Elsevier

European Journal of Mechanics - B/Fluids

Volume 96, November–December 2022, Pages 15-25
European Journal of Mechanics - B/Fluids

Influence of surface roughness on the wake structure of a circular cylinder at Reynolds number 5 × 103 to 12 × 103

https://doi.org/10.1016/j.euromechflu.2022.06.003Get rights and content

Highlights

  • Added surface roughness decreases the wake length and increases the wake width at lower Reynolds numbers.

  • Added surface roughness decreases the Strouhal numbers at lower Reynolds numbers.

  • The cylinder drag was increased at higher surface roughnesses.

  • The kinetic energy of the first two POD modes cover up to 50% of the total kinetic energy.

  • The absolute kinetic energy is increasing with increasing surface roughness.

Abstract

The flow around roughened circular cylinders and the wake behind them were studied in a wind tunnel flow using the Particle Image Velocimetry (PIV) method. The Reynolds number ranged from 5140 to 11800, and the real diameter of the cylinders including surface roughness ranged from 15.06 to 17.34 mm. The flow properties and forces around the roughened cylinders was evaluated by means of Strouhal number and coefficient of drag. The wake was analysed by means of mean velocities along the cylinder centreline, its width and the Proper Orthogonal Decomposition (POD) method. The added surface roughness was found to decrease the wake length and increase the wake width at lower Reynolds numbers. At higher Reynolds numbers, the added surface roughness did not decrease the wake length, but did increase the wake width, although with less effect. The POD analysis showed changes in the higher modes of the flow. The kinetic energy of the first two modes covers up to 50% of the total kinetic energy; the first two modes in the case of the smooth cylinder have the lowest kinetic energy, whereas the first two modes in the case of the roughest cylinder have the highest kinetic energy. The similarity between the POD modes of the smooth and roughened cylinders might be due the fact that the actual Reynolds number range was below the transitional one.

Introduction

Circular cylinders often find applications in the form of cables, rods, etc. as parts of structural or mechanical engineering structures. These cylinders as so-called substructures are subjected to fluid and air loading, and their response to this loading can have in the worst cases destructive effects. The cables or rods are often assumed to have perfect circular cross-sections. However, in many cases the circular cross-section is disturbed by various influences, e.g. manufacturing processes. Either way, the perfect circularity disappears and some kind of surface roughness is created. In particular, climatic conditions (ice accretion) define the so-called climatic roughness, and geometrical deviations, corrosion and other surface changes define the so-called technological roughness.

Flow around circular cylinders with various surface changes has been studied by many authors. Zdravkovich  [1] made an overview of flow past smooth and rough cylinders by means of Strouhal number and drag analysis. Adachi [2] measured Strouhal numbers on eight cylinders with various surface roughness over a wide range of Reynolds numbers. Buresti [3] studied the influence of surface roughness on transition between subcritical up to postcritical flow regimes around circular cylinders. Various surface roughness profiles of cylinders were tested by Fuss [4], and significant changes in drag were observed in the critical Reynolds number range. Many researchers have tried to increase the cylinder surface roughness with grooves, fabric covering or other methods in order to shift the drag crisis into lower Reynolds numbers and therefore reduce the drag. Skeide et al. [5] reduced the drag of the cylinder with the use of hydrophobised sand. Sooraj et al. [6] studied the flow over a superhydrophobic and smooth cylinder using particle image velocimetry (PIV). They found that superhydrophobicity substantially affects the flow in terms of drag reduction and changes in near-wake coherent structures.

Besides experimental work, a growing number of numerical simulations are also being done. Rodrigues et al. [7], [8] employed large-eddy simulation (LES) of the roughness effects on the wake of a circular cylinder. Michalcova and Lausova [9] numerically determined the aerodynamic roughness of industrial chimneys. Flow over rough cylinders is also applicable in ocean engineering, as underwater surfaces are often covered with aquatic organisms which change the flow regime around the structure. Zeinoddini et al. [10] investigated the effects of vortex-induced vibrations (VIV) on circular cylinders with regular pyramidal roughness. They found that VIV amplitude, drag coefficient and other parameters were decreased by the roughness. Gao et al. [11] studied the effects of surface roughness on the VIV of a flexible cylinder with the finding that rough cylinders have a smaller displacement response and a higher vortex shedding frequency than smooth cylinders. Drag reduction was also studied for the purpose of sports aerodynamics. Terra et al. [12] measured the drag of the surfaces of cylinders roughened with fabric used in sports clothing. Khashehchi et al. [13] investigated the wake behind finned and foamed cylinders and incorporated the POD method. The climatic roughness caused mainly by ice accretion was in focus as well, e.g. with Gorski et al. [14] and Pospisil et al. [15], who studied ice accretion on bridge cables and its influence on Strouhal number and wind-induced vibrations. They concluded that ice accreted on the circular bridge cables has a significant impact on the dependence of Strouhal number on Reynolds number throughout the entire studied Reynolds number range. Xu et al. [16] conducted pressure and force measurements on a large diameter circular cylinder with accreted ice to determine its aerodynamic characteristics. They observed that the mean aerodynamic coefficients of the ice-accreted cylinders exhibited significant Reynolds number effects closely related to the shape of the ice. Zhou et al. [17] demonstrated that grooved and dimpled cylinders produce lower mean drag than smooth cylinders. Aiman and Samion [18] found that grooved cylinders produce lower drag the smooth ones.

The aim of this paper was to investigate the effect of surface roughness on flow around and in the wake of roughened circular cylinders and evaluate its drag and Strouhal numbers as well as the occurrence of coherent structures in the flow. These measurements were carried out in the subcritical Reynolds number range 5.14 × 103–1.18 × 104, and the research was concentrated upon the analysis of the flow around roughened circular cylinders using Proper Orthogonal Decomposition, which is still less common even today, as can be also seen from selected references in Table 1, where a summary of most relevant literature is presented. There, following abbreviations were used: AR — aspect ratio of the cylinder (AR = l/D, where l is cylinder length and D is cylinder diameter), NA — information is not available, TH — thermocouple(s), PT — pressure taps, HWA — hot wire anemometry, FB — force balance technique, LC — load cells and Vis. – visualization technique.

Section snippets

Experiment instrumentation and setup

The measurements were carried out at the Laboratory of Turbulent Shear Flows of the Institute of Thermomechanics of The Czech Academy of Sciences (IT CAS). A blow-down wind tunnel with a test section 250 × 250 mm2 in cross section was used. The inlet velocities at the test sections were U1=5.2 m/s and U2=10.4m/s. The flow in the facility contraction outlet was measured using the hot-wire anemometry technique. The flow apart from the boundary layers on the walls (thickness about 2–3 mm) was

Proper Orthogonal Decomposition (POD)

The POD method is an important tool in the study of dynamical systems. It was first introduced in 1967 by J. L. Lumley [20], and its principle was described, e.g., by Uruba  [21]. The POD method provides a tool for identification of deterministic structures from a random turbulent flow. It is a systematic way to localize organized motions, so-called coherent structures in a stochastic signal. Uruba and Prochazka [22] analysed the POD spectrum of the wake behind a smooth circular cylinder. Other

Mean flow field

The resulting mean flow field images for the flow perpendicular to the cylinder axis are presented in the following figures. The figures depict the mean flow velocity of the streamwise velocity component U and its standard deviation. The values in the colour bars are dimensionless [–]. Fig. 3a and b present mean streamwise velocity U of the case SC (smooth cylinder) and its standard deviation for the reference velocity Uref1=5.2m/s. Fig. 3c and d present the mean streamwise velocity of the case

Conclusions

The aim of this work was an investigation of the effect of cylinder surface roughness on the flow both around and in the wake of a circular cylinder. These measurements were carried out in the subcritical Reynolds numbers range 5.14 × 103 – 1.18 × 104, and the research concentrated on the analysis of the flow around roughened circular cylinders using Proper Orthogonal Decomposition, which is currently still less common. The study was conducted in a wind tunnel using the PIV method. The Strouhal

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research was supported by the Czech Academy of Sciences , institutional support RVO:68378297 and RVO:61388998.

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