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.

On the Effects of Incidence Angle on the Mean Wake of a Surface-Mounted Finite-Height Square Prism

2015 ◽  
Author(s):  
Ayodele Ogunremi ◽  
David Sumner
Keyword(s):  
Incidence Angle ◽  
Flow Direction ◽  
Ground Plane ◽  
Vortex Pair ◽  
Tip Vortex ◽  
Pressure Probe ◽  
Mean Velocity ◽  
Square Prism ◽  
Finite Height ◽  
The Mean

The wake of a surface-mounted finite-height square prism of sub-critical aspect ratio AR = 3 was studied experimentally in a low-speed wind tunnel at a Reynolds number of Re = 3.7×104. The ratio of the boundary layer thickness on the ground plane, to the width of the prism, was δ/D = 1.5. The incidence angle of the prism was varied from α = 0° to 45°. Wake mean velocity measurements were made in vertical planes normal to and parallel to the main flow direction using a seven-hole pressure probe. As the prism is rotated from α = 0° to 45°, the mean wake progressively widens and the maximum streamwise extent of the mean recirculation zone increases. The mean streamwise tip vortex pair is symmetric at 0° and 45°, but becomes strongly asymmetric at intermediate α, where the tip vortex is found higher above the ground plane on the wider side of the wake. The wake and tip vortex asymmetry is most pronounced near the critical incidence angle.

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.

scholarly journals Direct Numerical Simulation of Flow around a Circular Cylinder Controlled Using Plasma Actuators

2014 ◽  
Vol 2014 ◽  
pp. 1-13 ◽  
Author(s):  
Taichi Igarashi ◽  
Hiroshi Naito ◽  
Koji Fukagata
Keyword(s):  
Numerical Simulation ◽  
Circular Cylinder ◽  
Direct Numerical Simulation ◽  
Drag Reduction ◽  
Three Dimensional ◽  
Reduction Rate ◽  
Plasma Actuators ◽  
Two Dimensional ◽  
Freestream Velocity ◽  
The Mean

Flow around a circular cylinder controlled using plasma actuators is investigated by means of direct numerical simulation (DNS). The Reynolds number based on the freestream velocity and the cylinder diameter is set atReD=1000. The plasma actuators are placed at±90° from the front stagnation point. Two types of forcing, that is, two-dimensional forcing and three-dimensional forcing, are examined and the effects of the forcing amplitude and the arrangement of plasma actuators are studied. The simulation results suggest that the two-dimensional forcing is primarily effective in drag reduction. When the forcing amplitude is higher, the mean drag and the lift fluctuations are suppressed more significantly. In contrast, the three-dimensional forcing is found to be quite effective in reduction of the lift fluctuations too. This is mainly due to a desynchronization of vortex shedding. Although the drag reduction rate of the three-dimensional forcing is slightly lower than that of the two-dimensional forcing, considering the power required for the forcing, the three-dimensional forcing is about twice more efficient.

Fluctuating forces on a rigid circular cylinder in confined flow

1976 ◽  
Vol 78 (3) ◽  
pp. 561-576 ◽  
Author(s):  
A. Richter ◽  
E. Naudascher
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Critical Value ◽  
Experimental Information ◽  
Rectangular Duct ◽  
Wide Range ◽  
Confined Flow ◽  
The Mean ◽  
Lift And Drag

The fluctuating lift and drag acting on a long, rigidly supported circular cylinder placed symmetrically in a narrow rectangular duct were investigated for various blockage percentages over a wide range of Reynolds numbers around the critical value. The data obtained permit a full assessment of the effect of confinement on the mean-drag coefficient, the root-mean-square values of both the drag and the lift fluctuations, the Strouhal number of the dominant vortex shedding, and the Reynolds number marking transition from laminar to turbulent flow separation. Besides experimental information on a subject on which little is known so far, the paper provides a basis for the deduction of better correction procedures concerning the effects of blockage.

Influence of a Magnetic Obstacle on Forced Convection in a Three-Dimensional Duct With a Circular Cylinder

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Xidong Zhang ◽  
Hulin Huang ◽  
Yin Zhang ◽  
Hongyan Wang
Keyword(s):  
Pressure Drop ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Three Dimensional ◽  
Separated Flow ◽  
Optimum Conditions ◽  
Nusselt Numbers ◽  
Maximum Value ◽  
Wide Range ◽  
The Mean

The predictions of flow structure, vortex shedding, and drag force around a circular cylinder are promoted by both academic interest and a wide range of practical situations. To control the flow around a circular cylinder, a magnetic obstacle is set upstream of the circular cylinder in this study for active controlling the separated flow behind bluff obstacle. Moreover, the changing of position, size, and intensity of magnetic obstacle is easy. The governing parameters are the magnetic obstacle width (d/D = 0.0333, 0.1, and 0.333) selected on cylinder diameter, D, and position (L/D) ranging from 2 to 11.667 at fixed Reynolds number Rel (based on the half-height of the duct) of 300 and the relative magnetic effect given by the Hartmann number Ha of 52. Results are presented in terms of instantaneous contours of vorticity, streamlines, drag coefficient, Strouhal number, pressure drop penalty, and local and average Nusselt numbers for various magnetic obstacle widths and positions. The computed results show that there are two flow patterns, one with vortex shedding from the magnetic obstacle and one without vortex shedding. The optimum conditions for drag reduction are L/D = 2 and d/D = 0.0333–0.333, and under these conditions, the pressure drop penalty is acceptable. However, the maximum value of the mean Nusselt number of the downstream cylinder is about 93% of that for a single cylinder.

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