Showing papers on "Vortex shedding published in 1978"

Flow around a circular cylinder near a plane boundary

Abstract: The flow around a circular cylinder placed at various heights above a plane boundary has been investigated experimentally. The cylinder spanned the test section of a wind tunnel and was aligned with its axis parallel to a long plate and normal to the free stream. It was placed 36 diameters downstream of the leading edge of the plate and its height above the plate was varied from zero, the cylinder lying on the surface, to 3·5 cylinder diameters. The thickness of the turbulent boundary layer on the plate at the cylinder position, but with it removed from the tunnel, was equal to 0·8 of the cylinder diameter. Distributions of mean pressure around the cylinder and along the plate were measured at a Reynolds number, based on cylinder diameter, of 4·5 × 104. Spectral analysis of hot-wire signals demonstrated that regular vortex shedding was suppressed for all gaps less than about 0·3 cylinder diameters. For gaps greater than 0·3 the Strouhal number was found to be remarkably constant and the only influence of the plate on vortex shedding was to make it a more highly tuned process as the gap was reduced. Flow-visualization experiments in a smoke tunnel revealed the wake structure at various gap-to-diameter ratios.

Turbulence memory in self-preserving wakes

Abstract: The persistence of the large vortices formed at the origin of wakes and mixing layers constitutes a kind of memory of initial conditions by the turbulence. In order to study the fading of this turbulence memory, and its effect on the rate of approach to the fully developed state, two wakes with different initial conditions have been examined experimentally. The wake of a sphere was compared with the wake of a porous disk which had the same drag, but did not exhibit vortex shedding. Measurements were made of the mean and fluctuating velocities, the anisotropy of the turbulence, and the intermittency. It was found that the wake of the sphere developed self-preserving behaviour more rapidly than the wake of the disk, and that even after both wakes became self-preserving there were differences between them in the structure of the turbulence and the scale of the mean flow. From this it is concluded that the behaviour of self-preserving wakes does not depend on the drag alone, but also on the structure of the dominant eddies. Generalizing these results, it is suggested that reported differences in the value of the entrainment constant of jets, wakes, and mixing layers are due to differences in the structure of the dominant eddies, rather than differences in the type of flow.

Flows around Two Tandem Circular Cylinders at Very High Reynolds Numbers

Abstract: Flows around two tandem circular cylinders in a stream up to the range of much higher Reynolds numbers than the critical one were investigated in a low speed wind tunnel. By making the surface of the upstream circular cylinder rough with two types of surface roughness, the critical Reynolds number of the cylinders was reduced. Drag coefficients and Strouhal numbers of the two tandem circular cylinders were measured and the flow patterns on the cylinders were visualized by the surface oil-flow technique, in the subcritical flow regime, in the supercritical one where there were laminar bubbles followed by turbulent separation and in the transcritical one where purely turbulent separation occurred without laminar bubbles but with an extremely regular vortex shedding behind the cylinders. The flow characteristics of the two tandem circular cylinders were discussed and the effects of Reynolds numbers and the gap spacing between the two cylinders were made clear.

A universal Strouhal number for the ‘locking-on’ of vortex shedding to the vibrations of bluff cylinders

Abstract: It is well known that the vortices shed from a circular cylinder lock on in frequency to the vibrations when the cylinder is forced to vibrate or is naturally excited to sufficient amplitudes by flow-induced forces. This paper presents a model for a universal wake Strouhal number, valid in the subcritical range of Reynolds numbers, for both forced and vortex-excited oscillations in the locking-on regime. The Strouhal numbers thus obtained are constant at St* = 0·178 over the range of wake Reynolds numbers Re* = 700-5 × 104. This value is in good agreement with the results obtained by Roshko (1954a) and Bearman (1967) for stationary circular cylinders and other bluff bodies in the same range of Reynolds numbers. A correspondence between the amplification of the cylinder base pressure, drag and vortex circulation is demonstrated over a wide range of frequencies and for vibration amplitudes up to a full cylinder diameter (peak to peak). The fraction e of the shed vorticity in the individual vortices is found to be dependent upon the base-pressure parameter K = (1 − Cpb)½. Consequently, e is also a function of the amplitude and frequency of the vibrations in the locking-on regime.

Detailed Measurements on a Circular Cylinder in Cross Flow

Abstract: ASERIES of wind tunnel tests covering a range of Mach numbers and Reynolds numbers in subsonic and transonic flows was conducted on a circular cylinder placed normal to the flow. Form drag coefficients were determined from surface pressure measurements and displayed as a function of Mach number to show the transonic drag rise phenomenon. Buried wire gages arranged on the model surface were used to measure skin friction distributions and vortex shedding frequencies at different flow conditions. It was found that detectable periodic shedding ceases above M= 0.9. The measured skin friction distributions indicate the positions of mean separation points clearly; these values are documented for different flow conditions. Contents Flow past circular cylinders is of interest because of its relevance to various problem areas such as aerodynamics of aircraft and missiles at high angles of attack, 1>2 wind effects on tall chimney structures, flow past tube banks of heat exchangers, hydrodynamics of towing and mooring cables for undersea applications, etc. Recently, there has been some interest in developing numerical methods for solving flow past circular cylinders so that predictions can be made for flow regimes that cannot be easily attained in test facilities. Success in such attempts depends heavily on extended flow documentation, especially on boundary-laye r development. At present, only very limited data exist in this regard. The present investigation was concerned with three main features of flow past cylinders: skin friction distribution, form drag coefficients, and vortex shedding frequencies. All tests were conducted in the 2-x2-ft (61x61-cm) transonic wind tunnel at NASA Ames Research Center. Perforated walls in the test section allowed good control of Mach number through the transonic range. Different values of Reynolds numbers were achieved by altering the pressure level in the

Study of Flow past a Circular Cylinder by an Inviscid Model

Abstract: Two-dimensional vortex shedding behind a circular cylinder is simulated by a point vortex array separating from the boundary layer. The flow round the body is impulsively started from rest. The boundary layer is divided into partitions and each of them is replaced by a point vortex with the same circulation as the corresponding partition. This point vortex array separates from the surface of the cylinder, forming Karman's type wake. The calculated drag coefficients have a peak initially and decrease gradually to the value of about 0.6 and begin to oscillate around the value between 0.6 and 0.8. The lift also begins to oscillate more clearly than the drag. The results are in good agreement with those obtained by much more elaborate finite-difference methods.

Vortex-shedding flowmeter having a sensing vane

Abstract: A vortex-shedding flowmeter in which a shedder is transversely mounted in a flow tube through which the fluid to be metered is conducted, the shedder dividing the flow and causing vortices to be shed alternately on either side thereof at a repetition rate proportional to flow rate, thereby producing downstream trains of vortices moving along the right and left sides of the tube. The wave span between the eyes of successive vortices in the trains has a substantially constant length determined by the geometry of the shedder. At a downstream position in the flow tube is a vane flanked by the trains of vortices and having a length no less than one-half the length of the span and no greater than the full length thereof. The vane is pivoted on a torsion shaft on its central axis which is perpendicular to the tube axis and is balanced with respect thereto. The moving vortices on either side of the vane produce fluidic forces giving rise to alternate clockwise and counterclockwise torques, causing the vane to vibrate at a frequency in accordance with the repetition rate. These vibrations are sensed to provide an output signal indicative of flow rate.

Vortex shedding flowmeter construction

Abstract: A vortex flowmeter having a flow obstruction body or vortex forming bar (16, 17, 18) formed in a manner to provide enhanced generation of vortices across a wide range of Reynold numbers of the fluid being measured. The bar includes projections (16A, 17A, 18A) adjacent the lateral edges thereof which tend to cause formation of vortices at the leading lateral edges with respect to the direction of flow and enhance the formation of such vortices along the surfaces extending generally parallel to the flow. Additionally, the flowmeter disclosed provides a mounting (34, 36) for a sensor (37) which makes the device relatively insensitive to acceleration in the mounting structure or supporting pipe of the flowmeter.

The Game of Passing-Through of a Pair of Vortex Rings

Abstract: When two similar vortex rings travel along the common axis of symmetry in the same direction, the game of passage of one vortex ring through the other was successfully realized both in air and in water. The phenomenon depends on the Reynolds number, Re , based on the initial traveling speed of vortex rings and the diameter of the ejecting orifice, and also depends on the time interval between two puffs of fluid by which the two vortex rings are produced. The region in the Re - T plane in which the game of passing-through is observed was experimentally determined both for air and for water, where T denotes a non-dimensional time interval. It was found that the region for water was smaller than that for air.

Vortex cloud street during AMTEX 75

Abstract: Strong northerly flow across Cheju Island, Korea, during the 1975 Air Mass Transformation Experiment (AMTEX 75) resulted in a pronounced vortex cloud street to the lee of the island on February 17 1975. This pattern has been studied and explained in terms of classical von Karman eddies shed in laminar free flow moving past a cylindrical obstacle under subcritical Reynolds number conditions. DMSP satellite imagery and AMTEX aerological data have given the shedding frequency of the vortices that are contained in the mixed boundary layer below the capping inversion to be one per 3 h. The island extends to nearly 2 km above sea level and penetrates the base of the inversion layer at a height of approximately 600 m. At this height the island diameter is about 20 km, a figure in good agreement with the independently inferred vortex shedding diameter. DOI: 10.1111/j.2153-3490.1978.tb00868.x

Maximum Vortex-Induced Side Force

Abstract: A method of determining the maximum vortex-induced side force on slender bodies at high angle of attack and zero sideslip is presented. The maximum stationary vortex asymmetry is related to the maximum instantaneous asymmetry for nonstationary separation on a cylinder normal to the flow. Thus, the twodimensional peak nonsteady lift to steady drag ratio for a cylinder is indicative of the maximum possible stationary side-force to normal-force ratio on a slender body. An effective crossflow Reynolds number is used to relate three-dimensional Reynolds number effects to two-dimension al cylinder results. Similar techniques are used to predict the maximum additional lift generated by asymmetric vortex shedding. c d d* D2 M I IN

Experimental investigation of transonic flow past a blunt cone-cylinder

Abstract: The flow past a blunt cone-cylinder at transonic speeds and at moderate angles of incidence has been studied experimentally by recording (1) the longitudinal and circumferential pressure distribution, (2) the surface oil flow patterns using a suspension of titanium dioxide in oleic acid. The results indicate some interesting three-dimensional flow separation phenomenon at transonic speeds.

Investigation of Rotor Noise Source Mechanisms with Forward Speed Simulation

Abstract: The noise of a model propeller was measured with and without forward speed simulation in the open test section of a low-speed wind tunnel at velocities up to 60 m/s and blade tip Mach numbers up to 0.6. Compared to the static tests, even low wind velocities reduced the propeller noise by up to 20 dB. Strong sound radiation occurred due to interaction of the propeller blades with the wake of the supporting beam when the propeller was mounted as a pusher propeller. At wind velocities above 20 m/s, the high-frequency part of the propeller noise depends strongly on the angle of attack of the blades. At moderate angles of attack, laminar vortex shedding noise dominates the high-frequency sound radiation. This noise component could be eliminated by tripping the boundary layer on the suction surface of the blades. The frequencies of maximum noise radiation were predicted fairly well by existing theories. The laminar vortex shedding noise disappears at higher angles of attack of the propeller blades. If the angle of attack is further increased, strong braodband noise occurs which is generated probably by the turbulent boundary layer and local flow separations.

Vortex shedding flowmeter

Abstract: A vortex shedding flowmeter is provided with a unitary obstacle element for shedding vortices and response element reacting to forces produced by the shedding vortices. A cylindrical element may be used for omni-directional measurement capability, with a pair of strain gauge bridges arranged in quadrature to provide a linearly variable frequency output proportional to flow velocity. Electronic circuitry transforms the frequency signal to an analog amplitude signal proportional to flow velocity. Sample and hold circuitry maintains a stable output under transient loss-of-input signal conditions.

The Influence of Yaw Angle Upon the Vortex Wakes of Stationary and Vibrating Cylinders.

Abstract: : This report investigates the influences of yaw angle and end condition upon the vortex wakes of finite-length, yawed circular cylinders. The standard for comparison is the Independence Principle which produces the so-called Cosine Laws for yawed body flows. These expressions have found widespread use even though their validity cannot be proved theoretically for separated, unsteady or turbulent flows. Many previous empirical checks of the Principle under these conditions have produced conflicting reports. Measurements of the vortex shedding frequency, the shedding angle, the base pressure, the vortex formation length and the wake width were obtained for both stationary and vibrating yawed cylinders in the range of Reynolds numbers Re = 160 to 1100. The influence of end conditions was assessed for free-ended cylinders of varying end geometries and for cylinders fitted with adjustable endplates. The cylinder lengths varied from about 20 to nearly 100 diameters. Based upon the experimental results and upon some fundamental considerations of the vorticity discharged at separation it is concluded that the use of the Independence Principle is not valid for separated, yawed cylinder flows except in two special cases. For the first time the 'universal' Strouhal number concept for scaling the vortex wakes of bluff bodies at normal incidence is found to be valid for inclined cylinders.

Spectral and Probability-Density Nature of Square-Prism Separation-Reattachment Wall Pressures

Abstract: The statistical nature of the fluctuating pressures associated with the separation--reattachment flow were studied for a two-dimensional square prism in uniform flow for low (0.33 percent) and high (10.4 percent) turbulence levels. Studies were also made with a splitter plate to inhibit the feedback effect arising from vortex shedding. The nature of the separation--reattachment flow was characterized by use of the measured value of the mean and fluctuating pressure fields. Spectral distribution of the unsteady pressures reveals strong energy spikes at the Strouhal frequency which are eliminated by the presence of the splitter plate. Probability density distributions indicate appreciably non-Gaussian nature only in the wake. Additional information is presented on the variation with angle of the Strouhal frequency for the wake flow.

Devices for detecting fluid flow

Abstract: In a device for detecting fluid flow, an elongated body, such as a wire, is supported so that when a substantial portion of its length is exposed to a flowing fluid with the longitudinal axis of said portion arranged substantially transverse to the fluid flow, said portion can be caused to vibrate by vortex shedding; an electromechanical transducer is coupled to the body so as to produce an electrical signal in response to the vibration; and either the presence of the electrical signal is detected, for operation as a safety switch, or the frequency of the vibrations is detected, for measurement of fluid flow rate, fluid velocity etc.

Response of a pendulum spar to 2-dimensional random waves and a uniform current

Abstract: A linearized theory for the response of a circular pendulum spar in 2-dimensional waves and a uniform current is developed. The linear forces on the cylinder are predicted using an approximate potential flow theory for slender bodies. The dynamic equations are then amended to account for the wake effects of viscous bluff body flow by including a quadratic drag law and neglecting wave damping. A spectral model for the forces on a cylinder due to an oscillating wake, modeling the force as a frequency modulation process, is proposed. The non-linear equations of motion which result are then solved, assuming constant force coefficients, by linearization for use with a Gaussian random sea. The method of equivalent linearization is extended to include mean flow effects and a spatially distributed process. Some numerical experiments are then used to test the performance of the linearization. For a variety of environments, the linearization predicts the standard deviation of the simulation response to within 10% and the mean angle of inclination to within 30%. Results of the numerical experiments indicate that there is significant variation (order of magnitude changes) in both response and mean angle of inclination. Thus, significant changes are followed by the linearization. A laboratory experiment was carried out to test the linearized spar model in a realistic fluid environment. Only the low Keulegan Carpenter number regime was investigated. With some minimal manipulations, good agreement is obtained between the experiment and the linearized estimates. It appears that the drag coefficients for vortex induced in-line forces may be an order of magnitude larger than those reported in the literature, .5 instead of .06, and that the shedding of vortices due to steady flow may reduce the added mass coefficient significantly, as observed in oscillating flows with significant vortex shedding.

Wave loads on horizontal cylinders

Abstract: The problem of predicting wave induced loads on cylinders is an enormously complex one. It is clear from the scatter present in most experimental determinations of force coefficients that there are many individual factors which influence the mechanisms of flow induced loading. Among these are some, for instance Reynolds number, separation and periodic vortex shedding, which are inter-related and whose influences cannot be studied in isolation. Others, such as shear flow, irregular waves and free surface effects, can at least be eliminated in the laboratory, in order to approach an understanding of the more fundamental characteristics of the flow. A vertical cylinder in uniform waves experiences an incident flow field which can be described in terms of rotating velocity and acceleration vectors, always in the same vertical plane, containing also the cylinder axis, whose magnitudes are functions of time and of position along the length of the cylinder. Some of the essential features of this flow can be studied under two-dimensional oscillatory conditions, in which either the cylinder or the fluid is oscillated relative to the other along a straight line (planar oscillatory flow). The incident velocity and acceleration vectors are then always concurrent, normal to the cylinder axis, and oscillating in magnitude with time.

Mixing two fluids in vortex - by injecting fluids from above and below centre of vortex, axially or tangentially for use in cement calculation

Abstract: Method of mixing >=2 fluids in a vortex formed between upper and lower ends of the mixing chamber, having the shape of a revolution about an axis is described. The first fluid is introduced at the top to form the vortex and the second fluid is introduced into the vortex, to be mixed into the first fluid, the mixt. being removed from the lower end of the vortex. Specifically, the second fluid is introduced into the vortex in its axis from the lower end at a speed high enough for the axial jet thus formed to penetrate the interior of the vortex in a direction opposite to its axial component as far as a reversal zone where the second fluid is absorbed by the vortex to be mixed with the first fluid. Improved mixing occurs, hence the dimensions of the mixing chamber can be reduced.

Split bar vortex shedding flowmeter

Abstract: A vortex shedding flowmeter assembly which utilizes at least one vibrating body that is positioned in relation to an upstream facing vortex forming plate so that vibration of the body caused by vortices formed at the edges of the plate is enhanced. The body is separated from the plate so it will vibrate without causing vibration of the plate. The vibration body may be split transversely generally in the midsection of the body so that the body forms two cantilevered portions.

Magnetic vortex generator with a vortex pool and single vortex transfer

Abstract: For storing digital data e.g. in magnetic vortex shift registers, a single flux quantum vortex is injected into a superconductive layer by a vortex generator consisting of an emitter and a transfer station. The emitter generates a pool of vortices. The transfer station upon a signal transfers one single vortex at the time. In a particular embodiment, the transfer station consists of a Josephson interferometer at the entrance of a transfer channel. Switching of the interferometer passes a flux vortex from a vortex pool to a shifting and storage apparatus. Other embodiments do not use interferometers but include transfer channels with separation corners and vortex spacing means which insure that upon application of appropriate signals one and only one vortex is passed from the vortex pool to the shifting and storage means.

Vortex Shedding from `Taylor Columns'

Abstract: Visual observations are presented of the vortex wakes behind `Taylor columns' generated by a short upright cylinder moving horizontally through water in rapid rotation about a vertical axis. They demonstrate the periodic separation and rolling-up of the free shear layers around the Taylor column. A remarkable feature is that in a certain parameter range the separated shear layers form into the symmetric double row of vortices.