Reynolds Number Dependence of the Flow Structure Behind Two Side-by-Side Cylinders

2002 ◽  
Author(s):  
S. J. Xu ◽  
Y. Zhou ◽  
R. M. C. So
Keyword(s):  
Reynolds Number ◽  
Flow Structure ◽  
Free Stream ◽  
Present Finding ◽  
Dominant Frequency ◽  
Wind Tunnels ◽  
Stream Velocity ◽  
Single Vortex ◽  
The One ◽  
Reynolds Number Dependence

The wake structure of two side-by-side cylinders was experimentally investigated using flow visualization and hotwire techniques. The investigation was focused on the asymmetrical flow regime, i.e., T/d = 1.2 – 1.6, where T is the center-to-center cylinder spacing and d is the cylinder diameter. Experiments were conducted in both water and wind tunnels at a Reynolds number (Re) range of 150 – 14300. It has been found that, as Re increases, the flow structure behind the cylinders would change from one single vortex street to two streets with one narrow and one wide, for the same T/d. The one-street flow structure is dominated by one frequency ƒ0* = ƒ0d/U∞ ≈ 0.09, where ƒ0 is the dominant frequency and U∞ is the free-stream velocity. On the other hand, two frequencies, ƒ0* ≈ 0.3 and 0.09, characterized the two-street flow structure. These are associated with the narrow and wide street frequency, respectively. It is further observed that the critical Re, at which transition from single to two streets occurs, increases as T/d decreases. The present finding help clarify previous scattered reports for 1.2 < T/d < 1.5: detection of one dominant frequency by some but two by others.

Further Studies of the Axisymmetric Disk Wake Using the “Slice POD”

2002 ◽  
Author(s):  
Peter B. V. Johansson ◽  
William K. George
Keyword(s):  
Reynolds Number ◽  
Strouhal Number ◽  
Free Stream ◽  
Stream Velocity ◽  
Azimuthal Mode ◽  
Wire Arrays ◽  
Mode 2 ◽  
High Reynolds ◽  
The One ◽  
Zero Frequency

This paper presents the findings of three experiments using multi-point hot-wire arrays in the high Reynolds number axisymmetric turbulent wake behind a disk. The purpose of the multiple experiments was to validate earlier and less extensive experiments. The ‘slice POD’ was applied to all sets to examine the effects of array coverage and the disk support system. The Reynolds number based on the free stream velocity and disk diameter was kept constant at 28,000. The investigated region spanned from 10 to 60 disk diameters downstream. These results confirm the earlier findings. In particular, the eigenvalues integrated over frequency show a azimuthal mode-1 dominance at x/D = 10 which evolves to a mode-2 dominance by x/D = 50. For all downstream positions, two distinct peaks were found in the first eigenspectrum: one for azimuthal mode-2 at near zero frequency, and another for azimuthal mode-1 at a Strouhal number (fd/U∞) of 0.126. Both peaks decrease in magnitude as the flow evolves downstream, but the peak at the Strouhal number 0.126 decrease more rapidly then the one at near-zero frequency, leaving the latter to eventually dominate.

Experimental Study on the Effect of Reynolds Number Variation on Drag Force for Various Cut Angle on D-Type Cylinders

2014 ◽  
Vol 493 ◽  
pp. 140-144
Author(s):  
Astu Pudjanarsa ◽  
Ardian Ardawalika
Keyword(s):  
Experimental Study ◽  
Reynolds Number ◽  
Drag Force ◽  
Free Stream ◽  
Force Balance ◽  
Stream Velocity ◽  
Turning Angle ◽  
Subsonic Wind Tunnel ◽  
Minimum Drag ◽  
Number Variation

Experimental study on the effect of Reynolds number variation on drag force for various cut angles on D-type cylinders was performed. Five different cut angles on different cylinders were applied including: 35o, 45o, 53o, 60o, and 65o. The free stream velocity was varied so the Reynolds number also varied.The experiment was carried out at a subsonic wind tunnel. Drag force for a cut D-type cylinder (for example 35o) was measured using a force balance and wind speed was varied so that corresponding Reynolds number of 2.4×104÷5.3×104 were achieved. Wind turning angle was kept at 0o (without turning angle). This experiment repeated for other D-type cylinders.Experiment results show that, for all D-type cylinders, drag force decreased as the Reynolds number increased, then it was increased after attain minimum drag force. For all D-type cylinders and all variations of Reynolds number the drag minimum is attained at cut angle of 53o. This value is appropriate with previous experiment results.

CFD Study of the Pressure Distribution on the Back of a 3-D Bluff Body (CUBE) of Air Flow in Different Reynolds Numbers

2009 ◽  
Author(s):  
Mohammad Javad Izadi ◽  
Pegah Asghari ◽  
Malihe Kamkar Delakeh
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Free Stream ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
The Body ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Unsteady Forces ◽  
Flow Round

The study of flow around bluff bodies is important, and has many applications in industry. Up to now, a few numerical studies have been done in this field. In this research a turbulent unsteady flow round a cube is simulated numerically. The LES method is used to simulate the turbulent flow around the cube since this method is more accurate to model time-depended flows than other numerical methods. When the air as an ideal fluid flows over the cube, flow separate from the back of the body and unsteady vortices appears, causing a large wake behind the cube. The Near-Wake (wake close to the body) plays an important role in determining the steady and unsteady forces on the body. In this study, to see the effect of the free stream velocity on the surface pressure behind the body, the Reynolds number is varied from one to four million and the pressure on the back of the cube is calculated numerically. From the results of this study, it can be seen that as the velocity or the Reynolds number increased, the pressure on the surface behind the cube decreased, but the rate of this decrease, increased as the free stream flow velocity increased. For high free stream velocities the base pressure did not change as much and therefore the base drag coefficient stayed constant (around 1.0).

An experimental investigation of turbulent spots and breakdown to turbulence

1960 ◽  
Vol 9 (2) ◽  
pp. 235-246 ◽  
Author(s):  
J. W. Elder
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Hydrodynamic Stability ◽  
Free Stream ◽  
Stream Velocity ◽  
Turbulent Spot ◽  
Turbulent Spots ◽  
The Impact ◽  
Critical Intensity

The theory of hydrodynamic stability and the impact on it of recent work with turbulent spots is discussed. Emmons's (1951) assumptions about the growth and interaction of turbulent spots are found experimentally to be substantially correct. In particular it is shown that the region of turbulent flow on a flat plate is simply the sum of the areas that would be obtained if all spots grew independently.An investigation of the conditions required for breakdown to turbulence near a wall, that is, to initiate a turbulent spot, suggests that regardless of how disturbances are generated in a laminar boundary layer and independent of both the Reynolds number and the spatial extent of the disturbances, breakdown to turbulence occurs by the initiation of a turbulent spot at all points at which the velocity fluctuation exceeds a critical intensity. Over most of the layer this intensity is about 0·2 times the free-stream velocity. The Reynolds number is important merely in respect of the growth of disturbances prior to breakdown.

A Closed Formula for the Drag of a Flat Plate with Transition in the Absence of a Pressure Gradient

1960 ◽  
Vol 64 (589) ◽  
pp. 38-39 ◽  
Author(s):  
A. R. Collar
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Transition Point ◽  
Free Stream ◽  
Analytical Form ◽  
Stream Velocity ◽  
Closed Formula ◽  
Friction Drag ◽  
Do So

Most University students of fluid mechanics are familiar with the problem of the evaluation of the skin friction drag of a flat plate in the absence of a pressure gradient: transition is specified, usually by the free stream velocity and the transition point, or directly by the transition Reynolds number. The solution is normally obtained numerically: so far as the writer is aware, the processes used have not been written down in closed analytical form, though to do so presents no difficulty.

Unsteady drag on a sphere at finite Reynolds number with small fluctuations in the free-stream velocity

1991 ◽  
Vol 233 ◽  
pp. 613-631 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence ◽  
Ronald J. Adrian
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Low Frequency ◽  
Reynolds Numbers ◽  
Free Stream Velocity ◽  
Creeping Flow ◽  
Stream Velocity ◽  
Velocity Difference ◽  
Long Time ◽  
The Time Domain

Unsteady flow over a stationary sphere with small fluctuations in the free-stream velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady Stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the force has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & Mckee (1984) wherein for a particle introduced to a turbulent flow the initial velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effects of Reynolds number on the unsteady drag due to the fluctuating free-stream velocity are presented. The implications for particle motion in turbulence are discussed.

Transition to chaos in the wake of a rolling sphere

2012 ◽  
Vol 695 ◽  
pp. 135-148 ◽  
Author(s):  
A. Rao ◽  
P.-Y. Passaggia ◽  
H. Bolnot ◽  
M.C. Thompson ◽  
T. Leweke ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Low Frequency ◽  
Quantitative Information ◽  
Hairpin Vortices ◽  
Rotating Sphere ◽  
Transition To Chaos ◽  
Rolling Sphere ◽  
The One ◽  
Planar Symmetry

AbstractThe wake of a sphere rolling along a wall at low Reynolds number is investigated numerically and experimentally. Two successive transitions are identified in this flow, as the Reynolds number is increased. The first leads to the periodic shedding of planar symmetric hairpin vortices. The second and previously unknown transition involves a loss of planar symmetry and a low-frequency lateral oscillation of the wake, exhibiting a surprising 7:3 resonance with the hairpin vortex shedding. The two transitions are characterized by dye visualizations and quantitative information obtained from numerical simulations, such as force coefficients and wake frequencies (Strouhal numbers). Both transitions are found to be supercritical. Further increasing the Reynolds number, the flow becomes progressively more disorganized and chaotic. Overall, the transition sequence for the rolling sphere is closer to the one for a non-rotating sphere in a free stream than to that of a non-rotating sphere close to a wall.

The Magnus Effect: A Summary of Investigations to Date

1961 ◽  
Vol 83 (3) ◽  
pp. 461-470 ◽  
Author(s):  
W. M. Swanson
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Mathematical Problem ◽  
Experimental Results ◽  
Stream Velocity ◽  
Magnus Force ◽  
Drag Forces ◽  
Number Ratio ◽  
Magnus Effect ◽  
Lift And Drag

The Magnus force on a rotating body traveling through a fluid is partly responsible for ballistic missile and rifle shell inaccuracies and dispersion and for the strange deviational behavior of such spherical missiles as golfballs and baseballs. A great deal of effort has been expended in attempts to predict the lift and drag forces as functions of the primary parameters, Reynolds number, ratio of peripheral to free-stream velocity, and geometry. The formulation and solution of the mathematical problem is of sufficient difficulty that experimental results give the only reliable information on the phenomenon. This paper summarizes some of the experimental results to date and the mathematical attacks that have been made on the problem.

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