Vortex shedding by a circular cylinder in lock-exchange density current 

2021 ◽  
Author(s):  
Ana M Ricardo ◽  
Giovanni Di Lollo ◽  
Moisés Brito ◽  
Claudia Adduce ◽  
Rui M.L. Ferreira
Keyword(s):  
Circular Cylinder ◽  
Fresh Water ◽  
Vortex Shedding ◽  
Gravity Current ◽  
Shear Instability ◽  
Density Current ◽  
Bluff Bodies ◽  
Density Currents ◽  
Ambient Fluid ◽  
Dense Fluid

<p>Flow around bluff bodies have been attracting the interest of the research community for more than a century. The physical mechanisms associated with the vortex shedding in the wake of bluff bodies is still of fundamental research interest. However, flow-structure interaction in density currents has not received enough attention. The transient nature of the interaction between the density driven flow and a stationary object constitutes the motivation for the present laboratory study aiming at investigating the vortex generation and fate on the wake of a circular cylinder in a density current.</p><p>The experiments were conducted in a horizontal and rectangular cross-section channel with 3.0 m long, 0.175 m wide and 0.4 m deep. The gravity current was generated using the classic lock-exchange configuration. A sliding stainless-steel gate with 1 mm thickness, sealed by PVC board glued in the sidewall, was positioned at 0.3 m from the left hand side of the channel. The experiment starts when the gate is suddenly removed, leaving the dense fluid to flow along the bottom of the channel, while the ambient fluid moves above in the opposite direction. The dense fluid consists in a mixture of fresh water and salt while the ambient fluid is a solution fresh water and ethanol (96%). The amount of salt and alcohol added in each mixture was determined in order to obtain a given density difference and to ensure the same refractive index in both fluids. Two different currents were tested with reduced gravity equal to 0.06 ms<sup>-2</sup> and 0.24 ms<sup>-2</sup>. For each test ten repetitions were carried out. Instantaneous velocity maps were acquired with a Particle Image Velocimetry system at 15 Hz. Polyamide seeding particles of density equal to 1.03 were added in both dense and ambient fluids.</p><p> The Reynolds number varied between 1500 and 4000. The results show that vortex shedding varies as the current reaches and overtakes the cylinder. Boundary layer detachment and shear instability is initiated shortly before the snout reaches the cylinder. A pattern of well-defined symmetrical vortexes is formed as a result of the initial shear instability. As the head of the current engulfs the cylinder, stronger turbulence diffusion contributes to reduce vortex coherence. Vortexes are smaller and detach sooner, while is not clear if shedding is alternate or simply random. The formation length is smaller than that of a steady flow with the same Re. When the back of the current passes, the formation length is increased and vortex shedding becomes periodical again. A striking feature is that the Von Kármán street is frequently symmetrical rather than exhibiting a pattern of alternate vortices.</p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

3-D Simulation of Turbulent Density

2006 ◽  
Author(s):  
S. Hormozi ◽  
B. Firoozabadi ◽  
H. Ghasvari Jahromi ◽  
H. Afshin
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Fine Particles ◽  
Density Current ◽  
Data Sets ◽  
Density Currents ◽  
Ambient Fluid ◽  
Sluice Gate ◽  
Dense Fluid ◽  
Sloping Surface

Density current is a dense fluid, which is continuously released from a source and spreads down a sloping surface inside a lighter, motionless fluid. A low-Reynolds number k-ε model (Launder and Sharma, 1974) has been used to simulate the behavior of 3-D density currents. Density current with a uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. The model has been verified with the experimental data sets. Although the k-ε Launder and Sharma model is applied here to a conservative density current, it seems the analysis is valid in general for turbidity current laden with fine particles.

scholarly journals Effect of Cylinder Gap Ratio on The Wake of a Circular Cylinder Enclosed by Various Perforated Shrouds

CFD letters ◽  
2021 ◽  
Vol 13 (4) ◽  
pp. 51-68
Author(s):  
Nurul Azihan Ramli ◽  
Azlin Mohd Azmi ◽  
Ahmad Hussein Abdul Hamid ◽  
Zainal Abidin Kamarul Baharin ◽  
Tongming Zhou
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Control Method ◽  
Lift Coefficient ◽  
Base Case ◽  
Bluff Bodies ◽  
Ansys Fluent ◽  
Gap Ratio ◽  
Spacing Ratio

Flow over bluff bodies produces vortex shedding in their wake regions, leading to structural failure from the flow-induced forces. In this study, a passive flow control method was explored to suppress the vortex shedding from a circular cylinder that causes many problems in engineering applications. Perforated shrouds were used to control the vortex shedding of a circular cylinder at Reynolds number, Re = 200. The shrouds were of non-uniform and uniform holes with 67% porosity. The spacing gap ratio between the shroud and the cylinder was set at 1.2, 1.5, 2, and 2.2. The analysis was conducted using ANSYS Fluent using a viscous laminar model. The outcomes of the simulation of the base case were validated with existing studies. The drag coefficient, Cd, lift coefficient, Cl and the Strouhal number, St, as well as vorticity contours, velocity contours, and pressure contours were examined. Vortex shedding behind the shrouded cylinders was observed to be suppressed and delayed farther downstream with increasing gap ratio. The effect was significant for spacing ratio greater than 2.0. The effect of hole types: uniform and non-uniform holes, was also effective at these spacing ratios for the chosen Reynolds number of 200. Specifically, a spacing ratio of 1.2 enhanced further the vortex intensity and should be avoided.

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.

Suppression of Vortex Shedding Downstream of a Circular Cylinder Using Permeable Cylinders in Shallow Water

2013 ◽  
Author(s):  
Göktürk Memduh Özkan ◽  
Hüseyin Akıllı
Keyword(s):  
Circular Cylinder ◽  
Flow Control ◽  
Shallow Water ◽  
Flow Structure ◽  
Vortex Shedding ◽  
Outer Cylinder ◽  
Vortex Formation ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Different Diameters

The characteristics of the flow around a 50mm circular cylinder surrounded by a permeable outer cylinder were investigated by Particle Image Velocimetry (PIV) and flow visualization techniques in order to control the unsteady flow structure downstream of the cylinder in shallow water. The effect of outer permeable cylinder with a porosity of β = 0.4 on the flow control was studied using five different diameters; D = 60, 70, 80, 90, 100mm. Depth-averaged free stream velocity was kept constant as U = 170mm/s corresponding to a Reynolds number of Re = 8500 and the water height was adjusted to hw = 25mm throughout the study. The results clearly showed that the outer permeable cylinder significantly affects the flow structure of the inner cylinder. It was found that by the existence of outer cylinder, the frequency of unsteady vortex shedding is reduced, vortex formation region is elongated and fluctuations are attenuated which are good indications of effective flow control. Owing to the results, optimum parameters were defined and suggested for the suppression of vortex-induced vibrations on bluff bodies.

Inclined gravity currents filling basins: the impact of peeling detrainment on transport and vertical structure

2017 ◽  
Vol 820 ◽  
pp. 400-423 ◽  
Author(s):  
Charlie A. R. Hogg ◽  
Stuart B. Dalziel ◽  
Herbert E. Huppert ◽  
Jörg Imberger
Keyword(s):  
Gravity Current ◽  
Uniform Density ◽  
Ambient Fluid ◽  
Transport Pathway ◽  
Density Profiles ◽  
Model Match ◽  
Dense Fluid ◽  
Vertical Density ◽  
Multiprocess Models ◽  
The Impact

Transport of dense fluid by an inclined gravity current can control the vertical density structure of the receiving basin in many natural and industrial settings. A case familiar to many is a lake fed by river water that is dense relative to the lake water. In laboratory experiments, we pulsed dye into the basin inflow to visualise the transport pathway of the inflow fluid through the basin. We also measured the evolving density profile as the basin filled. The experiments confirmed previous observations that when the turbulent gravity current travelled through ambient fluid of uniform density, only entrainment into the dense current occurred. When the gravity current travelled through the stratified part of the ambient fluid, however, the outer layers of the gravity current outflowed from the current by a peeling detrainment mechanism and moved directly into the ambient fluid over a large range of depths. The prevailing model of a filling box flow assumes that a persistently entraining gravity current entrains fluid from the basin as the current descends to the deepest point in the basin. This model, however, is inconsistent with the transport pathway observed in visualisations and poorly matches the stratifications measured in basin experiments. The main contribution of the present work is to extend the prevailing filling box model by incorporating the observed peeling detrainment. The analytical expressions given by the peeling detrainment model match the experimental observations of the density profiles more closely than the persistently entraining model. Incorporating peeling detrainment into multiprocess models of geophysical systems, such as lakes, will lead to models that better describe inflow behaviour.

scholarly journals An Analysis of Two-Dimensional Stratified Gravity Current Flow using Open FOAM

2018 ◽  
Vol 7 (4.35) ◽  
pp. 589
Author(s):  
W.K Lam ◽  
L. Chan ◽  
H. Hasini ◽  
A. Ooi
Keyword(s):  
Current Flow ◽  
Gravity Current ◽  
Propagation Speed ◽  
Ambient Air ◽  
Front Velocity ◽  
Test Case ◽  
Two Dimensional ◽  
Ambient Fluid ◽  
Dense Fluid ◽  
Reduced Density

Direct numerical simulations (DNSs) of two-dimensional stratified gravity-current are simulated using OpenFOAM. Three different aspect ratio, h0/l0 (where h0 is the height of the dense fluid and l0 is the length of the dense fluid) are simulated with stratification ranging from 0 (homogenous ambient) to 0.2 with a constant Reynolds number (Re) of 4000. The stratificationof the ambient air is determined by the density difference between the bottom and the top walls of the channel (ρb- ρ0, where ρb is the density at the bottom of the domain and ρ0 is the density at the top). The magnitude of the stratification (S=ԑb/ԑ) can be determined by calculating the reduced density differences of the bottom fluid with the ambient fluid (ԑb = (ρb- ρ0)/ ρ0) and the dense fluid with the ambient fluid (ԑ = (ρc-ρ0)/ ρ0, where ρc represents the density of the dense fluid). The configuration of the simulation is validated with a test case from Birman, Meiburg & Ungraish and the contour and front velocity (propagation speed) were in good agreement. The gravity current flow in the stratified ambient is analyzed qualitatively and compared with the gravity current in the homogenous ambient. Gravity current in homogenous ambient (S=0) and weak stratification (S=0.2) are supercritical flow where the flow is turbulent and Kelvin-Helmholtz (K-H) billow formed behind the gravity current head. The front location of the gravity is reduced as the stratification increase and denotes that the front velocity of the gravity current is reduced by the stratification.

Vortex shedding and lock-on of a circular cylinder in oscillatory flow

1986 ◽  
Vol 170 ◽  
pp. 527-544 ◽  
Author(s):  
C. Barbi ◽  
D. P. Favier ◽  
C. A. Maresca ◽  
D. P. Telionis
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Oscillatory Flow ◽  
Published Data ◽  
Bluff Bodies ◽  
Driving Frequency ◽  
Frequency Limit ◽  
Mean Velocity ◽  
Shedding Frequency ◽  
Friction Measurements

An experimental study has been made of a circular cylinder in steady and oscillatory flow with non-zero mean velocity up to a Reynolds number of 40000. The results for the stationary cylinder are in close agreement with previously published data. Skin-friction measurements revealed the amplitude of fluctuation of the boundary layer for different angular locations. It has been universally accepted that bluff bodies shed vortices at their natural frequency of shedding (Strouhal frequency), or, when synchronized with an external unsteadiness, at the frequency of the disturbance or half of it, depending of the direction of the unsteadiness. Our findings, instead, indicate that the shedding frequency may vary smoothly with the driving frequency before locking on its subharmonic. Moreover, the present results indicate that, at the lowest frequency limit of lock-on, vortices are shed simultaneously on both sides of the model. A more traditional alternate pattern of vortex shedding is then recovered at higher driving frequencies.

A flow-front instability in viscous gravity currents

1998 ◽  
Vol 369 ◽  
pp. 1-21 ◽  
Author(s):  
DON SNYDER ◽  
STEPHEN TAIT
Keyword(s):  
Gravitational Instability ◽  
Gravity Current ◽  
High Viscosity ◽  
Gravity Currents ◽  
Ambient Fluid ◽  
Cell Width ◽  
Dense Fluid ◽  
Unstable Layer ◽  
Gravitational Mechanism ◽  
Hele Shaw Cell

We describe an instability that appears at the front of laminar gravity currents as they intrude into a viscous, miscible ambient fluid. The instability causes a current to segment into fingers aligned with its direction of flow. In the case of currents flowing along a rigid floor into a less dense fluid, the case of primary interest here, two mechanisms can produce this instability. The first is gravitational and arises because the nose of the gravity current is elevated above the floor and overrides a buoyantly unstable layer of ambient liquid. The second is a form of viscous fingering analogous to a Saffman–Taylor instability in a Hele-Shaw cell. Whereas the ambient fluid must be more viscous than the current in order for the latter instability to occur, the gravitational instability can occur even if the ambient fluid is less viscous, as long as it is viscous enough to elevate the nose of the current and trap a layer of ambient fluid. For the gravitational mechanism, which is most important when the current and ambient fluids have comparable viscosities, the wavelength when the instability first appears is proportional to a length scale constructed with the viscosity, the flux and the buoyancy. The Saffman–Taylor-type mechanism is most important when the ambient liquid is much more viscous than the current. We have carried out experiments with miscible fluids in a Hele-Shaw cell that show that, at the onset of instability, the ratio of the finger wavelength to the cell width is a constant approximately equal to 2. This result is explained by using the principle that the flow tends to minimize the dissipation associated with the finger perturbation. For the gravity currents with high viscosity ratios, the ratio of the wavelength to the current thickness is also a constant of about 2, apparently consistent with the same mechanism. But, further analysis of this instability mechanism is required in order to assess its role in wavelength selection for gravity currents.

scholarly journals The Effect of Slat Opening on Vortex Shedding Behind a Circular Cylinder

2019 ◽  
Vol 8 (4) ◽  
pp. 6879-6885
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Governing Equation ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Bluff Bodies ◽  
Vortex Induced Vibrations ◽  
Better Than ◽  
Laminar Model

Add-on devices are widely used as one of the means of suppressing vortex induced vibrations from bluff bodies. The present study numerically investigates flow over a circular cylinder attached by an axial slat. The axial slat were of uniform and non-uniform openings of 67% and 44% porosity. The governing equation was solved using viscous-laminar model at Reynolds number, Re=300. It was found that the presence of the axial slats significantly suppressed vortex shedding behind the circular cylinder. The non-uniform slats showed longer vortex formation length with lower drag, in comparison to that of the uniform slats. In addition, the slats with 67% porosity of both uniform and non-uniform openings suppressed vortex better than that of 44% porosity slats, indicated by the longer vortex formation length and weaker intensity of vortices.

The evaluation of a density current experiment as a verification tool of a secondary clarifier model

2003 ◽  
Vol 47 (12) ◽  
pp. 113-118 ◽  
Author(s):  
J. Kim ◽  
S. Kim ◽  
J. Yoon
Keyword(s):  
Steady State ◽  
Activated Sludge ◽  
Gravity Current ◽  
Settling Tank ◽  
Sewage Treatment Plant ◽  
Treatment Plant ◽  
Bottom Layer ◽  
Density Current ◽  
Density Currents ◽  
The Impact

The prediction of the spatial and temporal flow regime is important for the efficient design and operation of the settling tank. The impact of the gravity current has been investigated by the temperature difference or artificial glass particles. This experimental study is designed to obtain the steady state of stratified flow with the consideration of the heavy density flow. Two different approaches were performed at lab scale considering the hydraulic similarity. One is the investigation of a heavy density current of saline water intrusion. The characteristics of the internal hydraulics are monitored using Particle Image Velocimetry (PIV). As a result of the saline intrusion, the internal flow established a continual density stratification of 3-layers in steady state. The other is the movement of activated sludge from a sewage treatment plant. The video image is captured to analyze the behavior of activated sludge in the settling tank. The experiments with various inlet baffle conditions suggest that the hydraulic characteristics of the two experiments are significantly different. As the distance between the baffle and inlet point was decreased, significant strength of saline density currents in the bottom layer was observed. However, the strength of the velocity in the bottom density currents appeared to be the opposite in the case of the activated sludge. The reason for the slower density current in the bottom layer is associated with the compressibility of activated sludge characteristics.

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