Interactions of Vortices of a Square Cylinder and a Rectangular Vortex Generator Under Couette–Poiseuille Flow

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Dilip K. Maiti ◽  
Rajesh Bhatt
Keyword(s):  
Poiseuille Flow ◽  
Lift Coefficient ◽  
Sudden Change ◽  
Vortex Generator ◽  
Wake Flow ◽  
Square Cylinder ◽  
Operational Parameters ◽  
Critical Spacing ◽  
Spacing Distance ◽  
Nonlinear Velocity

This study focuses on interactions of vortices generated by a family of eddy-promoting upstream rectangular cylinders (of different heights a* and widths b*) with the shear layers of a downstream square cylinder (of height A*) placed near a plane in an in-line tandem arrangement under the incidence of Couette–Poiseuille flow based nonuniform linear/nonlinear velocity profile. The dimensionless operational parameters are cylinders spacing distance S, ratio of heights r2=a*/A* (≤1), aspect ratio r1=b*/a* (≤1), Reynolds number Re (based on the velocity at height A* for Couette flow), ReU2 (based on the velocity at height 10A* for Couette–Poiseuille flow), and nondimensional pressure gradient P at the inlet. The governing equations are solved numerically through a pressure-correction-based iterative algorithm (SIMPLE) with the quadratic upwind interpolation for convective kinematics (QUICK) scheme for convective terms. The major issue of appearing multiple peaks in the spectrum of the fluctuating lift coefficient of the downstream cylinder is addressed and justified exhibiting the flow patterns. While considering the rectangular shape (for the upstream cylinder) and nonlinear velocity (at the inlet), the possibility of generating the unsteadiness in the steady wake flow of the downstream cylinder at a Re (based on height a*) less than the critical Re for the downstream cylinder is documented here. The dependence of flow characteristics of the downstream cylinder on the angle of incident linear velocity at specific S and r1 is also demonstrated here. It is observed that the discontinuous jump in the aerodynamic characteristics (due to a sudden change from one distinct flow pattern to the other in the critical spacing distance regime) is directly proportional to the height of the vortex generator. Increasing P under the same characteristic velocity causes the steady flow of cylinder(s) to convert to a periodic flow and reduces the critical spacing distance for the vortex generator.

Pulsating Flow Past a Square Cylinder: Analysis of Force Coefficient Spectra and Vortex Structure Development

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Thomas S. Fowler ◽  
Freddie D. Witherden ◽  
Sharath S. Girimaji
Keyword(s):  
Vortex Structure ◽  
Lift Coefficient ◽  
Excitation Frequency ◽  
Wake Flow ◽  
Square Cylinder ◽  
Spectral Character ◽  
Force Coefficient ◽  
Force Coefficients ◽  
Shedding Frequency ◽  
Lock In

Abstract This study examines the changes in force coefficients and wake flow structures of a square cylinder subject to pulsating in-flow at different frequencies. The Reynolds number is 200, according to previous literature. Over a range of forcing frequencies, a regime is observed where the shedding frequency scales with the forcing frequency rather than the natural shedding frequency, known as the lock-in phenomenon in literature. The change in spectral character across three frequency regimes—pre-lock-in, lock-in, and post-lock-in—are examined and characterized. During pre-lock-in, the shedding frequency remains equal to the natural shedding frequency. However, the corresponding peak in lift coefficient (CL) power spectral density (PSD) is a single decade larger than that of neighboring minima. This contrasts greatly with subsequent regimes where the amplitudes of the peaks are observed to be substantially larger than the amplitudes of neighboring minima. The onset of lock-in is sharp, and the corresponding excitation frequency is identified. The shedding frequency becomes a function of the forcing frequency within this regime, and the corresponding CL PSD peak is four decades larger than that of neighboring minima. The transition beyond the lock-in regime is gradual with peaks of the spectra broadening until separating into multiple discrete peaks. To comprehend the changes in the force coefficients, the vortex structure in the wake is characterized at different frequencies. The connection between the vortex development sequence and force profile is investigated, and z-vorticity probes are utilized to correlate these qualitative observations with prior quantitative analysis. Three-dimensional flow effects are also examined.

An Experimental Investigation of the Wake of an Axisymmetric Body with a Slanted Base

1983 ◽  
Vol 34 (1) ◽  
pp. 24-45 ◽  
Author(s):  
X.J. Xia ◽  
P.W. Bearman
Keyword(s):  
Drag Coefficient ◽  
Vortex Shedding ◽  
Free Stream ◽  
Sudden Change ◽  
Base Pressure ◽  
Wake Flow ◽  
Main Body ◽  
Angle Of Incidence ◽  
Slant Angle ◽  
Free Stream Turbulence

SummaryThe effect of base slant on the base pressure distribution, drag coefficient and vortex shedding characteristics of a model consisting of an axisymmetric main body with an ellipsoidal nose have been investigated for three fineness ratios; 3, 6 and 9. A sudden change in the drag coefficient and separated flow pattern is observed at a critical slant angle (for constant incidence) or at a critical angle of incidence (for a constant base slant angle). The tests confirm that the value of the maximum drag coefficient is extremely sensitive to angle of incidence. Measurements of the frequency of vortex shedding are presented and the structure of the wake is investigated using smoke visualization and hot-wire correlation measurements. The wake is found to be far less stable than that from a two-dimensional bluff body and the vortex structures are sometimes in-phase and sometimes out of phase across the wake. The effect of free-stream turbulence on this family of body shapes is observed to be different to that on three-dimensional blunt-faced bluff bodies. Free-stream turbulence is found to have a minimal effect on base pressure for slant angles giving a recirculating type near wake flow. When longitudinal vortices are present the addition of free-stream turbulence slightly reduces the magnitude of the peak suctions recorded on the base but has little effect on base drag.

scholarly journals Dynamical effect of the total strain induced by the coherent motion on local isotropy in a wake

2013 ◽  
Vol 720 ◽  
pp. 393-423 ◽  
Author(s):  
F. Thiesset ◽  
L. Danaila ◽  
R. A. Antonia
Keyword(s):  
Circular Cylinder ◽  
Large Scale ◽  
Bluff Body ◽  
Initial Conditions ◽  
Scale Up ◽  
Wake Flow ◽  
Total Strain ◽  
Coherent Motion ◽  
Square Cylinder ◽  
Local Isotropy

AbstractWe assess the extent to which local isotropy (LI) holds in a wake flow for different initial conditions, which may be geometrical (the shape of the bluff body which creates the wake) and hydrodynamical (the Reynolds number), as a function of the dynamical effects of the large-scale forcing (the mean strain, $ \overline{S} $, combined with the strain induced by the coherent motion, $\tilde {S} $). LI is appraised through either classical kinematic tests or phenomenological approaches. In this respect, we reanalyse existing LI criteria and formulate a new isotropy criterion based on the ratio between the turbulence strain intensity and the total strain ($ \overline{S} + \tilde {S} $). These criteria involve either time-averaged or phase-averaged quantities, thus providing a deeper insight into the dynamical aspect of these flows. They are tested using hot wire data in the intermediate wake of five types of obstacles (a circular cylinder, a square cylinder, a screen cylinder, a normal plate and a screen strip). We show that in the presence of an organized motion, isotropy is not an adequate assumption for the large scales but may be satisfied over a range of scales extending from the smallest dissipative scale up to a scale which depends on the total strain rate that characterizes the flow. The local value of this scale depends on the particular nature of the wake and the phase of the coherent motion. The square cylinder wake is the closest to isotropy whereas the least locally isotropic flow is the screen strip wake. For locations away from the axis, the study is restricted to the circular cylinder only and reveals that LI holds at scales smaller than those that apply at the wake centreline. Arguments based on self-similarity show that in the far wake, the strength of the coherent motion decays at the same rate as that of the turbulent motion. This implies the persistence of the same degree of anisotropy far downstream, independently of the scale at which anisotropy is tested.

Improving Ships’ Efficiency Using Energy Saving Devices (ESDs)

2015 ◽  
Author(s):  
Charinda L. Perera ◽  
Ema Muk-Pavic
Keyword(s):  
Boundary Layer ◽  
Potential Energy ◽  
Energy Saving ◽  
Flow Pattern ◽  
Energy Savings ◽  
Vortex Generator ◽  
Wake Flow ◽  
Container Ship ◽  
Trapezoidal Shape ◽  
Before And After

This research paper describes the CFD work carried out by the authors to investigate the potential energy savings achieved by attaching a Vortex Generator to the hull of a container ship. This is done by computing the flow pattern at the propeller plane before and after the addition of a Vortex Generator, to determine if the addition of the mentioned device presents the propeller with a more favourable inflow. The Vortex Generator is a trapezoidal shape fin attached to the hull which works by inducing vorticity and deflecting streamlines within the boundary layer, thus diverting and equalizing wake flow into the propeller.

Wake Interaction of a Square Cylinder With a Splitter Plate Boundary Layer at a Low Reynolds Number

2015 ◽  
Author(s):  
Smriti Srivastava ◽  
Sudipto Sarkar
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Bluff Body ◽  
Acoustic Noise ◽  
Vortex Formation ◽  
Plate Boundary ◽  
Wake Flow ◽  
Stream Velocity ◽  
Square Cylinder ◽  
Plate Length

One of the most important researches in bluff body aerodynamics is to control the shear layer evolution leading to vortex formation. This kind of research is closely associated with reduction of aerodynamics forces and acoustic noise. Passive and active control of wake-flow from bluff bodies have received a great deal of attention in the last few decades [1–4]. Keeping this in mind, authors investigate the interaction of a square cylinder (side of the square = a) wake with a flat plate (length L = a, width w = 0.1a) boundary layer positioned at various downstream locations close to the cylinder. The gap-to-side ratios are maintained at G/a = 0, 0.5, 1 and 2 (where G is the gap between square cylinder and plate), and the simulation is performed at a Reynolds number, Re = 100 (Re = U∞a/v, where U∞ is free stream velocity and v is kinematic viscosity). Instantaneous flow visualization, aerodynamic forces and vortex shedding frequencies for all cases are described to gain insight about the changes associated with wake of the cylinder when a short plate is kept in its downstream.

Flow Past Square Cylinders of Different Size With and Without Corner Modification

2015 ◽  
Author(s):  
Y. T. Krishne Gowda ◽  
Ravindra Holalu Venkatdas ◽  
Vikram Chowdeswarally Krishnappa
Keyword(s):  
Drag Coefficient ◽  
Flow Velocity ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Research Work ◽  
Tall Buildings ◽  
Convergence Criterion ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Flow Past

In many mechanical engineering applications, separated flows often appear around any object such as tall buildings, monuments, and towers are permanently exposed to wind. Similarly, piers, bridge pillars, and legs of offshore platforms are continuously subjected to the load produced by maritime or fluvial streams. These bodies usually create a large region of separated flow and a massive unsteady wake region in the downstream. The highly asymmetric and periodic nature of flow in the downstream has attracted the attention of physicists, engineers and CFD practitioners. A lot of research work is carried out for a square cylinder but flow past square cylinders with and without corner modification work is not taken up. This motivated to take up the task of flow past two different sized square cylinders, numerically simulated. A Reynolds number of 100 and 200 is considered for the investigation. The flow is assumed to be two dimensional unsteady and incompressible. The computational methodology is carried out once the problem is defined the first step in solving the problem is to construct a geometry on which the simulation is planned. Once the geometry is constructed, proper assignment of its boundaries in accordance to the actual physical state is to be done. The various boundary options that are to be set. After setting the boundary types, the continuum type is set. The geometry is discretized into small control volumes. Once the surface mesh is completed, the mesh details are exported to a mesh file, then exported to Fluent, which is CFD solver usually run in background mode. This helps to prioritize the execution of the run. The run would continue until the required convergence criterion is reached or till the maximum number of iterations is completed. Results indicate, in case of chamfered and rounded corners in square cylinder, there is decrease in the wake width and thereby the lift and drag coefficient values. The form drag is reduced because of a higher average pressure downstream when separation is delayed by corner modification. The lift coefficients of Square cylinder with corner modification decreases but Strouhal number increases when compared with a square cylinder without corner modification. Strouhal number remains same even if magnitude of oscillations is increased while monitoring the velocity behind the cylinder. Frequency of vortex shedding decreases with the introduction of second cylinder either in the upstream or downstream of the first cylinder. As the centre distance between two cylinders i.e., pitch-to-perimeter ratio is increased to 6,the behavior of the flow almost approaches to that of flow past a square cylinder of with and without modification of same condition. When the perimeter of the upstream cylinder with and without modification is larger than the downstream cylinder, the size of the eddies is always bigger in between the cylinders compared to the downstream of the second cylinder. The flow velocity in between the cylinders with and without corner modification are less compared to the downstream of the second cylinder. As the distance increases, the flow velocity in between the cylinders become almost equal to the downstream of the second cylinder. The results are presented in the form of streamlines, flow velocity, pressure distribution. drag coefficient, lift coefficient and Strouhal number.

scholarly journals Numerical Analysis of Wind Turbine Airfoil Aerodynamic Performance with Leading Edge Bump

2015 ◽  
Vol 2015 ◽  
pp. 1-8 ◽  
Author(s):  
Majid Asli ◽  
Behnam Mashhadi Gholamali ◽  
Abolghasem Mesgarpour Tousi
Keyword(s):  
Wind Turbine ◽  
Control Method ◽  
Flow Behavior ◽  
Renewable Energy Sources ◽  
Lift Coefficient ◽  
Turbine Blades ◽  
Vortex Generator ◽  
Leading Edge ◽  
Suction Side ◽  
Aerodynamic Performance

Aerodynamic performance improvement of wind turbine blade is the key process to improve wind turbine performance in electricity generated and energy conversion in renewable energy sources concept. The flow behavior on wind turbine blades profile and the relevant phenomena like stall can be improved by some modifications. In the present paper, Humpback Whales flippers leading edge protuberances model as a novel passive stall control method was investigated on S809 as a thick airfoil. The airfoil was numerically analyzed by CFD method in Reynolds number of 106and aerodynamic coefficients in static angle of attacks were validated with the experimental data reported by Somers in NREL. Therefore, computational results for modified airfoil with sinusoidal wavy leading edge were presented. The results revealed that, at low angles of attacks before the stall region, lift coefficient decreases slightly rather than baseline model. However, the modified airfoil has a smooth stall trend while baseline airfoil lift coefficient decreases sharply due to the separation which occurred on suction side. According to the flow physics over the airfoils, leading edge bumps act as vortex generator so vortices containing high level of momentum make the flow remain attached to the surface of the airfoil at high angle of attack and prevent it from having a deep stall.

Effect of Dissimilar Leading Edges on the Flow Structures Around a Square Cylinder

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
R. Ajith Kumar ◽  
K. Arunkumar ◽  
C. M. Hariprasad
Keyword(s):  
Amplitude Ratio ◽  
Vortex Formation ◽  
Leading Edge ◽  
Wake Flow ◽  
Practical Significance ◽  
Flow Structures ◽  
Square Cylinder ◽  
Near Wake ◽  
Study Results ◽  
Leading Edges

In the present study, results of a flow visualization study on the flow around a square cylinder with dissimilar leading edges are presented. The radii of the leading edges of the cylinder “r1” and “r2” are such that the ratio r1/r2 is systematically varied from 0 to 1. The flow structures around the cylinder with different leading edge radii particularly the vortex shedding mode and mechanism are investigated. For studies with stationary as well as oscillated cylinder cases, the results are taken at a Reynolds number value of 2100. For the oscillated case, a special mechanism is made to oscillate the cylinders at a desired amplitude and frequency. That is, the cylinder undergoes forced oscillation in this case. Results indicate that dissimilar leading edges bring notable changes in the near-wake flow structures of a square cylinder. For the stationary cylinder cases, the vortex formation length decreases with increase in the r1/r2 ratio. Flow structures are also found to be influenced by the amplitude ratio (amplitude to body size ratio); the higher the amplitude, the larger the size of vortices shed per cycle of cylinder oscillation. In view of marine structures and building sections with similar geometries, the present results carry considerable practical significance.

Lift Augmentation between Passive and Active Vortex Generator

2016 ◽  
Vol 851 ◽  
pp. 532-537
Author(s):  
Nur Faraihan Zulkefli ◽  
Zulhilmy Sahwee ◽  
Nurhayati Mohd Nur ◽  
Muhamad Nor Ashraf Mohd Fazil ◽  
Muaz Mohd Shukri
Keyword(s):  
Reynolds Number ◽  
Lift Coefficient ◽  
Vortex Generator ◽  
Leading Edge ◽  
Plasma Actuator ◽  
Triangular Shape ◽  
Subsonic Wind Tunnel ◽  
Different Types ◽  
Drag Ratio ◽  
Lift To Drag Ratio

This study was conducted to investigate the performance of passive and active vortex generator on the wing’s flap. The triangular shape of passive vortex generator (VG) was developed and attached on the wing’s flap leading edge while the plasma actuator performed as active vortex generator. The test was carried out experimentally using subsonic wind tunnel with 300 angles extended flap. Three different types of turbulent flow; with Reynolds number 1.5 x105, 2.0 x105, and 2.6x105 were used to study the aerodynamics forces of airfoil with plasma actuator OFF. All Reynolds number used were below 1x106. The result indicated that airfoil with plasma actuator produced higher lift coefficient 12% and lift-to-drag ratio 5% compared to airfoil with passive vortex generator. The overall result showed that airfoil with plasma actuator produced better lift forces compared to passive vortex generator.

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