Effect of an Oscillating Free Stream on the Unsteady Pressure on a Circular Cylinder

1973 ◽  
Vol 95 (2) ◽  
pp. 249-252 ◽  
Author(s):  
H. M. Hatfield ◽  
M. V. Morkovin
Keyword(s):  
Circular Cylinder ◽  
High Frequency ◽  
Free Stream ◽  
Vortex Street ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Resonant Coupling ◽  
Unsteady Pressure ◽  
The Mean ◽  
Coupling Mechanisms

An experimental investigation was made of the unsteady pressure on a circular cylinder in a free stream of constant and oscillating velocity. Consideration of possible resonant coupling mechanisms between the Karman vortex street fluctuations and free-stream velocity oscillations showed such coupling to, be insignificant. The frequency of the vortex street fluctuations scales with the mean velocity for high frequency (30 to 50 Hz at mean velocity of 40 fps) oscillation of the free stream.

Active Control Methods for Drag Reduction in Flow Over Bluff Bodies (Keynote)

2002 ◽  
Author(s):  
Haecheon Choi
Keyword(s):  
Circular Cylinder ◽  
Drag Reduction ◽  
Active Control ◽  
High Frequency ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Control Methods ◽  
Periodic Forcing ◽  
Time Periodic

In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40∼3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is 105 based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing with a high frequency (larger than 20 times the vortex shedding frequency) produces 50% drag reduction for the flow over a sphere at Re = 105. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

A Numerical Investigation of the Effects of Flow Pulsations on the Drag Force Over a Cylinder

2011 ◽  
Author(s):  
Eric D’herde ◽  
Laila Guessous
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Natural Frequency ◽  
Vortex Shedding ◽  
Free Stream ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency fs when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag force over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies f varying between 0.25 and 5 times the natural shedding frequency were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. This drop in the drag coefficient is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder.

A Numerical Investigation of the Effects of Flow Pulsations on the Vortex Shedding, Drag and Lift Forces Over a Cylinder

2011 ◽  
Author(s):  
Eric D’herde ◽  
Laila Guessous
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Free Stream ◽  
Lift Coefficient ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency ◽  
Drag And Lift

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag and lift forces over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies varying between 0.25 and 5 times the natural shedding frequency fs were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. The first drop in the drag coefficient, i.e. near f = fs, is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder. This change in vortex shedding pattern manifests itself as a departure from symmetrical shedding, and in a non-zero mean lift coefficient value. The second drop, i.e. near f = 2 fs, has similar characteristics, except that the mean lift coefficient remains at zero.

Free convection effects on the oscillatory flow past an infinite, vertical, porous plate with constant suction. I

1973 ◽  
Vol 333 (1592) ◽  
pp. 25-36 ◽  
Keyword(s):  
Free Convection ◽  
Prandtl Number ◽  
Skin Friction ◽  
Free Stream ◽  
Reverse Flow ◽  
Porous Plate ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Mean Temperature ◽  
The Mean

An analysis of the two-dimensional flow of an incompressible, viscous fluid past an infinite porous plate is presented under the following conditions: (i) the suction velocity normal to the plate is constant, (ii) the free stream velocity oscillates in time about a constant mean, (iii) the plate temperature is constant, (iv) the difference between the temperature of the plate and the free stream is moderately large causing the free convection currents. Approximate solutions for the coupled nonlinear equations are obtained for velocity and temperature field. Expressions for the mean velocity, the mean temperature and the mean skin-friction are derived in part I. The mean velocity, the mean temperature are shown on graphs and the numerical values of the skin friction are entered in table 1. The effects of G (the Grashof number), P (the Prandtl number) and E (the Eckert number), on the mean motion of air and water are described during the course of discussion. Some of the important observations are as follows. There is a reverse flow of the mean velocity profile of fluids, with small Prandtl number, in the boundary layer close to a plate which is being heated by the free convection currents. The mean skin friction increases with more cooling of the plate and decreases with more heating of the plate. In part II of the paper, the fluctuating flow is described.

scholarly journals Added mass of a circular cylinder oscillating in a free stream

2013 ◽  
Vol 469 (2156) ◽  
pp. 20130135 ◽  
Author(s):  
Efstathios Konstantinidis
Keyword(s):  
Circular Cylinder ◽  
Free Stream ◽  
Transverse Velocity ◽  
Inertial Force ◽  
Added Mass ◽  
Stream Velocity ◽  
Virtual Experiment ◽  
Classical Formulation ◽  
Fundamental Understanding ◽  
Two Dimensional Flow

The fundamental understanding of the added mass phenomenon associated with the motion of a solid body relative to a fluid is revisited. This paper focuses on the two-dimensional flow around a circular cylinder oscillating transversely in a free stream. A virtual experiment reveals that the classical approach to this problem leads to a paradox. The inertial force is derived afresh based on analysis of the motion in a frame of reference attached to the cylinder centroid, which overcomes the paradox in the classical formulation. It is shown that the inertial force depends not only on the acceleration of the cylinder per se , but also on the relative motion between body and fluid embodied in a parameter called alpha, α , which represents the ratio of the maximum transverse velocity of the cylinder to the free-stream velocity; the induced inertial force is directionally varying and non-harmonic in time depended on the alpha parameter. It is further shown that the component of the inertial force in the transverse direction is negligible for α <0.1, increases quadratically for α <0.5, and tends asymptotically to the classical result as , i.e. in still fluid.

Sting-free measurements on a magnetically supported right circular cylinder aligned with the free stream

2008 ◽  
Vol 596 ◽  
pp. 49-72 ◽  
Author(s):  
HIROSHI HIGUCHI ◽  
HIDEO SAWADA ◽  
HIROYUKI KATO
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Free Stream ◽  
Aerodynamic Force ◽  
Cross Flow ◽  
Leading Edge ◽  
Magnetic Suspension ◽  
Recirculation Region ◽  
Mean Velocity ◽  
Wide Range

The flow over cylinders of varying fineness ratio (length to diameter) aligned with the free stream was examined using a magnetic suspension and balance system in order to avoid model support interference. The drag coefficient variation of a right circular cylinder was obtained for a wide range of fineness ratios. Particle image velocimetry (PIV) was used to examine the flow field, particularly the behaviour of the leading-edge separation shear layer and its effect on the wake. Reynolds numbers based on the cylinder diameter ranged from 5×104 to 1.1×105, while the major portion of the experiment was conducted at ReD=1.0×105. For moderately large fineness ratio, the shear layer reattaches with subsequent growth of the boundary layer, whereas over shorter cylinders, the shear layer remains detached. Differences in the wake recirculation region and the immediate wake patterns are clarified in terms of both the mean velocity and turbulent flow fields, including longitudinal vortical structures in the cross-flow plane of the wake. The minimum drag corresponded to the fineness ratio for which the separated shear layer reattached at the trailing edge of the cylinder. The base pressure was obtained with a telemetry technique. Pressure fields and aerodynamic force fluctuations are also discussed.

scholarly journals On the streamwise evolution of turbulent boundary layers in arbitrary pressure gradients

2002 ◽  
Vol 461 ◽  
pp. 61-91 ◽  
Author(s):  
A. E. PERRY ◽  
IVAN MARUSIC ◽  
M. B. JONES
Keyword(s):  
Boundary Layers ◽  
Scaling Laws ◽  
Power Spectra ◽  
Adverse Pressure Gradient ◽  
Turbulent Boundary Layers ◽  
Stream Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
Law Of The Wall ◽  
The Mean

A new approach to the classic closure problem for turbulent boundary layers is presented. This involves, first, using the well-known mean-flow scaling laws such as the log law of the wall and the law of the wake of Coles (1956) together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used for establishing a framework for closure. Initially closure is achieved here empirically and the potential for achieving closure in the future using the wall-wake attached eddy model of Perry & Marusic (1995) is outlined. Comparisons are made with experiments covering adverse-pressure-gradient flows in relaxing and developing states and flows approaching equilibrium sink flow. Mean velocity profiles, total shear stress and Reynolds stress profiles can be computed for different streamwise stations, given an initial upstream mean velocity profile and the streamwise variation of free-stream velocity. The attached eddy model of Perry & Marusic (1995) can then be utilized, with some refinement, to compute the remaining unknown quantities such as Reynolds normal stresses and associated spectra and cross-power spectra in the fully turbulent part of the flow.

Free Span Vibrations of Submarine Pipelines in Steady Flows—Effect of Free-Stream Turbulence on Mean Drag Coefficients

1985 ◽  
Vol 107 (4) ◽  
pp. 415-420 ◽  
Author(s):  
A. To̸rum ◽  
N. M. Anand
Keyword(s):  
Free Stream ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
Steady Flows ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
The Mean ◽  
Elastically Supported ◽  
Wave Induced ◽  
Rigid Smooth

In this paper part of the results of a laboratory study related to free span vibrations of submarine pipelines in steady and wave-induced fluid flows are summarized. Tests have been carried out using an elastically supported rigid smooth circular cylinder close to a plane smooth boundary in steady flows with turbulence intensities of 3.4, 5.5, and 9.5 percent for four cylinder gap to diameter ratios, G/D equal to 0.5, 0.75, 1.0, and 3.0. The range of Reynolds numbers based on mean velocity of flow and cylinder diameter was 0.65·104 to 0.35·105. Effect of turbulence intensity on the mean drag force and vibration amplitudes are discussed.

A vortex-street model of the flow in the similarity region of a two-dimensional free turbulent jet

1982 ◽  
Vol 123 ◽  
pp. 523-535 ◽  
Author(s):  
J. W. Oler ◽  
V. W. Goldschmidt
Keyword(s):  
Experimental Data ◽  
Reynolds Stress ◽  
Turbulent Jet ◽  
Vortex Street ◽  
Two Dimensional ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Core ◽  
The Mean ◽  
Similarity Relationships

The mean-velocity profiles and entrainment rates in the similarity region of a two-dimensional jet are generated by a simple superposition of Rankine vortices arranged to represent a vortex street. The spacings between the vortex centres, their two-dimensional offsets from the centreline, as well as the core radii and circulation strengths, are all governed by similarity relationships and based upon experimental data.Major details of the mean flow field such as the axial and lateral mean-velocity components and the magnitude of the Reynolds stress are properly determined by the model. The sign of the Reynolds stress is, however, not properly predicted.

The Effects of Surface Roughness on the Mean Velocity Profile in a Turbulent Boundary Layer

2002 ◽  
Vol 124 (3) ◽  
pp. 664-670 ◽  
Author(s):  
Donald J. Bergstrom ◽  
Nathan A. Kotey ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Friction Velocity ◽  
Outer Region ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Outer Flow ◽  
The Mean

Experimental measurements of the mean velocity profile in a canonical turbulent boundary layer are obtained for four different surface roughness conditions, as well as a smooth wall, at moderate Reynolds numbers in a wind tunnel. The mean streamwise velocity component is fitted to a correlation which allows both the strength of the wake, Π, and friction velocity, Uτ, to vary. The results show that the type of surface roughness affects the mean defect profile in the outer region of the turbulent boundary layer, as well as determining the value of the skin friction. The defect profiles normalized by the friction velocity were approximately independent of Reynolds number, while those normalized using the free stream velocity were not. The fact that the outer flow is significantly affected by the specific roughness characteristics at the wall implies that rough wall boundary layers are more complex than the wall similarity hypothesis would allow.

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