scholarly journals Large structure in the far wakes of two-dimensional bluff bodies

1988 ◽  
Vol 190 ◽  
pp. 265-298 ◽  
Author(s):  
John M. Cimbala ◽  
Hassan M. Nagib ◽  
Anatol Roshko
Keyword(s):  
Circular Cylinder ◽  
Hydrodynamic Instability ◽  
Broad Band ◽  
Parametric Instability ◽  
Driving Mechanism ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Large Structure ◽  
Flat Plates ◽  
Far Wake

Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Kármán-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased.The far-wake structure does not depend directly on the scale or frequency of Kármán vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re ≈ 6000), which do not initially shed Kármán-type vortices into the wake.Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.

The vortex-shedding process behind two-dimensional bluff bodies

1982 ◽  
Vol 116 ◽  
pp. 77-90 ◽  
Author(s):  
A. E. Perry ◽  
M. S. Chong ◽  
T. T. Lim
Keyword(s):  
Circular Cylinder ◽  
Flow Visualization ◽  
Vortex Shedding ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Closed Cavity ◽  
Time Exposure ◽  
Starting Flow ◽  
Visualization Techniques ◽  
Insight Into

Using a variety of flow-visualization techniques, the flow behind a circular cylinder has been studied. The results obtained have provided a new insight into the vortex-shedding process. Using time-exposure photography of the motion of aluminium particles, a sequence of instantaneous streamline patterns of the flow behind a cylinder has been obtained. These streamline patterns show that during the starting flow the cavity behind the cylinder is closed. However, once the vortex-shedding process begins, this so-called ‘closed’ cavity becomes open, and instantaneous ‘alleyways’ of fluid are formed which penetrate the cavity. In addition, dye experiments also show how layers of dye and hence vorticity are convected into the cavity behind the cylinder, and how they are eventually squeezed out.

A Free-Streamline Model for Bluff Bodies in Confined Flow

1977 ◽  
Vol 99 (3) ◽  
pp. 585-592 ◽  
Author(s):  
V. J. Modi ◽  
S. E. El-Sherbiny
Keyword(s):  
Potential Flow ◽  
Bluff Body ◽  
Flow Model ◽  
Circular Cylinders ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Flat Plates ◽  
Surface Loading ◽  
Confined Flow ◽  
Potential Flow Model

A potential flow model is presented for two-dimensional symmetrical bluff bodies under wall confinement. It provides a procedure for predicting surface loading on a bluff body over a range of blockage ratios. Experimental results with normal flat plates and circular cylinders for blockage ratios up to 35.5 percent substantiate the validity of the approach.

scholarly journals Transition to the secondary vortex street in the wake of a circular cylinder

2019 ◽  
Vol 867 ◽  
pp. 691-722 ◽  
Author(s):  
Hongyi Jiang ◽  
Liang Cheng
Keyword(s):  
Circular Cylinder ◽  
Transverse Velocity ◽  
Broad Band ◽  
Flow Characteristics ◽  
Vortex Street ◽  
Bluff Bodies ◽  
Secondary Vortex ◽  
Frequency Spectra ◽  
Vortex Streets ◽  
Merging Process

Instabilities and flow characteristics in the far wake of a circular cylinder are examined through direct numerical simulations. The transitions to the two-layered and secondary vortex streets are quantified by a new method based on the time-averaged transverse velocity field. Two processes for the transition to the secondary vortex street are observed: (i) the merging of two same-sign vortices over a range of low Reynolds numbers ($Re$) between 200 and 300, and (ii) the pairing of two opposite-sign vortices, followed by the merging of the paired vortices into subsequent vortices, over a range of $Re$ between 400 and 1000. Single vortices may be generated between the merging cycles due to mismatch of the vortices. The irregular merging process results in flow irregularity and an additional frequency signal $f_{2}$ (in addition to the primary vortex shedding frequency $f_{1}$) in the two-layered and secondary vortex streets. In particular, a gradual energy transfer from $f_{1}$ to $f_{2}$ with distance downstream is observed in the two-layered vortex street prior to the merging. The frequency spectra of $f_{2}$ are broad-band for $Re=200$–300 but become increasingly sharp-peaked with increasing $Re$ because the vortex merging process becomes increasingly regular. The ratio of the sharp-peaked frequencies $f_{2}$ and $f_{1}$ is equal to the ratio of the numbers of vortices observed after and before the merging. The general conclusions drawn from a circular cylinder are expected to be applicable to other bluff bodies.

Two-Dimensional Numerical Simulation of Vortex Induced Vibration of a Circular Cylinder

2007 ◽  
Author(s):  
Juan B. V. Wanderley ◽  
Gisele H. B. Souza ◽  
Carlos Levi
Keyword(s):  
Circular Cylinder ◽  
Quantitative Agreement ◽  
Curvilinear Coordinates ◽  
Petroleum Exploration ◽  
Bluff Bodies ◽  
Offshore Platforms ◽  
Two Dimensional ◽  
Governing Equations ◽  
Vortex Induced Vibration ◽  
Boussinesq Hypothesis

Vortex induced Vibration (VIV) plays a very important role in the offshore petroleum exploration. For example, risers used in oil extraction from the bottom of the sea to the offshore platforms are subjected to marine flows that may trigger dangerous VIV oscillations. Many researches have been spending a lot of efforts to understand the complicated flow around bluff bodies to control or even eliminate the VIV occurrence. Numerical simulations have been unsuccessful to predict the VIV amplitudes mainly because of the diffusive nature of the numerical methods. The present two-dimensional numerical investigation is a continuation of previous efforts trying to predict correct amplitudes of the VIV oscillations. The Beam and Warming scheme is used to solve the governing equations written in general curvilinear coordinates and the Boussinesq Hypothesis and the K-ε turbulence model are used to simulate the turbulent flow in the wake of a circular cylinder. The numerical results agreed qualitatively well with other data from the literature, but poor quantitative agreement was obtained confirming the difficulty of predicting amplitudes reported by other authors.

scholarly journals Parametric instability in a free-evolving warped protoplanetary disc

2020 ◽  
Vol 500 (3) ◽  
pp. 4248-4256
Author(s):  
Hongping Deng ◽  
Gordon I Ogilvie ◽  
Lucio Mayer
Keyword(s):  
Wave Equations ◽  
Hydrodynamic Instability ◽  
Parametric Instability ◽  
Low Viscosity ◽  
Crossing Time ◽  
Particle Hydrodynamics ◽  
Smoothed Particle ◽  
First Time ◽  
Bending Wave ◽  
Grid Based

ABSTRACT Warped accretion discs of low viscosity are prone to hydrodynamic instability due to parametric resonance of inertial waves as confirmed by local simulations. Global simulations of warped discs, using either smoothed particle hydrodynamics or grid-based codes, are ubiquitous but no such instability has been seen. Here, we utilize a hybrid Godunov-type Lagrangian method to study parametric instability in global simulations of warped Keplerian discs at unprecedentedly high resolution (up to 120 million particles). In the global simulations, the propagation of the warp is well described by the linear bending-wave equations before the instability sets in. The ensuing turbulence, captured for the first time in a global simulation, damps relative orbital inclinations and leads to a decrease in the angular momentum deficit. As a result, the warp undergoes significant damping within one bending-wave crossing time. Observed protoplanetary disc warps are likely maintained by companions or aftermath of disc breaking.

The response of a turbulent boundary layer to lateral divergence

1979 ◽  
Vol 94 (2) ◽  
pp. 243-268 ◽  
Author(s):  
A. J. Smits ◽  
J. A. Eaton ◽  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Circular Cylinder ◽  
Turbulent Boundary Layer ◽  
Structural Parameters ◽  
Axial Flow ◽  
Turbulence Structure ◽  
Two Dimensional ◽  
Transition Section ◽  
Two Dimensional Flow ◽  
Made In

Measurements have been made in the flow over an axisymmetric cylinder-flare body, in which the boundary layer developed in axial flow over a circular cylinder before diverging over a conical flare. The lateral divergence, and the concave curvature in the transition section between the cylinder and the flare, both tend to destabilize the turbulence. Well downstream of the transition section, the changes in turbulence structure are still significant and can be attributed to lateral divergence alone. The results confirm that lateral divergence alters the structural parameters in much the same way as longitudinal curvature, and can be allowed for by similar empirical formulae. The interaction between curvature and divergence effects in the transition section leads to qualitative differences between the behaviour of the present flow, in which the turbulence intensity is increased everywhere, and the results of Smits, Young & Bradshaw (1979) for a two-dimensional flow with the same curvature but no divergence, in which an unexpected collapse of the turbulence occurred downstream of the curved region.

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