Generation and Suppression of a Karman Vortex Street by Negative Buoyancy in Wake from a Cylinder with Fixed Separation Points.
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A wake from an elliptic cylinder with fixed separation points is analyzed numerically, and is clarified as follows : An isothermal Karman vortex street never occurs at Re<36, but a cooled Karman vortex street is generated by cooling a progressive wavy wake at 26≤ Re<36, where Re is the Reynolds number. The cooled Karman vortex street can be generated at smaller value of the absolute Richardson number in an elliptic cylinder wake than a circular cylinder wake. By cooling the Karman vortex street, a suppressed flow Is generated. Both the Karman vortex street and the suppressed flow have a double structure combined the motions due to both negative buoyancy and the forced flow. The dominant motion is due to the forced now at the Karman vortex street, and due to negative buoyancy for the suppressed flow. At the boundary between the Karman vortex street and the suppressed now, the dominant motion is exchanged, and the wake frequency and the mean Nusselt number decrease abruptly.Keywords:
Kármán vortex street
The application of a flapping wing mechanism offers a vast range of development possibilities for unmanned aerial vehicles (UAVs) and autonomous underwater vehicles (AUVs). The influence of wake transitions on flapping wing mechanism’s capabilities is not fully understood particularly at low Reynolds numbers. The numerical investigation of a symmetric airfoil performing sinusoidal heaving oscillations is performed to explore the wake transitions. The influence of heaving parameters on wake transitions when exposed to a constant velocity flow is investigated. The existence of reverse von Karman vortex street, deflected wake and chaotic wake is observed. The wake deflection is found to switch its direction before transforming into a chaotic wake. The coherent structures and its evolution with the flow are presented using proper orthogonal decomposition (POD). The underlying structures and their interactions for different wake situations are identified. Correlations for the nondimensional maximum velocity in the wake in terms of frequency and amplitude is proposed. The wake dynamics is found to depend significantly on the leading edge vortices. The time-varying velocity fluctuations in the flow field are presented and discussed in detail. The velocity fluctuation contours are used to identify the regions of momentum transfer. The transient nature of the flow field is studied using the phase plot. A transition route from the periodic to chaotic regime though a quasi-periodic regime is established using time series analysis. The wake transitions are observed to be more sensitive towards heaving frequency than the heaving amplitude.
Kármán vortex street
Flapping
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The process of the formation of a vortex street in the wake behind a flat plate set parallel to a uniform flow was investigated in a low speed wind tunnel. The vorticity distributions in the wake were calculated from the measured velocities using Taylor's hypothesis.
Kármán vortex street
Wake turbulence
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The effect of a discontinuous cylinder geometry on the near wake structures was investigated experimentally. This 'discontinuous' circular cylinder has gaps so that solid segments 5D long are followed by gaps 2.5D long, in a repeating pattern, where D is the diameter of the cylinder. A thin steel plate was used to hold all of the cylinder pieces together. Thus, a three-dimensional (3D) wake was created at the origin with the intent to force the near wake flow to have similar structural characteristics to the far wake behind an 'infinite' cylinder, i.e., a near wake flow with horseshoes or double rollers formed by rapid kinking of Karman-like vortices. Since the kinetic energy associated with the fluctuations of these near-wake 3D vortical structures was high, the flow system was considered suitable to clarify the role of these velocity patterns in the entrainment process of wake flows, which is still the subject of controversy. Particle image velocimetry (PIV) measurements were made in the wake behind the discontinuous cylinder up to x/D=56, at a Reynolds number Re=10,000. The development of double rollers resulting from the interaction between the high momentum flow through the gaps and the Karman-like vortices formed behind the solid cylindrical segments was confirmed. They have a dominant role in the initial wake growth which occurs under significant momentum transfer in the spanwise direction. This overall flow dynamics is similar to the momentum transfer that takes place at the scale of the intermittent turbulent bulges that protrude from the wake in the far region and that were reported to be associated with double rollers.
Kármán vortex street
Momentum (technical analysis)
Entrainment (biomusicology)
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In this paper an experiment on measurements of the wake of Goldfish carassius auratus swimming unrestricted was conducted in a water tunnel. Color liquid was used to visualize the wake of the fish and PIV was used to measure velocity field of the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the fish's wake and the Strouhal frequency of the fish is about 0.35 udner the different experimental conditions. The distribution of velocity and vorticity in the wake of Goldfish was measured by PIV and formation of reverse Karman vortex street in the wake was studied in a model experiment.
Kármán vortex street
Strouhal number
Water tunnel
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The formation of the Karman street of alternating vortices in the wake of cylinders is well known. It is important to clarify the flow characteristics in the wake for the field of engineering. An experimental study was performed to investigate the mechanism of flows around three rectangular cylinders with an in-line arrangement. The flow patterns were visualized by particles of the fluorescent dye and the ion exchange resin. The oscillation in the wake of rectangular cylinders was also visualized by the string method. The mechanism of the wake of cylinders with different lengths and gaps are clarified.
Kármán vortex street
Line (geometry)
Oscillation (cell signaling)
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The second wake transition occurs in the far wake of a bluff body. This transition destroys the Bénard–von Kármán vortex street originating in the near wake and produces a secondary vortex street with a lower characteristic frequency. We characterize the onset of the second wake for Reynolds numbers 50<Re<800 in a nearly two-dimensional soap film flow. The dimensionless distance between the cylinder and the onset of the second wake decreases with Reynolds number consistently with Re−1/2 power law. Our two-dimensional far-wake numerical simulations are in good agreement with the experiment.
Kármán vortex street
Dimensionless quantity
Vortex shedding
Bluff
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The motion of two rigid spheres settling in a viscous fluid inside a circular cylinder is investigated both theoretically and experimentally. The ratio of the particle radius to the cylinder radius, and the particle Reynolds number are small. The cylinder Reynolds number based on the cylinder radius is finite. At the cylinder Reynolds number 6.74,two settling spheres change the motion to unsymmetrical one from the symmetrical motion with respect to the cylinder axis. As cylinder Reynolds number become large, two spheres are forced towards the equilibrium positions. Method of matched asymptotic expansions was used.
Settling
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The flow visualization for the
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Vortex generator
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In this paper an experiment on wake of Goldfish swimming unrestricted was conducted in a water tunnel. Method of color liquid was used to visualize the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the wake and the Strouhal frequency of the fish is in the range 0.25-0.35. A 3D vortex ring chain model was presented.
Strouhal number
Kármán vortex street
Wake turbulence
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The influence of a glow electrical discharge on the wake behind two cylinders was studied. This effect consists in a redistribution of the power in the velocity pulsation spectrum due to the discharge action on fluid flow. To treat this observation, we need a simple model of intermittent turbulent wake in the vicinity of cylinders. A variant of such model has been developed in the form of two coupled van-der-Pole oscillators representing two interacting von Karman vortex streets behind the cylinders. According to the model, the set of the global wake modes and its concurrence are discussed. Accordingly, a mechanism of the glow discharge effect on the cylinders wake has been proposed.
Kármán vortex street
Wake turbulence
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