Two-Dimensional Study of the Turbulent Wake Behind a Square Cylinder Subject to Uniform Shear

2001 ◽  
Vol 123 (3) ◽  
pp. 595-603 ◽  
Author(s):  
A. K. Saha ◽  
G. Biswas ◽  
K. Muralidhar
Keyword(s):  
Drag Coefficient ◽  
Strouhal Number ◽  
Turbulence Model ◽  
Lift Coefficient ◽  
Turbulent Wake ◽  
Square Cylinder ◽  
Uniform Shear ◽  
Shear Parameter ◽  
Drag And Lift ◽  
Direct Calculations

The flow past a square cylinder at a Reynolds number of 20,000 has been simulated through direct calculations and through the calculations using turbulence model. The present investigation highlights significant differences between the two approaches in terms of time-averaged flow, Strouhal number, and aerodynamic forces. The time-averaged drag coefficient and the rms fluctuations due to the direct calculations are higher than those due to the turbulence model. However, Strouhal number is underpredicted in the direct calculations. The effect of shear on the flow has also been determined using the turbulence model. The time-averaged drag coefficient is found to decrease with the increase in shear parameter up to a certain value. Then it increases with the further increase in the shear parameter. On the other hand, lift coefficient increases with the increase in shear parameter. Strouhal number shows a decreasing trend with the increase in shear parameter whereas the rms values of drag and lift coefficients increase with the shear parameter. The Ka´rma´n vortex street, mainly comprising clockwise vortices due to shear, decays slowly compared to the uniform flow condition.

Two-Dimensional Study of the Turbulent Wake Behind a Square Cylinder Subject to Uniform Shear

2000 ◽  
Author(s):  
A. K. Saha ◽  
G. Biswas ◽  
K. Muralidhar
Keyword(s):  
Drag Coefficient ◽  
Strouhal Number ◽  
Turbulence Model ◽  
Lift Coefficient ◽  
Turbulence Models ◽  
Direct Calculation ◽  
Square Cylinder ◽  
Shear Parameter ◽  
High Reynolds ◽  
Direct Calculations

Abstract The flow past a square cylinder at a high Reynolds number has been simulated through direct calculations and through the calculations using turbulence models. The present investigation highlights significant differences between the two approaches in terms of instantaneous flow, Strouhal number and the aerodynamic forces. The time-averaged drag coefficient and the rms fluctuations due to the direct calculation are higher than those due to the turbulence model. However, Strouhal number is underpredicted in direct calculations. The effect of shear on the flow has also been determined using the turbulence model. The time-averaged drag coefficient is found to decrease with the increase in shear parameter up to a certain value. Then it increases with the further increase in the shear parameter. On the other hand, the lift coefficient increases with the increase in shear parameter. Strouhal number shows a decreasing trend with the increase in shear parameter whereas the rms values of the drag and lift coefficients increase with the shear parameter. Kármán Vortex Street, mainly comprising of clockwise vortices due to shear, decays slowly compared to uniform flow condition.

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.

Numerical Analysis of Two Square Cylinders of Different Sizes With and Without Corner Modification

2016 ◽  
Author(s):  
Y. T. Krishne Gowda ◽  
Holalu Venkatdas Ravindra ◽  
Vikram Chowdeswarally Krishnappa
Keyword(s):  
Drag Coefficient ◽  
Flow Velocity ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Research Work ◽  
Convergence Criterion ◽  
Back Side ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Flow Past

Flow past square cylinders has attracted a great deal of attention because of its practical significance in engineering e.g., High rise buildings, monuments and towers. Similarly, bridge pillars, and legs of offshore platforms are continuously subjected to the load produced by maritime or fluvial streams. The presence of separated flows, reattachment, formation the vortices, un steadiness of flow, mass and momentum transfer across shear layer makes the flow field quite complex. Many research work was carried out for a single square cylinder and flow past two square cylinders, but with corner medications in square cylinder of different size arranged in tandem was not taken up. This has motivated to take up the flow past two different sized square cylinders i.e., smaller in upstream and larger in downstream which is numerically simulated by using Fluent software. 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 then proper assignment of boundaries are set. After setting the boundary types, the geometry is discretized into small control volumes. Once the surface mesh is completed by using Gambit software, the mesh along with boundary conditions are exported to fluent, which is CFD solver usually run in background mode. 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 cylinders of different size, there is decrease in the wake width and thereby the lift and drag coefficient values. The lift coefficients in Square cylinders of different size with corner modifications decreases but Strouhal number increases when compared with a single square cylinder without corner modifications. 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 square cylinders i.e., PPR (pitch to perimeter ratio) with and without corner modifications is increased to 6, the flow velocity almost approaches to flow past a single square cylinder with and without modifications for same condition. When the size of the upstream square cylinder with and without modifications is smaller than that of the downstream square cylinder, the size of the eddies is always smaller in between the cylinders compared to the downstream of the second cylinder. The flow velocity in between the cylinders with and without corner modifications are less compared to the downstream of the second cylinder. Pressure on the downstream side of the cylinder is smaller than that on the upstream side of the cylinder for with and without corner modifications. Also, the front portion of the cylinder is experiencing highest pressure compared to the second cylinder for all the three cases i.e., PPR = 2, 4 and 6. Pressure at the upper side, bottom side and back side of square cylinder with and without corner modifications is of negative pressure, it is because of vortices generated at that surfaces. The downstream cylinder is found to experience higher lift compared to the upstream cylinder. The results are presented in the form of while the downstream cylinder is found to experience higher drag compared to the streamlines, flow velocity, pressure distribution, drag coefficient, lift coefficient and strouhal number.

Numerical study of the rounded corners effect on flow past a square cylinder

2015 ◽  
Vol 25 (4) ◽  
pp. 686-702 ◽  
Author(s):  
Sajjad Miran ◽  
Chang Hyun Sohn
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Numerical Results ◽  
The Body ◽  
Square Cylinder ◽  
Content Type ◽  
Flow Past ◽  
Corner Radius

Purpose – The purpose of this paper is to numerically investigate the influence of corner radius on flow past a square cylinder at a Reynolds number 500. Design/methodology/approach – Six models were studied, for R/D=0 (square cylinder), 0.1, 0.2, 0.3, 0.4, and 0.5 (circular cylinder), where R is the corner radius and D is the characteristic dimension of the body. The transient two-dimensional (2D) laminar and large eddy simulations (LES) models were employed using finite volume code. The Strouhal number, mean drag coefficient (CD), and root mean square (RMS) value of lift coefficient (CL,RMS), for different R/D values, were computed and compared with experimental and other numerical results. Findings – The computational results showed good agreement with previously published results for a Reynolds number, Re=500. It was found that the corner effect on a square cylinder greatly influences the flow characteristics around the cylinder. Results indicate that, as the corner radius ratio, R/D, increases, the Strouhal number increases rapidly for R/D=0-0.2, and then gradually rises between R/D=0.2 and 0.5. The minimum values of the mean drag coefficient and the RMS value of lift coefficient were found around R/D=0.2, which is verified by the time averaged streamwise velocity deficit profile. Originality/value – On the basis of the numerical results, it is concluded that rounded corners on a square cylinder are useful in reducing the drag and lift forces generated behind a cylinder. Finally, it is suggested that with a rounded corner ratio of around R/D=0.2, the drag and oscillation of the cylinder can be greatly reduced, as compared to circular and square cylinders.

Experimental Investigation of Uniform-Shear Flow Past a Circular Cylinder

1992 ◽  
Vol 114 (3) ◽  
pp. 457-460 ◽  
Author(s):  
Tae Soon Kwon ◽  
Hyung Jin Sung ◽  
Jae Min Hyun
Keyword(s):  
Circular Cylinder ◽  
Shear Flow ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Laboratory Experiments ◽  
Processing Technique ◽  
Image Processing Technique ◽  
Uniform Shear ◽  
Uniform Shear Flow ◽  
Shear Parameter

Extensive laboratory experiments were carried out to investigate the uniform-shear flow approaching a circular cylinder. The aim was to present the Strouhal number (St)- Reynolds number (Re) diagrams over a broad range of the shear parameter K (0 ≤ K ≤ 0.25) and at higher values of Re (600 ≤ Re ≤ 1600). An image processing technique, in conjunction with flow visualization studies, was used to secure more quantitative depictions of vortex shedding from the cylinder. The Strouhal number increases with increasing shear parameter. The drag coefficient decreases with increasing Re; also, Cd decreases as the shear parameter K increases.

An Experimental Investigation Assessing the Validity of Quasi-Static Calculations for an Oscillating Hydrofoil

2011 ◽  
Author(s):  
Rajan Fernandez ◽  
Keith Alexander
Keyword(s):  
Steady Flow ◽  
Strouhal Number ◽  
Strong Dependence ◽  
Lift Coefficient ◽  
Experimental Program ◽  
Design Data ◽  
Drag And Lift ◽  
Drag And Lift Coefficient ◽  
Lift And Drag ◽  
Human Powered

Inspired by animals, flapping wing propulsion has been of interest since the early 1900s. Flapping hydrofoil propulsion has been attempted by designers of human powered watercraft because of the novelty and the apparent high theoretical efficiency, but with limited success. The earliest human powered hydrofoil, the Wasserlaufer, was invented by Julius Schuck in 1953. The first really successful human powered hydrofoil, the Trampofoil, was invented by Alexander Sahlin in 1998. While these craft function adequately the design data for flapping hydrofoils is inadequate or not available. This paper describes an experimental program and initial results for the required data. To design a vehicle with a lifting and thrusting oscillating hydrofoil the force that the hydrofoil will exert on the vehicle through its entire oscillating cycle must ideally be known. The force profiles could be estimated via quasi-static calculations based on steady flow lift and drag coefficients, but these often do not cover the full 360 degree range that can be required and there is doubt that the steady flow coefficients properly represent the dynamic situation of an oscillating hydrofoil. Hence a valuable process would be one that could determine dynamic drag and lift coefficient loops as function of the Strouhal number, heaving and pitching profiles. To work toward the collection of this information, experimental data is being recorded in a towing tank with an oscillating NACA4415 hydrofoil over a range of Strouhal numbers and types of oscillating profiles. While there are still some limitations to the experimental equipment preliminary experimental results show the limitations of using quasi-static calculations and go some way to providing the design data for the hydrofoil section tested. We conclude that quasi-static calculations based on the gliding coefficient curve for for an oscillating hydrofoil are only valid for very small Strouhal numbers (St≪0.05). We have shown that as the Strouhal number increases, the error in such calculations increases very rapidly. We also note that the lift coefficient of the hydrofoil has a strong dependence on the angle of attack and is not affected by the gliding stall.

scholarly journals Drag and lift forces on clean spherical and ellipsoidal bubbles in a solid-body rotating flow

2011 ◽  
Vol 682 ◽  
pp. 434-459 ◽  
Author(s):  
MARIE RASTELLO ◽  
JEAN-LOUIS MARIÉ ◽  
MICHEL LANCE
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Solid Body ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Uniform Flow ◽  
Rotating Flow ◽  
Wide Range ◽  
Spherical Bubbles ◽  
Drag And Lift

A single bubble is placed in a solid-body rotating flow of silicon oil. From the measurement of its equilibrium position, lift and drag forces are determined. Five different silicon oils have been used, providing five different viscosities and Morton numbers. Experiments have been performed over a wide range of bubble Reynolds numbers (0.7 ≤ Re ≤ 380), Rossby numbers (0.58 ≤ Ro ≤ 26) and bubble aspect ratios (1 ≤ χ ≤ 3). For spherical bubbles, the drag coefficient at the first order is the same as that of clean spherical bubbles in a uniform flow. It noticeably increases with the local shear S = Ro−1, following a Ro−5/2 power law. The lift coefficient tends to 0.5 for large Re numbers and rapidly decreases as Re tends to zero, in agreement with existing simulations. It becomes hardly measurable for Re approaching unity. When bubbles start to shrink with Re numbers decreasing slowly, drag and lift coefficients instantaneously follow their stationary curves versus Re. In the standard Eötvös–Reynolds diagram, the transitions from spherical to deformed shapes slightly differ from the uniform flow case, with asymmetric shapes appearing. The aspect ratio χ for deformed bubbles increases with the Weber number following a law which lies in between the two expressions derived from the potential flow theory by Moore (J. Fluid Mech., vol. 6, 1959, pp. 113–130) and Moore (J. Fluid Mech., vol. 23, 1965, pp. 749–766) at low- and moderate We, and the bubble orients with an angle between its minor axis and the direction of the flow that increases for low Ro. The drag coefficient increases with χ, to an extent which is well predicted by the Moore (1965) drag law at high Re and Ro. The lift coefficient is a function of both χ and Re. It increases linearly with (χ − 1) at high Re, in line with the inviscid theory, while in the intermediate range of Reynolds numbers, a decrease of lift with aspect ratio is observed. However, the deformation is not sufficient for a reversal of lift to occur.

Drag and lift measurements of solid sports balls in still air

2017 ◽  
Vol 232 (3) ◽  
pp. 255-263 ◽  
Author(s):  
Jeff R Kensrud ◽  
Lloyd V Smith
Keyword(s):  
Surface Roughness ◽  
Wind Tunnel ◽  
Drag Coefficient ◽  
Rough Surface ◽  
Lift Coefficient ◽  
Laboratory Setting ◽  
Air Drag ◽  
High Speeds ◽  
Drag And Lift ◽  
Lift And Drag

The following article considers lift and drag measurements of solid sports balls propelled through still air in a laboratory setting. The balls traveled at speeds ranging from 26 to 134 m/s with spin rates up to 3900 r/min. Light gates measured the speed and location of the balls at two locations from which lift and drag values were determined. Ball roughness varied from polished to rough surface protrusions, that is, seams as high as 1.5 mm. Lift and drag were observed to depend on speed, spin rate, surface roughness, and seam orientation. A drag crisis was observed on smooth balls as well as non-rotating seamed balls with seam heights less than 0.9 mm. The drag coefficient of approximately 0.42 was nearly constant with speed for spinning seamed balls with seam height greater than 0.9 mm. The still air drag coefficient of smooth balls was comparable to wind tunnel drag at low speeds ( Re < 2 × 105) and higher than wind tunnel results at high speeds ( Re > 2 × 105). The lift and drag coefficients of spinning balls increased with increasing spin rate. The lift coefficient of baseballs was not sensitive to ball orientation or seam height.

Three-dimensional numerical simulation of a flat plate perpendicularly submitted to current with a blockage ratio of 0.214: URANS and DES

2021 ◽  
pp. 1-29
Author(s):  
Hao Wu ◽  
Antonio Carlos Fernandes ◽  
Renjing Cao
Keyword(s):  
Free Surface ◽  
Drag Coefficient ◽  
Flat Plate ◽  
Strouhal Number ◽  
Turbulence Model ◽  
Low Frequency ◽  
Three Dimensions ◽  
Small Scale ◽  
Blockage Ratio ◽  
Detached Eddy Simulation

Abstract The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier–Stokes (URANS) based on the realizable k-epsilon (RKE) turbulence model, URANS based on the k–omega shear stress transport (SST) turbulence model and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate prediction of all. Moreover, by comparing the instantaneous vorticity contours and 3D turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF) based simulation of the free surface, it is demonstrated that the free surface has no significant effect on mean drag coefficient and Strouhal number.

Influence of incidence angle on the aerodynamic characteristics of square cylinders with rounded corners

2016 ◽  
Vol 26 (1) ◽  
pp. 269-283 ◽  
Author(s):  
Sajjad Miran ◽  
Chang Hyun Sohn
Keyword(s):  
Incidence Angle ◽  
Lift Coefficient ◽  
Square Cylinder ◽  
Aerodynamic Forces ◽  
Content Type ◽  
Minimum Drag ◽  
Rounded Corner ◽  
Drag And Lift ◽  
Drag And Lift Coefficient ◽  
Critical Incidence

Purpose – The purpose of this paper is to focus on the variation of wake structures and aerodynamic forces with changes in the cylinder corner radius and orientation. Design/methodology/approach – Numerical simulations were performed for flow past a square cylinder with different corner radii placed at an angle to the incoming flow. In the present study, the rounded corner ratio R/D=0 (square cylinder), 0.1, 0.2, 0.3, and 0.4 (where R is the corner radius and D is the characteristic dimension of the body) and the angle of incidence α in the range of 0°-45° were considered. Findings – The numerical model was validated by comparing the present results with results in the available literature, and they were found to be in good agreement. The critical incidence angle for the rounded corner cylinder – corresponding to the minimum mean drag coefficient (C D ), the minimum root mean square value of the lift coefficient C L,RMS), and the maximum Strouhal number – shifted to a lower incidence angle compared with the sharp corner square cylinder. The minimum drag and lift coefficient at R/D=0 were observed for the critical incidence angle αcri=12°, whereas for R/D=0.1-0.4, the minimum drag and lift coefficient were found to be within the range of 5°-10° for α. Originality/value – The presented results shows the importance of the incidence angle and rounded corners of the square cylinder for reduction of aerodynamic forces. The two parameters support the shear layer flow reattachment on the lateral surface of the cylinder, have a strong correlation with the reduction of the wake width, and hence reduced the values of C D and C L .

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