Effect of Aspect Ratio on the Flow Field Above the Free End of a Finite Circular Cylinder

2014 ◽  
Author(s):  
Noorallah Rostamy ◽  
David Sumner ◽  
Donald J. Bergstrom ◽  
James D. Bugg
Keyword(s):  
Wind Tunnel ◽  
Circular Cylinder ◽  
Recirculation Zone ◽  
Ground Plane ◽  
Symmetry Plane ◽  
Leading Edge ◽  
Velocity Measurements ◽  
Reattachment Point ◽  
Aspect Ratios ◽  
The Mean

The flow above the free end of a surface-mounted finite-height circular cylinder was studied in a low-speed wind tunnel using particle image velocimetry (PIV). The cylinder was mounted vertically in the wind tunnel, normal to a ground plane. The approaching flow was in the x-direction and the cylinder axis was aligned in the z-direction. Velocity measurements were made above the free-end surface in several vertical (x-z) planes and several horizontal (x-y) planes, for finite circular cylinders of aspect ratios AR = 9, 7, 5 and 3, at a Reynolds number of Re = 4.2×104. The relative thickness of the boundary layer on the ground plane was δ/D = 1.7. In the vertical symmetry plane, the mean velocity measurements show the prominent separation from the circumferential leading edge, the mean recirculation zone above the free-end surface, the arch vortex inside the recirculation zone, and reattachment of the flow onto the free-end surface. Experimental evidence is found for a leading-edge separation bubble, a flow structure which has been reported in some numerical simulations in the literature. As AR decreases, the reattachment point and the centre of the arch vortex move downstream, the recirculation zone becomes thicker, and the centre of the arch vortex moves higher above the free end. Away from the symmetry plane, the recirculation zone becomes thinner, the arch vortex centre moves upstream and closer to the free-end surface, and the reattachment point moves upstream. In the horizontal planes, measurements made very close to the surface can approximate the mean surface streamline topology, revealing the pair of foci representing the termination points of the arch vortex, the prominent curved reattachment line, reverse flow beneath the mean recirculation zone, and the reattachment and separation saddle points on the free-end centerline.

The Mean Flow Structure on the Symmetry Plane of a Turbulent Junction Vortex

1990 ◽  
Vol 112 (1) ◽  
pp. 16-22 ◽  
Author(s):  
F. J. Pierce ◽  
I. K. Tree
Keyword(s):  
Velocity Field ◽  
Vortex Structure ◽  
Symmetry Plane ◽  
Leading Edge ◽  
Surface Flow ◽  
The Body ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Vortex System ◽  
The Mean

The mean flow structure on the symmetry plane of a turbulent junction vortex is documented. A two-channel, two-color LDV system allowed nonintrusive measurements of the two velocity components on the symmetry plane. Extensive measurements were made in and around the separation point and within the junction vortex system, both very close to the floor and to the leading edge of the body generating the vortex system. Real-time smoke visualizations confirmed a region of strongly time-variant flow with large changes in the scale and position of the principal vortex structure. The extensive velocity field data are correlated with high quality surface visualizations and surface pressure measurements. The mean velocity measurements show one large well-defined vortex structure and one singular saddle point of separation on the symmetry plane. The transverse vorticity field computed from the extensive velocity field suggests a very strong but small second, counter rotating vortex located in the extreme corner formed by the floor and leading edge of the body. The surface flow visualization suggests only one clear separation line. The single pair of counter rotating vortices revealed by these detailed LDV velocity measurements is in agreement with two independent studies which used multiple orifice pressure probes. This measured two vortex model is not in agreement with the frequently pictured four vortex flow model, inferred from surface flow visualizations, showing two pairs of counter rotating vortices.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

Wake Interference Effects for Two Surface-Mounted Finite Cylinders in a Tandem Configuration

2014 ◽  
Author(s):  
David Sumner ◽  
He Li
Keyword(s):  
Circular Cylinder ◽  
Ground Plane ◽  
Flow Regimes ◽  
Vortex Structures ◽  
Finite Cylinder ◽  
Pressure Probe ◽  
Extended Body ◽  
Tandem Configuration ◽  
Finite Height ◽  
The Mean

The mean wake of two identical surface-mounted finite-height circular cylinders arranged in a tandem configuration was investigated in a low-speed wind tunnel using a seven-hole pressure probe. The Reynolds number was Re = 2.4×104, the cylinder aspect ratio was AR = 9, and the boundary layer thickness on the ground plane relative to the cylinder height was δ/H ≈ 0.4. Three centre-to-centre longitudinal pitch ratios of L/D = 1.125, 2, and 5 were examined, corresponding to the extended-body, reattachment, and co-shedding flow regimes, respectively. Reference measurements were also made in the wake of a single finite circular cylinder of AR = 9. For the tandem configurations, velocity measurements were made behind the downstream cylinder in two orthogonal vertical planes. Compared to the wake of the single surface-mounted finite-height circular cylinder, the mean downwash and upwash flows for the tandem cylinders, behind the downstream cylinder, were weaker, the mean recirculation zone behind the downstream cylinder was shorter, and the mean wake extended higher above the ground plane, for all three pitch ratios. Marked changes were also observed in the mean streamwise wake vortex structures, compared to the case of the single finite cylinder. For the extended-body and reattachment flow regimes, the tip vortex structures became elongated in the wall-normal direction. In the co-shedding regime, two sets of tip vortices were observed, with the second set possibly originating from the upstream cylinder.

Interactions Between the Shear Layer Emanating From Rectangular Cylinders and the Near Wake Region

2021 ◽  
Author(s):  
Sedem Kumahor ◽  
Samuel Addai ◽  
Mark F. Tachie
Keyword(s):  
Kinetic Energy ◽  
Shear Layer ◽  
Turbulent Kinetic Energy ◽  
Leading Edge ◽  
Wake Region ◽  
Mean Flow ◽  
Near Wake ◽  
Aspect Ratios ◽  
The Mean ◽  
Rectangular Cylinders

Abstract The interactions between the separated shear layer and the near wake region of rectangular cylinders of varying streamwise extents in a uniform flow are investigated using time resolved particle image velocimetry. The streamwise aspect ratios (AR) tested were 1 and 5, and the Reynolds number based on the oncoming flow velocity and cylinder height is 16200. The effects of varying AR on the mean flow, turbulent kinetic energy and Reynolds shear stresses are studied. Furthermore, the unsteady characteristics of the separation bubbles are examined in terms of frequency spectra analysis. The mean flow topology shows flow separation at the leading edge is not affected by the streamwise aspect ratios. However, the primary, secondary and wake vortexes show significant differences. Mean flow reattaches over the cylinder at 4.30 cylinder heights in the AR5 case while there is no mean reattachment in the AR1 case. The magnitudes of turbulent kinetic energy and Reynolds shear stress in the wake region are an order of magnitude higher in AR1 compared to AR5. Depending on the streamwise location, the vortex shedding motions in the near wake region reflect the dominant and second harmonic of the shear layer shedding frequency measured near the leading edge.

Energy Separation and Acoustic Interaction in Flow Across a Circular Cylinder

2001 ◽  
Vol 123 (4) ◽  
pp. 682-687 ◽  
Author(s):  
R. J. Goldstein ◽  
Boyong He
Keyword(s):  
Wind Tunnel ◽  
Circular Cylinder ◽  
Stagnation Point ◽  
Energy Separation ◽  
Acoustic Measurements ◽  
Velocity Measurements ◽  
Acoustic Interaction ◽  
Strong Energy ◽  
Separation Mechanisms ◽  
Rear Stagnation Point

Energy separation in a flow around an adiabatic circular cylinder is investigated using a surface-mounted thermocouple. Energy separation mechanisms in different regions around the cylinder are discussed. Velocity measurements near the rear stagnation point and acoustic measurements indicate that shedding vortices and the wind tunnel intrinsic resonant acoustics can strengthen each other when their frequencies match producing strong energy separation.

An Experimental Investigation of Aspect Ratio and Incidence Angle Effects for the Flow Around Surface-Mounted Finite-Height Square Prisms

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
J. F. McClean ◽  
D. Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted finite-height square prism was investigated using a low-speed wind tunnel. The experiments were conducted at a Reynolds number of Re = 7.3 × 104 for prism aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles from α = 0 deg to 45 deg. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the mean drag coefficient and Strouhal number were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The mean drag coefficient and Strouhal number for the finite prism were less sensitive to changes in incidence angle compared to the infinite square prism. The critical incidence angle, corresponding to minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number, shifted to a higher incidence angle compared to the infinite square prism, with values ranging from αcritical = 15 deg to 18 deg; this shift was greatest for the prisms of higher aspect ratio. The behavior of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of mean drag coefficient and mean lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0 deg and 15 deg. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30 deg and 45 deg, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

Aerodynamic Forces and Vortex Shedding for Surface-Mounted Finite Square Prisms and the Effects of Aspect Ratio and Incidence Angle

2012 ◽  
Author(s):  
John F. McClean ◽  
David Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of interest were the effects of aspect ratio and incidence angle on the mean aerodynamic forces and vortex shedding. Compared to the case of the “infinite” (or two-dimensional) square prism, the flow around the finite square prism has not been extensively studied. The experiments were conducted at a Reynolds number of Re = 7.2 × 104 for aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles of α = 0°, 15°, 30° and 45°. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe in the wake, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the Strouhal number and the mean drag coefficient were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The behaviours of the mean drag coefficient and Strouhal number with incidence angle were less sensitive compared to the case of the infinite square prism, although a minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number were found at α = 15°. The reduced sensitivity to incidence angle is attributed to the complex three-dimensional flow over the free end of the prism and the downwash flow that enters the near wake. The behaviour of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of drag coefficient and lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0° and 15°. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30° and 45°, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

Flow in the Wake of Surface-Mounted Pyramids

2002 ◽  
Author(s):  
Mazen AbuOmar ◽  
Robert J. Martinuzzi
Keyword(s):  
Boundary Layers ◽  
Vortex Shedding ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Reattachment Point ◽  
Flow Surface ◽  
Surface Patterns ◽  
The Mean ◽  
Shedding Frequency ◽  
Pressure And Velocity Measurements

The flow around square-based, wall-mounted pyramids in thin and thick boundary layer was experimentally investigated as a function of the pyramid apex angle, ζ, and, angle of attack, α, based on mean flow surface patterns, pressure and velocity measurements. For thin boundary layers, wake periodicity is observed. For slender pyramids (20° < ζ < 75°), periodic shedding of vortices is observed. The shedding frequency scales with the frontal (projected) width. For broad pyramids, wake periodicity cannot be related to vortex shedding. Vortex shedding appears related to the existence of a double vortex-structure along the side faces of the slender pyramids. For thick boundary layers, no wake periodicity is observed and the mean flow structure resembles that of broad pyramids. In all cases, the separation and reattachment point upstream and downstream of the obstacle scales with the projected frontal area.

Tomographic PIV investigation of vortex shedding topology for a cantilevered circular cylinder

2021 ◽  
Vol 931 ◽  
Author(s):  
R.J. Crane ◽  
A.R. Popinhak ◽  
R.J. Martinuzzi ◽  
C. Morton
Keyword(s):  
Circular Cylinder ◽  
Mean Field ◽  
Three Dimensional ◽  
Base Region ◽  
Aspect Ratios ◽  
Topological Features ◽  
Image Velocimetry ◽  
Side Walls ◽  
Order Representation ◽  
The Mean

The wake of a finite wall-mounted circular cylinder of diameter $D$ and height $H$ is investigated for aspect ratios $3\leq H/D \leq 7$ and boundary layer thickness of $\delta /D \approx 0.98$ using tomographic particle image velocimetry. The Reynolds number based on $D$ is $Re = 750$ . The mean wake topology is related to the evolution of the periodically shed vortices, educed from a low-order representation based on proper orthogonal decomposition of the three-dimensional velocity field. The main topological features are an arch vortex, defining the recirculating base region, and a quadrupole structure consisting of two pairs of opposite-sign vorticity concentrations extending downstream behind the obstacle-free end and wall junction. The quadrupole is the time-averaged signature of shed vortices. Vortex-tilting terms in the base region act to reorient flow-normal vorticity components streamwise, resulting in the reorientation of the ends of vortices initially shed parallel to the cylinder side walls. Through the action of the vortex-stretching terms, the bent ends connect successive vortices in a continuous chain. The influence of $H/D$ on the development of the quadrupole is characterized. The results demonstrate that the quadrupole in the mean field emerges as an imprint of the shed full-loop structures. This work reconciles mean and instantaneous interpretations satisfying the solenoidal condition on the vorticity field.

Flow Over a Blunt Disk With Incidence: Location of Stagnation Point and Flow Separation

2014 ◽  
Author(s):  
Christian Helcig ◽  
Stephan Uhkoetter ◽  
Stefan aus der Wiesche
Keyword(s):  
Stagnation Point ◽  
Laser Doppler Anemometry ◽  
Separation Bubble ◽  
Finite Thickness ◽  
Convective Heat Transfer Coefficient ◽  
Leading Edge ◽  
Angle Of Incidence ◽  
Velocity Measurements ◽  
Heated Disk ◽  
The Mean

The flow over a blunt disk placed in a stream of air was investigated by means of detailed velocity measurements employing Laser-Doppler-Anemometry (LDA). Special attention was spent to the effect of incidence. This parameter governed the location of the stagnation point and the length of the separation bubble. For a parallel disk with finite thickness, a large separation bubble was formed at the leading edge, followed by reattached regions of the turbulent flow. With increasing incidence, the length of the separation bubble significantly decreased, and the stagnation point moved from the blunt side to the leading edge. For angle of incidence higher than a special transition value, a stagnation flow over the disk surfaces resulted without significant turbulence. In case of a heated disk, the corresponding transition of the mean convective heat transfer coefficient as function of incidence was very sharp and discontinuous.

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