scholarly journals Beyond the Kármán gait: knifefish swimming in periodic and irregular vortex streets

2021 ◽  
pp. jeb.238808
Author(s):  
Victor M. Ortega-Jimenez ◽  
Christopher P. Sanford
Keyword(s):  
Vortex Shedding ◽  
Flow Speed ◽  
Laminar Flows ◽  
Oscillating Cylinder ◽  
Pectoral Fins ◽  
Left Behind ◽  
Moving Cylinder ◽  
Vortex Streets ◽  
Fixed Cylinder ◽  
Ribbon Fin

Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U∞∼1 – 5 Bl/s) and in periodic vortex streets (U∞∼2 – 4 Bl/s). Two different cylinders (∼2 and ∼3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (∼2 cm diameter; U∞∼2 Bl/s). In laminar flows knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U∞. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins and resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending, ribbon and pectoral fins movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder but, when placed one-cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

Lock-in regions of laminar flows over a streamwise oscillating circular cylinder

2018 ◽  
Vol 858 ◽  
pp. 315-351 ◽  
Author(s):  
Ki-Ha Kim ◽  
Jung-Il Choi
Keyword(s):  
Circular Cylinder ◽  
Aerodynamic Force ◽  
Lift Coefficient ◽  
Lower Boundary ◽  
Laminar Flows ◽  
Velocity Range ◽  
Frequency Range ◽  
Vortex Pattern ◽  
Moving Cylinder ◽  
Lock In

In this paper, flow over a streamwise oscillating circular cylinder is numerically simulated to examine the effects of the driving amplitude and frequency on the distribution of the lock-in regions in laminar flows. At $Re=100$, lock-in is categorized according to the spectral features of the lift coefficient as two different lock-in phenomena: harmonic and subharmonic lock-in. These lock-in phenomena are represented as maps on the driving amplitude–frequency plane, which have subharmonic lock-in regions and two harmonic lock-in regions. The frequency range of the subharmonic region is shifted to lower frequencies with increasing amplitude, and the lower boundary of this subharmonic region is successfully predicted. A symmetric harmonic region with a symmetric vortex pattern is observed in a certain velocity range for a moving cylinder. Aerodynamic features induced by different flow patterns in each region are presented on the driving amplitude–frequency plane. The lock-in region and aerodynamic features at $Re=200$ and $40$ are compared with the results for $Re=100$. A subharmonic region and two harmonic regions are observed at $Re=200$, and these show the same features as for $Re=100$ at a low driving amplitude. Lock-in at $Re=40$ also shows one subharmonic region and two harmonic regions. However, compared with the $Re=100$ case, the symmetric harmonic lock-in is dominant. The features of aerodynamic force at $Re=200$ and $40$ are represented on a force map, which shows similar characteristics in corresponding regions for the $Re=100$ case.

Turbulent vortex streets and the entrainment mechanism of the turbulent wake

1974 ◽  
Vol 62 (1) ◽  
pp. 11-31 ◽  
Author(s):  
Demosthenes D. Papailiou ◽  
Paul S. Lykoudis
Keyword(s):  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Turbulent Wake ◽  
Vortex Street ◽  
Experimental Measurements ◽  
Formation Region ◽  
Turbulent Vortex ◽  
Geometrical Characteristics ◽  
Vortex Streets ◽  
Longitudinal Distance

The results of an experimental investigation of a turbulent vortex street in the range 103 [lsim ] Re [lsim ] 2 × 104 are presented. The vortex street was created by the motion of a circular cylinder in a motionless fluid (mercury). Photographs obtained showed that the turbulent street, created by the vortex shedding behind the cylinder, persisted at longer downstream distances and higher Reynolds numbers than previously reported in the literature. A theory was developed to account for the experimental measurements pertaining to the change of the geometrical characteristics, h (distance between the two rows of vortices) and α (longitudinal distance between two consecutive vortices on the same row), of the street in the downstream direction. The implications of the structure of the vortex street on the entrainment mechanism of the turbulent wake are discussed. Some observations of the flow in the formation region of the vortices are discussed in relation to existing work.

Sources of Excitation in Tube Banks Due to Vortex Shedding

1986 ◽  
Vol 200 (4) ◽  
pp. 293-301 ◽  
Author(s):  
R S Hill ◽  
K C Shim ◽  
R I Lewis
Keyword(s):  
Vortex Shedding ◽  
Velocity Fluctuation ◽  
Power Spectra ◽  
Vortex Street ◽  
Experimental Investigations ◽  
Causal Mechanism ◽  
Hot Wire ◽  
Tube Bank ◽  
Vortex Streets ◽  
Tube Banks

This paper describes experimental investigations of vortex shedding patterns in staggered and in-line tube banks consisting of four rows with transverse pitch to diameter ratios PT/d of 2.67 and longitudinal pitch to diameter ratios PL/d of 2.31. Single hot wire probes were used to obtain velocity power spectra in order to identify discrete frequencies of velocity fluctuation. Double hot wire probes provided phase correlations which could indicate conclusively the presence of vortex streets. Quite different results were obtained for the staggered and in-line geometries. While vortex street fluctuations were observed in both, an additional higher frequency fluctuation was observed in the staggered tube bank, the causal mechanism for which remains obscure.

Modification of Vortex Shedding in the Synchronization Range

1982 ◽  
Vol 104 (4) ◽  
pp. 513-517 ◽  
Author(s):  
M. M. Zdravkovich
Keyword(s):  
Fluid Mechanics ◽  
Flow Visualization ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Maximum Amplitude ◽  
Oscillating Cylinder ◽  
Systematic Analysis ◽  
Lower Region ◽  
Simple Fluid ◽  
Synchronization Phenomenon

One aspect of the synchronization phenomenon, which has attracted little attention so far, is the timing of vortex shedding in relation to the displacement of a bluff body. Systematic analysis of flow visualization within the synchronization range revealed that the jump in fluctuating forces had a simple fluid mechanics origin. The oscillating cylinder imposed not only its frequency to the wake behind it but also the timing of the vortex shedding. In the lower region of the synchronization range, the vortex formed on one side of the cylinder was shed when the cylinder was near to the maximum amplitude on the opposite side. This timing changed suddenly in the upper synchronization range where the vortex of the same circulation as before was shed when the cylinder reached the maximum amplitude on the same side.

Vortex Formation in the Wake of a Vibrating, Flexible Cable

1974 ◽  
Vol 96 (4) ◽  
pp. 317-322 ◽  
Author(s):  
S. E. Ramberg ◽  
O. M. Griffin
Keyword(s):  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Hot Wire ◽  
Near Wake ◽  
Formation Region ◽  
Vortex Streets ◽  
Region Length ◽  
Karman Vortex ◽  
Flexible Cables

The von Karman vortex streets formed in the wakes of vibrating, flexible cables were studied using a hot-wire anemometer. All the experiments took place in the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the wake formation. Detailed measurements were made of the vortex formation flow for Reynolds numbers between 230 and 650. As in the case of vibrating cylinders, the formation-region length is dependent on a shedding parameter St* related to the natural Strouhal number and the vibrational conditions. Furthermore, the near wake configuration is found to be dependent on the local amplitude of vibration suggesting that the vibrating cylinder rseults are directly applicable in that region.

Influence of Unsteady Vortex Structures on Noise Reduction of Windscreens

2003 ◽  
Author(s):  
Z. Charlie Zheng ◽  
N. Zhang
Keyword(s):  
Fluid Dynamics ◽  
Noise Reduction ◽  
Vortex Shedding ◽  
Second Harmonic ◽  
Vortex Structures ◽  
Oscillating Cylinder ◽  
Wind Noise ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Wind Noise Reduction

The effects of windscreens on low-frequency wind noise reduction were previously investigated using a steady-state computational fluid dynamics model. The current concentration is on higher frequencies where the wind noise reduction is no longer independent of frequencies, and unsteady fluid dynamics is required to provide pressure fluctuation information on the windscreen surface. Flow across an oscillating cylinder is studied as a model problem. An immersed boundary method has been developed to compute the fluid flow. Pressure fluctuations on the surface of a rigid, impermeable windscreen are obtained from the flow computation. Noise reduction effects inside of the windscreen are then calculated based on the integration of surface pressure distributions caused by unsteady vortex structures. The results show that for a cylinder oscillating at a frequency close to the natural vortex shedding frequency, the peak noise sensed at the center of the cylinder is at twice of the oscillation frequency and its second and third harmonics. For a non-oscillating cylinder, the peak noise sensed at the center is at the vortex shedding frequency itself and its second harmonic.

scholarly journals Very Large-Eddy Simulations of the Flow Past an Oscillating Cylinder at a Subcritical Reynolds Number

2020 ◽  
Vol 10 (5) ◽  
pp. 1870
Author(s):  
Zhongying Xiong ◽  
Xiaomin Liu
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Lift Coefficient ◽  
Vortex Pair ◽  
Excitation Amplitude ◽  
Oscillating Cylinder ◽  
Flow Past ◽  
Large Eddy ◽  
Lift And Drag ◽  
Strong Vortex

This work focuses on flow past a circular cylinder at a subcritical Reynolds number. Although this classical study has been a concern for many years, it is still a challenging task due to the complexity of flow characteristics. In this paper, a high-efficiency very large-eddy simulation method is adopted and verified in order to handle the oscillating boundary. A series of numerical simulations are conducted to investigate the transient flow around the oscillating cylinder. The results show that the vortex shedding mode varies with an increase in the excitation amplitude and the excitation frequency. Vortex shedding is a lasting process under the condition of a low excitation amplitude that leads to irregular fluctuations of the lift and drag coefficients. For a vortex shedding mode that exhibits a strong vortex pair and a weak vortex pair or a weak single vortex, the temporal evolution of the lift coefficient of the oscillating cylinder shows irregular ”jumping” at a specific time per cycle corresponding to the shedding of the strong vortex pair. The vortex shedding mode and the frequency and time of the vortex shedding co-determine the temporal evolutions of the lift and drag coefficient.

Vortex Forces on an Oscillating Cylinder

2002 ◽  
Author(s):  
J. Carberry ◽  
J. Sheridan ◽  
D. Rockwell
Keyword(s):  
Vortex Shedding ◽  
Lift Force ◽  
Free Stream ◽  
Discontinuous Change ◽  
Near Wake ◽  
Oscillating Cylinder ◽  
Sinusoidal Oscillations ◽  
Lift And Drag ◽  
Wake State

The characteristic wake states of a cylinder undergoing forced sinusoidal oscillations transverse to the free-stream are studied by simultaneously measuring the structure of the near wake and the forces on the cylinder. The wake exhibits two distinctly different wake states. The transition between these states corresponds to a large discontinuous change in the phase of both the total and vortex forces on the cylinder, as well as the mode and phase of vortex shedding. Over the range of flow and oscillation parameters studied the vortex lift and drag phases collapse towards values that depend primarily on wake state. This is in contrast with the phase of the total lift force, which varied significantly with both A/D and Re.

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