Experimental Investigation of Three-Dimensional Modes in the Wake of a Rotationally Oscillating Cylinder (original) (raw)

Free end effects on the near wake flow structure behind a finite circular cylinder

Journal of Wind Engineering and Industrial Aerodynamics, 2000

The free end effect on the near wake of a finite circular cylinder in a cross flow has been investigated experimentally. Three finite cylinders with aspect ratios (L=D) of 6, 10 and 13 were tested in a subsonic wind tunnel at a Reynolds number of 20 000. A hot-wire anemometer was employed to measure the wake velocity. Mean pressure distributions on the cylinder surface were also measured. The flow near the free end was visualized to observe the flow structure qualitatively in a circulating water channel. The experimental results from these finite cylinder (FC) models were compared with those of a two-dimensional circular cylinder. The flow past the FC free end shows a complicated three-dimensional wake structure. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation region is elongated. The free end effect becomes dominant close to the FC free end. The threedimensionality of the FC wake may be attributed mainly to the strong entrainment of irrotational fluids, caused by the downwash of counter-rotating vortices separated from the FC free end. The downwash flow is concentrated in the central region of the wake. A peculiar flow structure having a 24 Hz frequency component was observed near the free end using spectral analysis and cross-correlation of the velocity signals. This 24 Hz frequency component is closely related to the counter-rotating twin vortices formed near the FC free end. #

Experimental Characterization of an Axisymmetric Swirling Wake

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), 2022

An experimental investigation of swirl and wake strength influence on axisymmetric turbulent wake evolution was conducted. A novel wake generator design was wire mounted in a wind tunnel test section with low free-stream turbulence produced wake Reynolds numbers based on momentum thickness and free-stream velocity in excess of 14,000 and Swirl numbers up to 0.4 with minimal blockage. Steady-state Blade Element Momentum simulations of reference wind turbine designs indicated that wind turbines operate in the flow regimes studied indicating the practical aspects of this work. Stereoscopic Particle Image Velocimetry was used to acquired three components of velocity in the swirling wake at locations up to approximately ten diameters downstream. Quantitative measures of wake growth and decay were deduced using available equilibrium similarity scaling for the swirling wake. The results show an increase above 50% in growth and axial velocity decay rate constants over the range of swirl strength studied compared to those of the non-swirling wake. Tangential velocity decay constants were shown to decrease with swirl strength over the range of conditions studied. Small changes in wake strength have little influence on all results observed for the flow regimes studied in this work.

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

Journal of Fluids Engineering, 2001

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 timeaveraged 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 Kármán vortex street, mainly comprising clockwise vortices due to shear, decays slowly compared to the uniform flow condition.

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).

Effects of crossflow pulsation intensity on wake properties of a circular cylinder

Experimental Thermal and Fluid Science, 2023

The effects of the crossflow pulsation intensity on the wake flow properties of a circular-cylinder were experimentally studied in a wind tunnel. A rotating plate actuator technique was used to generate periodic crossflow oscillations. The Reynolds number and oscillation Strouhal number of the crossflow were fixed at 3111 and 0.074, respectively. The crossflow pulsation intensities were varied within the range between 0.035 and 0.074. The wake evolution processes were captured by laser-light-sheet-assisted smoke flow visualization method. The wake instability frequencies and instantaneous oscillating turbulent velocities were detected by a one-component hot-wire anemometer along with a high-speed data acquisition system. The triple-decomposition technique was applied to the instantaneous velocities to consider the effects of the crossflow oscillations on turbulence. The statistical turbulence flow properties such as the Lagrangian integral time and length scales were estimated. The time-averaged flow structures and vorticity contours in the wake were obtained to analyze the wake geometric parameters by averaging instantaneously captured PIV images. The results revealed that at a fixed Reynolds number, the wake instability frequency of a circular cylinder in oscillating crossflow would present a value inbetween of the natural wake instability frequency and the crossflow oscillating frequency. The turbulence intensity and Lagrangian integral time and length scales in the cylinder wake of the oscillating crossflow were drastically larger than those in the natural crossflow. The time-averaged geometric parameters of the recirculation bubble in the circular-cylinder wake in oscillating crossflow were significantly smaller than those in the natural crossflow. The crossflow oscillations induced large scale flow motions and hence enhanced entrainment and mixing in the wake. These mechanisms expedited turbulent diffusion of momentum in the wake, and therefore the turbulence intensity and turbulence time and length scales were enlarged and the geometric parameters of recirculation bubble were shortened when compared with those in the natural crossflow.

Experimental study on the wake behind tapered circular cylinders

Journal of Fluids and Structures, 2011

The flow around tapered cylinders can act as basic models for numerous bluff body flows with a spanwise variation of either the body shape or the inflow conditions. The well-known vortex street is influenced by strong three-dimensional effects from the spanwise variation of the shedding frequency, namely oblique vortex shedding and vortex dislocations. Stereo-PIV was chosen to study these phenomena, since it allows analyzing planes with the full three-component, instantaneous velocity fields and local, time-dependent variations in the same setting. Hence, detailed aspects of the vortex dislocation phenomenon are presented. Single vortex dislocation events are presented through the local variation of the three measured velocity components u, v and w. Longer time-series reveal both period and location of these dislocation events, as well as quantity and sizes of the cells of constant shedding velocity in between them. The influence of the Reynolds number and the cylinder aspect ratio on the vortex cells could be shown. The analysis of the vortex shedding behavior shows good agreement with previously published results. At the same time, the applied PIV technique provides more spatial information than point-based measurements and offers insight into a Reynolds number range that is currently out of reach of Direct Numerical Simulations.

A numerical investigation of wake and mixing layer interactions of flow past a square cylinder

Meccanica, 2016

The aim of the present study is to simulate and analyze the interaction of two dimensional flow past a square cylinder in a laminar regime with an upstream mixing layer developed by an axis symmetrical horizontal splitter plate. The mixing layer is generated upstream of the square cylinder by mixing two uniform streams of fluid with different velocities above and below the splitter plate. A range of upstream domain lengths, distances between the splitter plate and the square cylinder, and upstream velocity ratios between the two streams of fluid are analyzed. Unconfined flow over a square cylinder placed in uniform upstream flow is initially analyzed as it plays a crucial role in understanding the properties of the wake. The results are compared with the existing literature to validate the code developed and they are found to be in very good agreement. It is observed from the present study that at smaller velocity ratios, the vortices shed into the wake consist of both clockwise and anticlockwise moment vortices. As velocity ratio is increased only clockwise moment vortices are shed downstream and anticlockwise vortices are shed as Kelvin-Helmholtz instabilities. In all simulations undertaken the same phenomena is observed independent of the upstream Reynolds number, indicating that velocity ratio is a primary parameter influencing the flow. Different instability modes were observed and they were highly dependent on the upstream velocity ratio.

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.

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.