Showing papers on "Vortex shedding published in 1982"

Strouhal numbers of rectangular cylinders

Abstract: Experiments on the vortex-shedding frequencies of various rectangular cylinders were conducted in a wind tunnel and in a water tank. The results show how Strouhal number varies with a width-to-height ratio of the cylinders in the range of Reynolds number between 70 and 2 × l04. There is found to exist a certain range of Reynolds number for the cylinders with the width-to-height ratios of 2 and 3 where flow pattern abruptly changes with a sudden discontinuity in Strouhal number. The changes in flow pattern corresponding to the discontinuity of Strouhal number have been confirmed by means of measurements of velocity distribution and flow visualization. These data are compared with those of other investigators. The experimental results have been found to show a good agreement with those of numerical calculations.

An experimental study of pressure fluctuations on fixed and oscillating square-section cylinders

Abstract: Measurements are presented of the pressure fluctuations acting on a stationary squaresection cylinder, with the front face normal to the flow, and one forced to oscillate, transverse to a flow, at amplitudes up to 25% of the length of a side. The range of reduced velocities investigated, 4–13, includes the vortex lock-in regime. At lock-in the amplification of the coefficient of fluctuating lift is found to be much less than that found for a circular cylinder. The variation of the phase angle, between lift and displacement, is also different from that measured on a circular cylinder, and vortex-induced oscillations are possible only at the high-reduced-velocity end of the lock-in range. At reduced velocities sufficiently far below lock-in the natural vortex-shedding mode is suppressed and vortices are found to form over the side faces at the body frequency. Intermittent reattachment occurs over the side faces and, for an amplitude of oscillation equal to 10% of the length of a side face, the time-mean drag coefficient can be reduced to 60% of its fixed-cylinder value.

A numerical study of vortex shedding from rectangles

Abstract: The purpose of this paper is to present numerical solutions for two-dimensional time-dependent flow about rectangles in infinite domains. The numerical method utilizes third-order upwind differencing for convection and a Leith type of temporal differencing. An attempted use of a lower-order scheme and its inadequacies are also described. The Reynolds-number regime investigated is from 100 to 2800. Other parameters that are varied are upstream velocity profile, angle of attack, and rectangle dimensions. The initiation and subsequent development of the vortex-shedding phenomenon is investigated. Passive marker particles provide an exceptional visualization of the evolution of the vortices both during and after they are shed. The properties of these vortices are found to be strongly dependent on Reynolds number, as are lift, drag, and Strouhal number. Computed Strouhal numbers compare well with those obtained from a wind-tunnel test for Reynolds numbers below 1000.

The vortex-shedding process behind two-dimensional bluff bodies

Abstract: Using a variety of flow-visualization techniques, the flow behind a circular cylinder has been studied. The results obtained have provided a new insight into the vortex-shedding process. Using time-exposure photography of the motion of aluminium particles, a sequence of instantaneous streamline patterns of the flow behind a cylinder has been obtained. These streamline patterns show that during the starting flow the cavity behind the cylinder is closed. However, once the vortex-shedding process begins, this so-called ‘closed’ cavity becomes open, and instantaneous ‘alleyways’ of fluid are formed which penetrate the cavity. In addition, dye experiments also show how layers of dye and hence vorticity are convected into the cavity behind the cylinder, and how they are eventually squeezed out.

Influence of end plates and free ends on the shedding frequency of circular cylinders

Abstract: The cylinder end boundaries, whether they be end plates or simple free ends, alter the vortex-shedding mechanism near these boundaries. This effect has in the past usually been overlooked. In a region near an end plate or a free end (ranging from 6 to 15 cylinder diameters in length), the shedding frequency f2 is found to be 10–15% less than the regular Strouhal frequency fs. The latter frequency is observed over the remaining cylinder length. The simultaneous occurrence of two frequencies results in a beat frequency, which is best observed at the junction of the two regions characterized by fs and f2 respectively. A third frequency f3 with fs > f3 > f2 is observed over the entire cylinder length when the cylinder is bounded by two end plates less than 20 to 30 cylinder diameters apart. Here the critical Reynolds number for the onset of shedding is shifted to about 60 and the laminar Reynolds-number range is extended from about 150 to about 250.

Some Recent Studies of Vortex Shedding With Application to Marine Tubulars and Risers

Abstract: A discussion is given in this paper of the problems caused by vortex shedding from flexible, bluff cylinders in steady current flows. In particular, recent measurements of the steady and unsteady deflections caused by the vortex-excited drag and lift forces are discussed. 48 refs.

Acoustic radiation from a semi-infinite annular duct in a uniform subsonic mean flow

Abstract: Using a Wiener-Hopf approach, ain analytical description is derived of the scattered field of a harmonic sound wave coming out of an open ended annular duct (a semi-infinite cylinder inside of which, coaxially, a doubly infinite hub), submerged in a subsonic, coaxial, uniform mean flow. The possibility of vortex shedding from the pipe exit edge is included.Explicit expressions are given of the acoustic power inside the pipe, in the acoustic far field, and, in the presence of vortex shedding, in the hydrodynamic far field and of the power absorbed by the vortex sheet. The formulae are evaluated with the aid of asymptotic expansions, and a method utilizing complex contour deformation, more convenient than those usually employed for this type of diffraction problems. The equality of power appeared to be an important check on the calculations. A numerical survey is made of the behaviour of the acoustic power loss, due to vortex shedding from the trailing edge, at frequencies near cut-off, as a function of Mach number, mode number of the incident wave, and hub radius. The power loss appears to increase with increasing Mach number, increasing hub radius and with decreasing frequency. Only in case of the plane wave (where k-K)) the ratio of radiated and transmitted power becomes zero, for the other modes (at their cutoff frequencies) this ratio tends to a finite value. Somewhat surprising is that, in comparison with the jet, the power loss in a uniform flow is much higher. As a typical example for higher frequencies, the far field radiation pattern of a k=50, m=U wave is considered as a function of Kutta condition and hub radius.

Viscous-Inviscid Interaction on Oscillating Airfoils in Subsonic Flow

Abstract: : Selected results from an extensive oscillating-airfoil experiment are analyzed and reviewed. Four distinct regimes of viscous-inviscid interaction are identified, corresponding to varying degrees of unsteady flow separation. The dominant fluid dynamic phenomena are described for each regime. Ten specific test cases, including the appropriate flow conditions and experimental results, are proposed for evaluating unsteady viscous theories and computational methods. (Author)

Flow Past a Circular Cylinder on a $\beta $-Plane

Abstract: With a view to obtaining a fuller understanding of the interactions between topography and large-scale geophysical flows, a series of laboratory investigations have been performed on the flow past a right circular cylinder in a rotating water channel. For large-scale flows on a spherical Earth the variation of the Coriolis parameter, F = 2Ωsinϕ , with latitude, ϕ, is commonly written (Pedlosky 1979) as F = f + β 0 y where f = 2Ωsinϕ o , β o = 2Ωcosϕ o /R E , y is the distance to the north from the reference latitude ϕ o , and R E and Ω( = 7.29 x 10 -5 s-1 ) are the radius and rotation rate of the Earth respectively. In this paper we shall discuss laboratory experiments in which the variation of F can be simulated. We shall refer to those studies in which β = 0 (i.e. the Coriolis parameter is uniform over the latitudinal extent of the region under investigation) as f-plane experiments. Models for which β o is non-zero will be referred to as β-plane experiments. In the experiments the β-effect has been simulated by tilting the upper and lower surfaces of the channel so that the depth of the fluid varies in the cross-stream direction. Flow patterns have been obtained over a range of five independent non-dimensional parameters: Rossby and Ekman numbers, cylinder aspect ratio, β-parameter and flow direction (‘eastward’ or ‘westward’). A dramatic difference in downstream behaviour is found between f-plane, β-plane westward and /plane eastward flows. In particular, the β-plane eastward flows are characterized by bunching and pinching of streamlines in the wake region, the generation of damped stationary Rossby waves and downstream acceleration. Compared with f-plane flows the β-effect is shown to inhibit boundary layer separation from the cylinder for eastward flow and to enhance the separation for westward flow. Data are presented from all cases to show the asymmetry of the downstream flows and the transitions from fully attached to unsteady flows. Under otherwise identical conditions the downstream extent of the separated bubble region is much greater for β-plane westward flow than, in turn, for f-plane and β-plane eastward flows. In addition, the data indicate that the size of the bubble increases with increasing Rossby number and decreases with increasing Ekman number and cylinder aspect ratio. For eastward flow the bubble size decreases with increasing β-parameter and for westward flow it increases with increasing β-parameter. Unsteady flows are investigated and instances of asymmetrical vortex shedding are presented.

An experiment on the flow past a finite circular cylinder at high subcritical and supercritical Reynolds numbers

Abstract: The complicated flow in the tip region of a finite circular cylinder in uniform cross flow has been examined at the Reynolds numbers 85,000, 180,000, and 770,000. Simultaneous measurements of the surface-pressure and wake-velocity fluctuations have revealed the existence of a shedding regime in the tip region that is distinct from the one prevailing on the main body of the cylinder. In particular, this regime can be unstable and intermittent, can have a cellular structure in the wake, or can be subcritical when the main flow is supercritical.

Vortex–leading-edge interaction

Abstract: Visualization of successive vortices impinging upon the leading edge of a wedge reveals patterns of deformation of each incident vortex; for certain offsets of the edge with respect to the incident vortex there is pronounced vortex shedding from the leading edge, whereby the shed vortex has a vorticity orientation opposite to that of the incident vortex. Simultaneous consideration of this visualization interaction and the force induced on the wedge gives the relation between the nature of the interaction mechanism and the relative magnitude and phase of the force exerted on the wedge. The amplitude of the induced force is found to be a strong function of the transverse offset of the leading edge with respect to the incident vortex and the degree of vorticity shedding from the leading edge. Application of Stuart's vortex model to the incident vortices provides a means for approximating the phase and relative amplitude of the induced force as a function of the transverse offset of the leading edge.

Stability of a vortex street of finite vortices

Abstract: The stability of the finite-area Karman ‘vortex street’ to two-dimensional disturbances is determined. It is shown that for vortices of finite size there exists a finite range of spacing ratio κ for which the array is stable to infinitesimal disturbances. As the vortex size approaches zero, the range narrows to zero width about the classical von Karman value of 0·281.

Chicago Monostatic Acoustic Vortex Sensing System. Volume IV. Wake Vortex Decay.

Abstract: : A Monostatic Acoustic Vortex Sensing System (MAVSS) was installed at Chicago's O'Hare International Airport to measure the strength and decay of aircraft wake vortices from landing aircraft. The MAVSS consists of an array of acoustic antennas which measure the vertical profile up to 60-m altitude of the vertical component of the wind. The decay in wake vortex strength is measured as the vortex passes over successive antennas in the array. Volume I (published in October 1979, 32 pages) described the MAVSS principles of operation, the hardware developed, and the data reduction methods employed. Volume II (published in September 1981, 162 pages) described the analysis of MAVSS data to examine whether landing B-707 and DC-8 aircraft need to remain divided into Heavy and Large categories on the basis of the wake vortex hazard. Volume III (published in January 1982, 25 pages) summarized the results of Volume II in terms of the saftey implications of categorizing all landing B-707s and DC-8s as Large aircraft. In this volume, the statistical methods used to understand wake vortex decay are described and the data on all common jet transport aircraft are presented. (Author)

Numerical Solution for Laminar Two Dimensional Flow About a Cylinder Oscillating in a Uniform Stream

Abstract: A finite difference model is presented for viscous two dimensional flow of a uniform stream past an oscillating cylinder. A noninertial coordinate transformation is used so that the grid mesh remains fixed relative to the accelerating cylinder. Three types of cylinder motion are considered: oscillation in a still fluid, oscillation parallel to a moving stream, and oscillation transverse to a moving stream. Computations are made for Reynolds numbers between 1 and 100 and amplitude-to-diameter ratios from 0.1 to 2.0. The computed results correctly predict the lock-in or wake-capture phenomenon which occurs when cylinder oscillation is near the natural vortex shedding frequency. Drag, lift, and inertia effects are extracted from the numerical results. Detailed computations at a Reynolds number of 80 are shown to be in quantitative agreement with available experimental data for oscillating cylinders.

Three-dimensional stability of vortex arrays

Abstract: The stability to three-dimensional disturbances of three classical steady vortex configurations in an incompressible inviscid fluid is studied in the limit of small vortex cross-sectional area and long axial disturbance wavelength. The configurations examined are the single infinite vortex row, the Karman vortex street of staggered vortices and the symmetric vortex street. It is shown that the single row is most unstable to a two-dimensional disturbance, while the Karman vortex street is most unstable to a three-dimensional disturbance over a significant range of street spacing ratios. The symmetric vortex street is found to be most unstable to three-dimensional or two-dimensional symmetric disturbances depending on the spacing ratio of the street. Short remarks are made concerning the relevance of the calculations to the observed instabilities in free shear layer, wake and boundary-layer type flows.

Dynamic Stall Due to Fluctuations of Velocity and Incidence

Abstract: Proper simulation of a retreating-blade stall requires considering incidence fluctuations combined withsimultaneous velocity oscillations of the incoming airstream. This paper reports experimental results obtained when simultaneous fluctuations of both velocity and incidence are generated on an airfoil executing oblique fore-and-aft translations. When dynamic stall occurs in out-of-phase or in-phase variations of incidence and velocity, strong unsteady effects are present. In the first case, an instantaneous lift overshoot and a time delay between incidences of dynamic and static stall are observed. In the second case, the main effect consists of an overshoot of the mean aerodynamic coefficients over the period, owing to a cyclic separation and reattachment process with a strong vortex shedding all along the upper surface during the separation phase.

Vortex shedding mass air flow sensor with stabilized fluid flow

Abstract: A device for measuring the velocity of fluid includes a vortex generating means positioned in the fluid stream so that vortices are formed in the wake of the generating means. Plates elongated in the direction of fluid flow are positioned adjacent the vortex generating means to improve detection of the vortices by increasing the signal to noise ratio.

Transport in the near aerodynamic wakes of flat plates

Abstract: An experimental investigation has been carried out into the nature of the transport of airborne material in the near aerodynamic wakes of bluff bodies with simple shapes. The main attention was focused on the essential differences existing between axi- symmetric flows (as about disks) and two-dimensional flows (as about rectangular long thin flat plates). Measurements were made for such bodies of the near-wake residence time of injected small particles, along with other and more familiar near- wake properties such as the vortex-shedding frequency and base pressure. It was concluded for disks that the transport of material into and out of the near-wake region is dominated by turbulent diffusion, and is strongly influenced by free-stream turbulence, especially for free-stream turbulence whose length scale is substantially smaller than the disk diameter. For rectangular flat plates, transport is dominated by the periodic shedding of vortices, and to only a secondary extent by turbulent motions, and is not strongly influenced by free-stream turbulence.

Recent advances in applying Free Vortex Sheet theory to the estimation of vortex flow aerodynamics

Abstract: Free Vortex Sheet theory has been applied to a variety of configurations for the estimation of three-dimensional pressure distributions for wings developing separation-induced leading-edge vortex flows. Correlations with experiment show reasonable estimates for the effects of compressibility, side-slip, side edges, swept-wing blast-induced loads, and leading-edge vortex flaps. Theoretical studies expand upon these correlations to show general aerodynamic trends. Consideration is also given to simple, yet effective techniques which expedite convergence and therefore reduce computational expense.

A Normal Flat Plate Close to a Large Plane Surface

Abstract: The flow around a normal flat plate close to a large plane surface has been investigated for a range of boundary-layer thickness on the plane surface of 0.72 to 2.53 times the plate height, h. As the gap between the plate and the plane surface is reduced below about 0.55h vortex shedding from the plate is inhibited by the presence of the plane surface. Boundary layer thickness has little effect on the gap at which this occurs, but does affect the strength and frequency of vortex shedding at greater gaps. As the gap between the plate lower edge and the plane surface is decreased towards the critical value of 0.55h, both vortex strength and Strouhal number are more significantly affected when the approaching boundary layer is thin. It is not clear whether this is a result of shear or increased turbulent intensity.

Numerical predictions of the laminar flow over a normal flat plate

Abstract: Finite-difference and finite-element techniques have been used to calculate the steady laminar flow over a flat plate normal to an air stream, up to a Reynolds number, Re, based on the plate half-width, of 100. The boundary conditions simulate a central splitter plate downstream of the body, to prevent vortex shedding, so the flow is characterized by a closed recirculation region which grows with increasing Re but at Re = O(100) is very similar in size to the turbulent recirculating region that occurs in the corresponding high Reynolds-number flow. Motivation came, in part, from the increasing efforts of turbulence modellers to calculate complex turbulent flows (containing elliptic regions) and our belief that the numerical methods commonly employed for such work can be inaccurate. The predictions are compared with each other and with some expectations based on classic solutions of the Navier-Stokes equations, and the nature of the numerical errors is demonstrated. It is concluded that effort comparable with that expended in developing turbulence models should be directed to developing higher-order numerical methods, before the numerical accuracy of predictions of, for example, bluff-body flows can be made sufficiently high to sustain detailed discussion of the adequacy of turbulence models in such situations.

Correlation of lift and boundary-layer activity on an oscillating lifting surface

Abstract: Boundary-layer and trailing-edge flow activities were recorded using hydrogen bubble flow visualization techniques on an oscillating lifting surface in a two-dimensional water tunnel. Simultaneous with flow documentation, unsteady lift was measured over a range of reduced frequencies of 0.5-10. Unsteady loads using classical, inviscid theories were predicted for the experimental conditions investigated. Reduced frequency bands exhibiting poor agreement between experiment and theory were identified and a correlation to observed flow phenomena was accomplished. The results support the utilization of a separate viscous model near the trailing edge coupled with an inviscid flowfield model to predict unsteady loads. The results further show that for certain reduced frequency bands, classical inviscid solutions may be applicable and adequate.

Flow Behind Two Coaxial Circular Cylinders

Abstract: The flow behind two circular cylinders of different diameters, which were joined coaxially has been examined at the Reynolds number of 8 × 104 . The detailed pressure spectral measurements indicate the shedding and the distributions of the pressure fluctuations of the vortex wakes associated with the big and small cylinders. The effect of the presence of the small vortex wake on the vortex shedding of the big cylinder and the interaction between them is presented. The wake formed behind the plane of discontinuity has been isolated and presented.

Chicago monostatic acoustic vortex sensing system. volume iii. executive summary: decay of b-707 and dc-8 vortices

Abstract: A Monostatic Acoustic Vortex Sensing System (MAVSS) was installed at Chicago's O'Hare International Airport to measure the strength and decay of aircraft wake vortices from landing aircraft. The MAVSS consists of an array of acoustic antennas which measure the vertical profile up to 60-m altitude of the vertical component of the wind. The decay in wake vortex strength is measured as the vortex passes over successive antennas in the array. In this volume, the results of Volume II are summarized in terms of the safety implications of categorizing all landing B-707s and DC-8s as Large aircraft.

Computational Model for Low Speed Flows Past Airfoils with Spoilers

Abstract: A computer model has been developed to simulate low-speed flow past an airfoil with a spoiler. An outer solution calculates the potential flow around an effective, closed-wake body. This body is formed by adding to the original airfoil and spoiler geometry: the boundary-layer displacement thickness; a closed wake behind the spoiler; and a trapped vortex at the spoiler hinge. An inner flow solution uses a turbulent jet mixing analysis and conservation of mass and momentum to simulate the time average flow within the wake. The final solution is obtained by iterative matching of the outer and inner solutions.