Showing papers on "Vortex shedding published in 1989"

Oblique and Parallel Modes of Vortex Shedding in the Wake of a Circular Cylinder at Low Reynolds Numbers

Abstract: Two fundamental characteristics of the low-Reynolds-number cylinder wake, which have involved considerable debate, are first the existence of discontinuities in the Strouhal-Reynolds number relationship, and secondly the phenomenon of oblique vortex shedding. The present paper shows that both of these characteristics of the wake are directly related to each other, and that both are influenced by the boundary conditions at the ends of the cylinder, even for spans of hundreds of diameters in length. It is found that a Strouhal discontinuity exists, which is not due to any of the previously proposed mechanisms, but instead is caused by a transition from one oblique shedding mode to another oblique mode. This transition is explained by a change from one mode where the central flow over the span matches the end boundary conditions to one where the central flow is unable to match the end conditions. In the latter case, quasi-periodic spectra of the velocity fluctuations appear; these are due to the presence of spanwise cells of different frequency. During periods when vortices in neighbouring cells move out of phase with each other, ‘vortex dislocations’ are observed, and are associated with rather complex vortex linking between the cells. However, by manipulating the end boundary conditions, parallel shedding can be induced, which then results in a completely continuous Strouhal curve. It is also universal in the sense that the oblique-shedding Strouhal data (S_θ) can be collapsed onto the parallel-shedding Strouhal curve (S_0) by the transformation, S_0 = S_θ/cosθ, where θ is the angle of oblique shedding. Close agreement between measurements in two distinctly different facilities confirms the continuous and universal nature of this Strouhal curve. It is believed that the case of parallel shedding represents truly two-dimensional shedding, and a comparison of Strouhal frequency data is made with several two-dimensional numerical simulations, yielding a large disparity which is not clearly understood. The oblique and parallel modes of vortex shedding are both intrinsic to the flow over a cylinder, and are simply solutions to different problems, because the boundary conditions are different in each case.

Airfoil self-noise and prediction

Abstract: A prediction method is developed for the self-generated noise of an airfoil blade encountering smooth flow. The prediction methods for the individual self-noise mechanisms are semiempirical and are based on previous theoretical studies and data obtained from tests of two- and three-dimensional airfoil blade sections. The self-noise mechanisms are due to specific boundary-layer phenomena, that is, the boundary-layer turbulence passing the trailing edge, separated-boundary-layer and stalled flow over an airfoil, vortex shedding due to laminar boundary layer instabilities, vortex shedding from blunt trailing edges, and the turbulent vortex flow existing near the tip of lifting blades. The predictions are compared successfully with published data from three self-noise studies of different airfoil shapes. An application of the prediction method is reported for a large scale-model helicopter rotor, and the predictions compared well with experimental broadband noise measurements. A computer code of the method is given.

The turbulent flow field around a circular cylinder

Abstract: The flow field around a circular cylinder mounted vertically on a flat bottom has been investigated experimentally. This type of flow occurs in several technical applications, e.g. local scouring around bridge piers. Hydrogen bubble flow visualization was carried out for Reynolds numbers ranging from 6,600 to 65,000. The main flow characteristic upstream of the cylinder is a system of horse-shoe vortices which are shed quasi-periodically. The number of vortices depends on Reynolds number. The vortex system was found to be independent of the vortices that are shed in the wake of the cylinder. The topology of the separated flow contains several separation and attachment lines which are Reynolds number dependent. In the wake region different flow patterns exist for each constant Reynolds number.

Vortex splitting and its consequences in the vortex street wake of cylinders at low Reynolds number

Abstract: Based on the observation of vortex splitting in the laminar wake of thin flat plates placed parallel to the flow, an investigation on the consequences of such events for the von Karman vortex street in the wake of circular cylinders was carried out. It was found that a ‘‘design break line’’ of vortex axes can lead to the decoupling of a wake flow from the always present disturbances deriving from the ends. The decoupling gives rise to parallel vortex shedding of a slightly higher frequency, instead of the oblique or slanted vortex shedding at a lower frequency usually observed.

A natural low-frequency oscillation of the flow over an airfoil near stalling conditions

Abstract: An experimental and computational study of the low-frequency oscillation observed in the flow over an airfoil at the onset of static stall is presented. Wind-tunnel results obtained with two-dimensional airfoil models show that this phenomena takes place only with a transitional state of the separating boundary layer. It is noted that the flowfield does not involve a Karman vortex street. The experimental results agree well with the results of a two-dimensional Navier-Stokes code. The present study demonstrates that the low-frequency oscillations produce intense flow fluctuations which impart much larger unsteady forces to the airfoil than experienced by bluff-body shedding and which may represent the primary aerodynamics of stall flutter of blades and wings.

Existence of steady vortex rings in an ideal fluid

Abstract: We prove the existence of global steady vortex rings in an ideal fluid with given propagation speed W > 0 , flux constant k ⩾ 0 and any bounded, positive, nondecreasing vorticity function.

Combustion-related shear-flow dynamics in elliptic supersonic jets

Abstract: : An elliptic jet having an aspect ratio of 3:1 was studied and compared to a circular jet at three Mach numbers: M = 0.15, 1 and 1.3. Hot-wire measurements and Schlieren photography were employed in this study. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works in subsonic jets, prevail in the sonic jet and are further augmented by the shock structures of the supersonic underexpanded jet. The major and minor axes switch at a distance of 3 diameters from the nozzle, and the spreading rate of the minor axis side is twice that of a subsonic jet. The experimental data are supported by results of the linear instability analysis of the supersonic elliptic jet which show that the initial vortices are bending at the major axis side in a similar way to the process which occurs in a subsonic elliptic jet.

A novel method to promote parallel vortex shedding in the wake of circular cylinders

Abstract: Slanted vortex shedding dominates the wake of circular cylinders of finite aspect ratio in the Reynolds number range of 72–158. Parallel vortex shedding can be induced in the wake of a circular cylinder by imposing a symmetric pressure boundary condition at the two ends of the cylinder. This condition can be achieved by positioning two upstream circular cylinders of larger diameter normal to it. The resulting Strouhal–Reynolds number curve shows no discontinuity. Also, the turbulent transition in the wake of a circular cylinder could be delayed by using this technique.

On atmospheric vortex streets in the wake of large islands

Abstract: Results from laboratory experiments on stably stratified flows over three-dimensional obstacles are related to atmospheric vortex streets formed in the lee of large islands. A quasi horizontal flow around the island can be expected if stable stratification favours the formation of a so-called dividing streamline below the islands top. The subsequent shedding of vortices with vertical axis from islands may then be due to viscous boundary layer separation, but also other possible mechanisms of vortex shedding in stably stratified flows are discussed.

Flow Induced Pulsations in Gas Transport Systems: Analysis of the Influence of Closed Side Branches

Abstract: A theoretical analysis is presented of the low frequency aero-acoustic behavior of closed side branches along a gas transport pipe. The theory predicts the hydrodynamic conditions for moderate and strong pulsations. A model is proposed which predicts the order of magnitude of the power generated by the aero-acoustic source. The theoretical analysis leads to the design of spoilers which reduce the pulsation level by 30 to 40 dB. The results obtained by theoretical analysis and model experiments (Reynolds number 10-6) have been confirmed in full scale tests (Reynolds number 10-8).

Absolute and convective instability of a cylinder wake

Abstract: The stability behavior of a circular cross-section cylinder's wake at Reynolds number values of up to 45 is presently investigated by means of local linear stability theory. The steady-wake profiles computed are Navier-Stokes solutions of a uniform, incompressible viscous flow around a cylinder obtained by a spectral method. An absolutely unstable region is found to begin to form at a Reynolds number of about 20, and grows with incresing Reynolds number. The onset of global instability response must be characterized by a critical length of an absolutely unstable region; a critical Reynolds number criterion and preferred frequency are proposed based on linear stability analysis.

The excitation and consequences of acoustic resonances in enclosed fluid flow around solid bodies

Abstract: A survey is made of enclosed flow systems in which acoustic and/or mechanical oscillations can be excited by vortex shedding from bodies immersed in the flow which becomes ‘locked’ to an acoustic resonance.

Phenomenology of Kármán vortex streets in oscillatory flow

Abstract: Vortex wakes of circular cylinders at low Reynolds numbers have been investigated Sound waves are superimposed on the flow in mean flow direction In this configuration the Karman vortices are shed at the sound frequency or at subharmonics of the sound frequency The Karman vortex street is treated as a nonlinear self-excited flow oscillator with forced oscillations Using a flow visualization technique a variety of wake structures has been identified as a function of sound frequency and sound amplitude, but independent of the Reynolds number The superimposed sound influences the distribution of circulation and accordingly the shedding mechanism Primary vortex and secondary vortex are shed simultaneously from one side of the cylinder The alternate vortex shedding is arranged spatially and temporally Structures along the vortex axes are revealed

Vortex ring dynamics at a free surface

Abstract: The results of a flow visualization study of the evolution of vortex rings near a free surface are presented. The vortex rings were formed underwater with their axis parallel to the free surface one ring diameter below the surface. It is shown that vortex lines in the ring open during the interaction with the surface. The resulting flow field consists of vortex lines, beginning and terminating at the free surface. Data are reported on several features of the vortex ring evolution, which show a large reduction of the propagation speed and an oscillation of the vortex eccentricity as it propagates downstream after the interaction.

Flow structure in the wake of an oscillating cylinder

Abstract: The numerical solution of the unsteady two-dimensional Navier-Stokes equations is used to investigate the vortex-shedding characteristics behind a circular cylinder immersed in a uniform stream and performing superimposed in-line or transversed oscillations of a given reduced amplitude

An experimental study of shock wave/vortex interaction

Abstract: The interaction of a supersonic streamwise vortices (of Mach number 2.2, 3.0, and 3.5) with a normal shock wave has been experimentally investigated, and is found to be highly unsteady. Five-hole pressure-probe and temperature measurements ahead of the interaction are used as initial conditions for an axisymmetric Navier-Stokes calculation. The numerical results supports the hypothesis that supersonic vortex breakdown is an important factor in the observed interaction flow pattern.

On flow separation and reattachment around a circular cylinder at critical reynolds numbers

Abstract: An experimental investigation of the flow around smooth circular cylinders in the Reynolds number range 0.8 x 10E5 < Re < 2.0 x 10E5 is presented. Measured quantities include spectra, spanwise correlations and cross correlations of cylinder-pressures and wake-velocity fluctuations, and low-frequency boundary-layer flow direction reversals near separation. The flow motion in the critical range is found to be characterised by intermittent symmetric boundary-layer reattachments, occurring in cells with a well-defined spanwise structure, accompanying a significant decrease in drag coefficient and a weakening of the vortex shedding.

Starting flow and structures of the starting vortex behind bluff bodies with sharp edges

Abstract: Experimental investigations were made in a water channel on the starting flow around several bluff bodies with sharp edges: flat plates, circular disks and hollow hemispheres. Details of the flow structures were visualized using the hydrogen bubble technique. Three-fold structures of the starting vortex behind flat plates were observed. The shedding of the vortex sheet from the edge was also studied.

Hydrodynamic Forces on Large Cylinders in Oscillatory Flow

Abstract: The hydrodynamic forces on a cylinder exposed to oscillatory flow have been determined at Reynolds numbers (Re) in the range 2.5×105 to 1.0×106 (supercritical and transcritical regimes) and Keulegan‐Carpter (KC) numbers in the interval from 1 to 16. One smooth and two rough cylinders were tested. Inline force coefficients and rms lift coefficients are given as a function of KC, Re, and surface roughness. Time series of both force components are shown, and different flow regimes are identified. These force traces are linked to characteristic vortex shedding modes in different KC‐intervals. Both force components are found to vary considerably in the KC‐range under investigation, with the transverse force sometimes being as large in magnitude as the in‐line force.

Noise reduction method

Abstract: When there is relative movement, at a speed above a critical value, between the trailing edge of a body, such as a hydrofoil or aerofoil section, and a fluid, the fluid flow tends to form a pattern of discrete swirling vortices. Vortex shedding at each side of the trailing edge alternately can cause resonance or "singing" of the section. The effect of this phenomenon is reduced by forming contiguous triangular flat-faced notches in the major surfaces of the section, adjacent the trailing edge, but leaving the profile of the leading edge substantially undistrubed. The notches in one major surface are preferably staggered relative to those in the other major surface.

Two-Component Phase-Averaged Turbulence Statistics Downstream of a Rotating Spoked-Wheel Wake Generator

Abstract: Flow-field measurements of unsteady turbulent flow downstream of a rotating spoked-wheel wake generator were performed in a short-duration light-piston tunnel, and the instantaneous-velocity data were phase averaged based on a signal synchronized with the bar-passing frequency. Mean axial velocities were found to agree well with those obtained from measurements behind a stationary cylinder and to be independent of both Reynolds and bar-passing Strouhal numbers. Reynolds stresses were found to be consistent with related cylinder-wake measurements, but were significantly higher than corresponding measurements obtained in large-scale research turbomachines. Phase-averaged triple velocity correlations were calculated from the digital velocity records, revealing the sign and the magnitude of skewness in the velocity probability density distributions for the two components.

Critical geometry of oscillating bluff bodies

Abstract: Measurements are presented of the mean pressures around rectangular and D-section cylinders, with a flat front face normal to the flow, forced to oscillate transversely at an amplitude of 10% of the length of the front face. The ratio of depth (streamwise dimension) to height (cross-stream dimension) of the cross-section ranges from 0.2 to 1.0 for rectangular cylinders and from 0.5 to 1.5 for D-section cylinders. The range of reduced velocities investigated, 3 to 11, includes the vortex-resonance region. When increasing the depth, an oscillating bluff cylinder shows a critical depth where base suction attains a peak. The value of a critical depth is lowered with decreasing reduced velocity. In particular, an extraordinarily low critical depth with a very high base suction is obtained on cylinders oscillating at vortex resonance. For cylinders with depths beyond the critical, a reattachment-type pressure distribution is established on the afterbody due to the shear-layer/edge direct interaction. The shear-layer/edge direct interaction can also occur on oscillating cylinders with a fixed splitter plate. At low reduced velocities, the reattachment-type pressure distributions on cylinders with and without a splitter plate are similar except for the mean level. A remark is made on the critical geometry of bluff bodies under various flow conditions.

Fluctuating forces acting on rectangular cylinders in uniform flow. (On rectangular cylinders with fully separated flow).

Abstract: Experimental investigations on the characteristics of the fluctuating forces acting on rectangular cylinders with fully separated flow were carried out at a Reynolds number of 5.5×104. The widty-to-height ratio B/H of the section was varied from 0.3 to 2.5. Phase-averaged lift and drag forces were obtained in terms of phase-averaged surface pressure. The phase-averaged fluctuating lift and drag coefficients take the respective maximum values in the same tendency as that of the time-mean drag coefficients for the critical geometry of B/H=0.7. In addition, the torsional moment forces associated with the phenomenon of the vortex-induced flutter were discussed in detail based on the characteristic of the fluctuating pressure, whose frequency equals to the vortex shedding frequency, acting on the surfaces of the rectangular cylinders.

Turbulent Structure of Backward-Facing Step Flow and Coherent Vortex Shedding from Reattachment in Open-Channel Flows

Abstract: The turbulent structure of the backward-facing step flow, including the reverse flow region, in open-channel flows was investigated by making use of two sets of two-component Laser Doppler anemometers (LDA). The turbulence characteristics and reattachment properties were revealed. The time-averaged reattachment length was dependent on both the Reynolds number and Froude number in open-channel flows. In particular, it should be noted that the reattachment length became smaller in supercritical flows than in subcritical flows. The instantaneous reattachment point moved over a distance of the time-averaged reattachment length. This suggested strongly that a coherent structure of the kolk-boil vortex might be generated due to the unsteady and low-frequency motions of the reattachment point.