the von Kármán street establishes itself as a self-perpetuating phenomenon.

scholarly journals Three-dimensional instabilities of a stratified cylinder wake

2014 ◽  
Vol 759 ◽  
pp. 149-180 ◽  
Author(s):  
M. Bosco ◽  
P. Meunier
Keyword(s):  
Tilt Angle ◽  
Unstable Mode ◽  
Three Dimensional ◽  
Axial Flow ◽  
Cylinder Wake ◽  
Homogeneous Fluid ◽  
Recirculation Bubble ◽  
Von Karman ◽  
Strong Shear ◽  
Von Kármán

AbstractThis paper describes experimentally, numerically and theoretically how the three-dimensional instabilities of a cylinder wake are modified by the presence of a linear density stratification. The first part is focused on the case of a cylinder with a small tilt angle between the cylinder’s axis and the vertical. The classical mode A well-known for a homogeneous fluid is still present. It is more unstable for moderate stratifications but it is stabilized by a strong stratification. The second part treats the case of a moderate tilt angle. For moderate stratifications, a new unstable mode appears, mode S, characterized by undulated layers of strong density gradients and axial flow. These structures correspond to Kelvin–Helmholtz billows created by the strong shear present in the critical layer of each tilted von Kármán vortex. The last two parts deal with the case of a strongly tilted cylinder. For a weak stratification, an instability (mode RT) appears far from the cylinder, due to the overturning of the isopycnals by the von Kármán vortices. For a strong stratification, a short wavelength unstable mode (mode L) appears, even in the absence of von Kármán vortices. It is probably due to the strong shear created by the lee waves upstream of a secondary recirculation bubble. A map of the four different unstable modes is established in terms of the three parameters of the study: the Reynolds number, the Froude number (characterizing the stratification) and the tilt angle.

scholarly journals THE EFFECTS OF WING ROTATION ON UNSTEADY AERODYNAMIC PERFORMANCE AT LOW REYNOLDS NUMBERS

1994 ◽  
Vol 192 (1) ◽  
pp. 179-206 ◽  
Author(s):  
M Dickinson
Keyword(s):  
Fluid Velocity ◽  
Angle Of Attack ◽  
Aerodynamic Performance ◽  
Force Generation ◽  
Rotational Axis ◽  
Kinematic Parameters ◽  
Small Component ◽  
Von Karman ◽  
Karman Street ◽  
Von Kármán

The downstroke-to-upstroke transition of many insects is characterized by rapid wing rotation. The aerodynamic consequences of these rapid changes in angle of attack have been investigated using a mechanical model dynamically scaled to the Reynolds number appropriate for the flight of small insects such as Drosophila. Several kinematic parameters of the wing flip were examined, including the speed and axis of rotation, as well as the duration and angle of attack during the wing stroke preceding rotation. Alteration of these kinematic parameters altered force generation during the subsequent stroke in a variety of ways. 1. When the rotational axis was close to the trailing edge, the model wing could capture vorticity generated during rotation and greatly increase aerodynamic performance. This vortex capture was most clearly manifested by the generation of lift at an angle of attack of 0°. Lift at a 0° angle of attack was also generated following rotation about the leading edge, but only if the downstroke angle was large enough to generate a von Karman street. The lift may be due to an alteration in the effective angle of attack caused by the inter-vortex stream in the downstroke wake. 2. The maximum lift attained (over all angles of attack) was substantially elevated if the wing translated backwards through a wake generated by the previous stroke. Transient lift coefficient values of nearly 4 were obtained when the wing translated back through a von Karman street generated at a 76.5° angle of attack. This effect might also be explained by the influence of the inter-vortex stream, which contributes a small component to fluid velocity in the direction of translation. 3. The growth of lift with angle of attack was significantly elevated following a 7.5 chord stroke with a 76.5° angle of attack, although it was relatively constant under all other kinematic conditions. 4. The results also indicate the discrepancies between transient and time-averaged measures of performance that arise when unsteady mechanisms are responsible for force generation. Although the influence of wing rotation was strong during the first few chords of translation, averaging the performance over as little as 6.5 chords of motion greatly attenuated the effects of rotation. 5. Together, these modeling results suggest that the unsteady mechanisms generated by simple wing flips could provide an important source for the production of aerodynamic forces in insect flight. Furthermore, the extreme sensitivity to small variations in almost all kinematic parameters could provide a foundation for understanding the aerodynamic mechanisms underlying active flight control.

Double Row of Vortices with Arbitrary Stagger

1929 ◽  
Vol 25 (2) ◽  
pp. 132-138 ◽  
Author(s):  
L. Rosenhead
Keyword(s):  
Detailed Account ◽  
Double Row ◽  
Von Karman ◽  
Karman Street ◽  
The Stability ◽  
Von Kármán

The investigations of von Karman dealing with the unsymmetrical double row of vortices in an infinite sea of liquid are well known. He found that the unsymmetrical double row is stable when, and only when, cosh2πa/b = 2, where 2a is the distance between the two rows and 2b is the distance between consecutive vortices on the same row. A detailed account of the stability of the Karman street and of the symmetrical double row has been given by Lamb, and it has been shown that the symmetrical double row is unstable for all values of the ratio a/b. The object of this paper is to investigate the stability of a double row of vortices of arbitrary stagger. We define a double row of stagger 2l to be the system formed by positive vortices at the points (2nb + l, a) and negative vortices at (2mb − l, − a), where m and n assume all integral values from − ∞ to + ∞. The vortices are thus neither exactly “in step” nor exactly “out of step.” When l = 0 the system reduces to the symmetrical double row and when the system is the unsymmetrical double row.

Characterization of von Karman street with seal whisker-like sensor

2012 ◽  
Author(s):  
Hendrik Hans ◽  
Jianmin Miao ◽  
Michael Triantafyllou
Keyword(s):  
Von Karman ◽  
Karman Street ◽  
Von Kármán
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