Effects of crossflow pulsation intensity on wake properties of a circular cylinder (original) (raw)
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A comparison between the wake behind finned and foamed circular cylinders in cross-flow
Experimental Thermal and Fluid Science, 2014
The flow pattern behind a circular cylinder is associated with various instabilities. These instabilities are characterized by the Reynolds number and include the wake, separated shear layer and boundary layer. Depending on the physical application of the cylinder, increasing the level of turbulence on the surface of the cylinder would be a target for drag reduction or heat transfer enhancement. Particle Image Velocimetry (PIV) has been carried out to investigate the wake region behind a foamed and a finned cylinder. The purpose of this analysis is to investigate the flow characteristics for these two cases. The experiments are conducted for a wide range of Reynolds numbers (based on the mean air velocity and the cylinder diameter) from 1000 to 10000. Two dimensional results of planar PIV reveal the important aspects of the local flow features of the circular finned and foamed cylinders. These include turbulent boundary layer development over the surface and a delayed separation of the flow resulting in a smaller wake size at each speed. The application of Proper Orthogonal Decomposition (POD) to the PIV velocity fields of the two cylinder types is also discussed. The POD computed for the measured velocity fields for all cases shows that the first two spatial modes contain most of the kinetic energy of the flow, irrespective to the cylinder type. These two modes are also responsible for the large-scale coherence of the fluctuations. For three different cylinder types, the first four eigenmodes of the flow field were calculated and their structures were analysed.
Turbulence properties in the cylinder wake at high Reynolds numbers
Journal of Fluids and Structures, 2006
The present contribution analyses the turbulence properties in unsteady flows around bluff body wakes and provides a database for improvement and validation of turbulence models, concerning the present class of nonequilibrium flows. The flow around a circular cylinder with a low aspect ratio and a high blockage coefficient is investigated. This confined environment is used in order to allow direct comparisons with realisable 3-D Navier-Stokes computations avoiding 'infinite' conditions. The flow is investigated in the beginning of the critical regime at Reynolds number 140 000. The analysis is carried out by means of 2-D PIV, of 3-C PIV and of high-frequency 2-D PIV. The experimental analysis contributes to confirm the validity of advanced statistical turbulence modelling for unsteady flows around bodies. r (M. Braza).
The Reynolds number effect on dynamics of the wake behind a circular cylinder
18TH CONFERENCE OF POWER SYSTEM ENGINEERING, THERMODYNAMICS AND FLUID MECHANICS, 2019
Circular cylinder in cross-flow was studied experimentally using time-resolved PIV technique. Reynolds numbers based on the cylinder diameter were in range from 3 850 up to 19 260. Effect of the Reynolds number on wake time-mean topology as well as the dynamical behavior is studied. The wake dynamics is analyzed using the OPD method. The Oscillation Pattern Decomposition method provides unique information on dynamical patterns topology and oscillations in the flow-field. The pseudo-periodical wavy patterns have been detected within the cylinder wake. The parameters of this behavior are Reynolds number dependent, especially their sizes and shapes. Typical flow-field topologies are presented in the paper. The study is of qualitative character as flow boundary conditions are not well defined.
Dependence of square cylinder wake on Reynolds number
Physics of Fluids, 2018
The wake of a square cylinder is investigated for Reynolds number Re < 10 7. Two-dimensional (2D) laminar simulation and three-dimensional (3D) large-eddy simulation are conducted at Re ≤ 1.0 × 10 3 , while experiments of hotwire, particle image velocimetry, and force measurements are carried out at a higher Re range of 1.0 × 10 3 < Re < 4.5 × 10 4. Furthermore, data covering a wide Re range, from 10 0 to 10 7 , in the literature are comprehensively collected for discussion and comparison purposes. The dependence on Re of the recirculation bubble size or vortex formation length, wake width, shear-layer transition, time-mean drag force, and Strouhal number is discussed in detail, revealing five flow regimes, each having distinct variations of the above parameters. With increasing Re, while the streamwise recirculation size enlarges at Re < 50 (steady flow regime), the vortex formation length reduces at 50 < Re < 1.6 × 10 2 (laminar flow regime), remains unchanged at 1.6 × 10 2 < Re < 2.2 × 10 2 (2D-to-3D transition flow regime), and decreases at 2.2 × 10 2 < Re < 1 × 10 3 (shear layer transition I regime), approaching asymptotically a constant at Re > 1.0 × 10 3 (shear layer transition II regime). Meanwhile, the wake width decreases with Re in the laminar flow regime, grows in 2D-to-3D transition and shear layer transition I regimes, and levels off in the shear layer transition II regime. The narrowest wake width is identified in the 2D-to-3D transition flow regime, corresponding to a minimum time-mean drag force and a largest Strouhal number. With increasing Re, the shear-layer transition length rapidly declines in the shear layer transition I regime where the transition occurs downstream of the trailing corner of the cylinder. On the other hand, it slowly tapers off in the shear layer transition II regime where the transition takes place upstream of the trailing corner. An extensive comparison is made between the dependence on Re of a circular cylinder wake and a square cylinder wake, with their distinct natures highlighted.
The effect of flow perturbations on the near wake characteristics of a circular cylinder
2003
The mean and fluctuating velocity fields in the near wake of a circular cylinder subjected to an incident mean flow with periodic velocity perturbations superimposed upon it were examined using laser Doppler anemometry. From these measurements the wake was characterized in terms of the recirculation bubble length, vortex formation length, maximum intensity of the velocity fluctuations and the wavelength of the vortex street. The well-known 'lock-on' phenomenon was observed for perturbation frequencies around two times the natural vortex shedding frequency. It is shown that the wake structure is modified in a systematic manner within the lock-on range. The forced wake shares many basic characteristics as that of a cylinder oscillating either transversely or in line, relative to the flow direction. These include the shortening of the recirculation bubble and the vortex formation region as well as the variation of the longitudinal vortex spacing with perturbation frequency. Differences but also similarities between forced wakes at low (less than 350) and relatively higher Reynolds numbers (greater than 350) are discussed.
2011
Large-eddy simulations are employed to investigate the dynamic response of the turbulent wake of a circular cylinder to sinusoidal perturbations in the inflow velocity superposed on a mean component. The perturbation frequency is varied across the vortex lock-on range at a constant amplitude of 5% of the mean velocity corresponding to a Reynolds number of 2580. The effect on the instantaneous, time-averaged and phase-averaged characteristics of the near-wake flow and fluid forces on the cylinder is reported. Comparisons of the present simulations to experimental realizations show that the physics of the unsteady three-dimensional separated flow are well reproduced. The simulations capture the modification of the wake structure including the shrinking of the recirculation bubble and vortex-formation region and the enhancement of the wake fluctuations and vortex strength in the lock-on regime. These wake effects are accompanied by an increase in the steady and unsteady drag and the unsteady lift acting on the cylinder. An empirical formula for the amplification of the mean drag coefficient due to inflow perturbations and equivalent oscillations of the cylinder in a steady flow is provided from compilation of available data. Particular attention is given to the change in the timing of vortex shedding with respect to the imposed perturbation across the lock-on range in order to reveal the link between the vortex dynamics and the fluid-induced forces on the cylinder. It is shown that the phase at which vortices are shed from the cylinder shifts monotonically as a function of the perturbation frequency resulting in corresponding changes in the phase of the unsteady forces. It is further shown that the phase of the lift is directly linked to that of vortex shedding but the phase of the drag is biased by inertial forces due to added mass and induced pressure waves. Decomposition of the total in-line force to inviscid "potential-flow" and viscous "vortex-drag" components indicates that the latter exhibits a behavior which is not physically consistent. The stochastic character of vortex synchronization in turbulent wakes and the implications of the present findings for vortex-induced free in-line vibrations are also discussed.
Experimental Investigation of 3D Dynamical Effects in a Wake behind Circular Cylinder
MATEC Web of Conferences
A circular cylinder in crossflow is subjected to the study of 3D dynamical structure of its wake. The typical dynamics is characterized by quasi-periodic behaviour called Kármán – Bénard vortex street with the typical frequency in dimensionless form known as Strouhal number. The experimental study relies on stereo Particle Image Velocimetry method, the plane of measurement is perpendicular to the flow in the distance 3.8 cylinder diameters in streamwise direction. Reynolds number was around 5 thousand. The structure and dynamical behaviour of the wake along the cylinder axis is studied in details. On the top of the known streamwise velocity deficit definition of the wake, the streamwise oriented dynamically evolving vortices are detected. For the detailed dynamics examination, the Oscillation Pattern Decomposition method was used. Waves of various structures travelling along the cylinder axis as well as some pulsations were identified. The wake dynamics is characterised by the strea...
Lecture Notes in Mechanical Engineering, 2021
Flume experiments were conducted to investigate the three-dimensional flow structure and turbulent flow mechanisms around a nonsubmerged, sidewall-attached rectangular spur dike with a low length-to-depth ratio. Velocity measurements show that the wake of the spur dike in the middepth region consists of a single, large recirculation zone, while that in the near-bed region is composed of a horizontal recirculation zone and a corner vortex with its axis perpendicular to the flume sidewalls. A horseshoe vortex system observed in front of the spur dike is found to interact with the downstream recirculation zone, which results in the increase in lateral turbulent mixing especially in near-bed regions. Another vortex, which rotates in the opposite direction to the horseshoe vortex, is found beneath the free surface in front of the spur dike, which is associated with increased free surface elevation and its fluctuation. Power spectra of the transverse velocity show evidence of a periodic behavior of the near-wake shear layer emanating from the tip of the spur dike with a Strouhal number of about 2. It is also shown that the two experiments conducted at different Reynolds and Froude numbers show very similar flow fields in spite of large differences in free surface elevations, which indicates that the effects of free surface deformation on mean velocities and Reynolds stresses are marginal.
Effect of orientation on the wake of a square cylinder at low Reynolds numbers
2004
Experimental investigation of flow past a square cylinder at Reynolds numbers of 97 and 187 is reported. Cylinder orientations of 0 to 60 o with respect to the mean flow have been considered. Two-component hotwire anemometry has been adopted for velocity measurements. The wake of the cylinder has been visualized using a pulsed laser sheet to understand the flow structure. Measurements have been carried out in the near wake, mid-wake and far wake of the cylinder. The effects of orientation and Reynolds number on Strouhal number, drag coefficient, time-average and rms velocity distributions, decay of velocity fluctuations and power spectra are of interest. There is a dominant peak of vortex shedding in the near wake velocity spectra over the range of Reynolds numbers studied. The Strouhal number and drag coefficient are correlated to the cylinder angle. A change in the cylinder orientation leads to an early appearance of quasi-periodicity and hence three-dimensionality, owing to the asymmetric nature of the wake. The shape of the mean velocity profiles, fluctuations and the rate of decay show a strong dependence on the cylinder orientation in the near wake, though the dependence weakens in the far wake. Among the angles studied, the wake of a cylinder whose orientation is 22.5 o with respect to the incoming flow is surprisingly vigorous, and shows strong three-dimensionality. The data of the present work is useful for assessing flow interaction with cylindrical structures of square cross-section.
On the characteristics of the turbulent wake behind a wall-mounted square cylinder
arXiv: Fluid Dynamics, 2020
The turbulent flow past a wall-mounted square cylinder with an aspect ratio of four was investigated with the aid of Spalart-Allmaras improved delayed detached-eddy simulation (S-A IDDES) and proper orthogonal decomposition (POD). The Reynolds number was equal to 12,000 (based on the free-stream velocity and obstacle width). The boundary layer thickness was approximately 0.18 of the obstacle height. This study focused on analysing the vortical structure of the wake and vortex shedding process along the obstacle height. A quantitative comparison of the first and second-order flow statistics with the available experimental and direct numerical simulation data was used to validate the numerical results. The numerical model coupled with the vortex method (VM) of generating the turbulent inflow conditions could successfully reproduce the flow field around and behind the obstacle with commendable accuracy. The flow structure and vortex shedding characteristics near the wake formation region have been discussed in detail using time-averaged and instantaneous flow parameters obtained from the simulation. Dipole type mean streamwise vortex and half-loop hairpin instantaneous vortices with energetic motions were identified. A coherent shedding structure was reported along the obstacle using two-point correlations. Two types of vortex shedding intervals were identified, namely, low amplitude fluctuations (LAFs) and high amplitude fluctuations (HAFs) (Sattari et al. 2012). The HAFs interval exhibits von Kármán like behaviour with a phase difference of approximately 180 • while the LAFs interval shows less periodic behaviour. It was observed that the effect of the LAFs interval tends to weaken the alternating shedding along the obstacle height. The POD analysis of the wake showed that for the elevations between 0.25 to 0.5 of the obstacle height, the first two POD modes represent the alternating shedding and the contribution to the kinetic energy is between 66.6% to 57.6%. However, at the free end of the obstacle, the first two modes have a symmetrical shedding nature and share 36.5% of the kinetic energy, while the rest of the energy is distributed between the alternating and the random shedding processes. A simple low-order model based on the vortex-shedding phase angle and the spectrum of the time coefficients obtained from POD was developed to predict the wake dynamics at the range of elevations where the alternating shedding is dominated.