Showing papers on "Vortex shedding published in 1980"

Spiral vortices in boundary layer transition regime on a rotating disk

Abstract: Behavior of spiral vortices on a disk rotating in still fluid is studied theoretically and experimentally in detail. A linear stability analysis, in which effects of streamline curvature and Coriolis force are considered, gives a critical Reynolds number at the onset of instability close to the one measured here by using a hot wire probe. Gradient of the vortex axis is determined under a condition of the maximum amplification. Flow patterns in the transition regime are experimentally visualized. The results show that the number of the spiral vortices is 31 or 32 as mean value and the gradient of the vortex axis decreases from 14° to 7° as the local Reynolds number is increased.

The forces on sharp-edged cylinders in oscillatory flow at low Keulegan–Carpenter numbers

Abstract: This paper describes an analysis of the forces induced by separation and vortex shedding from sharp-edged bodies in oscillatory flow at high Reynolds number. The analysis which is valid for the case of small oscillations of the fluid is compared with experimental data obtained at fairly low Keulegan–Carpenter numbers.The main conclusions of this paper were presented at the International Symposium on Wave Induced Forces on Structures at Bristol, 1978.

Coaxial jets with and without swirl

Abstract: Measured values of mean velocity, Reynolds stresses and probability density distributions of fluctuating velocity are reported for the turbulent coaxial jets, with and without swirl, emerging into stagnant surroundings from a long pipe and an annulus concentric with the pipe. They were obtained using hot-wire anemometry and on-line data processing with the aid of a mini-computer. The results show that non-swirling coaxial flow configurations approach a self-similar state in a much smaller distance than that of the round jet, for velocity ratios ranging between 0·65 and 1·5; this is due to the mixing layer and vortex shedding that occur in the region downstream of the separation wall between the two streams. In the presence of swirl, the coaxial jet was found to develop at a faster rate. An assessment of turbulence models, based on Reynolds stress closures, suggests that previous assumptions for turbulent diffusion of turbulent kinetic energy are in error.

Vortex Shedding From Two Circular Cylinders in Staggered Arrangement

Abstract: The frequency of vortex shedding from two circular cylinders of the same diameter in staggered arrangement is experimentally investigated at a Reynolds number of 1.58 × 104 . This Reynolds number is within the range where the flow around a circular cylinder is relatively insensitive to Reynolds number changes. The results are summarized in several figures from which one can obtain the Strouhal number of vortex shedding for all arrangements within distances between their centers less than 5 diameters.

Vortex shedding from a circular cylinder in moderate-Reynolds-number shear flow

Abstract: The frequency of vortex shedding from a circular cylinder in a uniform shear flow and the flow patterns around it were experimentally investigated. The Reynolds number Re, which was defined in terms of the cylinder diameter and the approaching velocity at its centre, ranged from 35 to 1500. The shear parameter, which is the transverse velocity gradient of the shear flow non-dimensionalized by the above two quantities, was varied from 0 to 0·25. The critical Reynolds number beyond which vortex shedding from the cylinder occurred was found to be higher than that for a uniform stream and increased approximately linearly with increasing shear parameter when it was larger than about 0·06. In the Reynolds-number range 43 < Re < 220, the vortex shedding disappeared for sufficiently large shear parameters. Moreover, in the Reynolds-number range 100 < Re < 1000, the Strouhal number increased as the shear parameter increased beyond about 0·1.

Karman Vortex Shedding and the Effect of Body Motion

Abstract: 'The unsteady aerodynamic characteristics of a circular cylinder in incompressible crossflow have been studied to obtain a better understanding of the coupling between body motion and Karman vortex shedding. It is found that the coupling is taking place through the effect of body motion on the boundary-layer development between stagnation and flow separation points. Using a moving wall/wall jet analogy, the experimentally observed effects of lateral and longitudinal oscillations on the Karman vortex shedding at subcritical Reynolds numbers can be explained. This permits an assessment to be made of the potential for self-excited oscillations in the different flow regimes. Where experimental results are available, they agree with this assessment. Basic flow similarity indicates that the unsteady flow concepts developed for prediction of dynamic stall of airfoils could be modified and applied to predict the observed effects of body motion on the Karman vortex shedding as well as the measured cylinder response. a c d

Vortex shedding from bluff bodies in oscillatory flow: A report on Euromech 119

Abstract: European Mechanics Colloquium number 119 was held at Imperial College on 16–18 July 1979, when the subject of vortex shedding from bodies in unidirectional flow and oscillatory flow, was discussed. A wide range of experimental work was presented including low-Reynolds-number flows around circular cylinders, the influence of disturbances on bluff body flow, the measurement of fluctuating forces and the influence of oscillations of the stream. About a third of the 33 papers presented concentrated on theoretical aspects and the majority of these were concerned with the ‘method of discrete vortices’.

Autorotation of an elliptic cylinder about an axis perpendicular to the flow

Abstract: Autorotation of an elliptic cylinder about an axis fixed perpendicular to a parallel flow is explained in this paper by means of numerical solutions of the Navier-Stokes equations. Potential-flow theory predicts, for constant angular velocity, half a period in which a torque supports rotation and half a period in which it opposes rotation, with vanishing torque in the average. This balance is disturbed by viscous-flow effects in such a way that, for a given angular velocity, vortex shedding either damps rotation or, under certain conditions, favours rotation. The proper interplay of those conditions, which include synchronization of vortex shedding and rate of rotation, results in auto-rotation. The numerical results for Re [les ] 400 are compared with experimental data for Re = 90000 from the literature. The agreement of the force coefficients and the large-scale flow patterns is surprisingly good.

Vortex shedding from cylinders at low Reynolds numbers

Abstract: The presence of discontinuities in the Strouhal-number–Reynolds-number relationship was observed in several flows for cylinders in the Reynolds-number range 50–175. An experimental technique was devised to monitor carefully the shedding frequency and free-stream velocity so that the transitions could be distinctly observed. Quantitative Strouhal-number–Reynolds-number data, for cylinders of length-to-diameter ratio exceeding 150, agree reasonably well with the results of Tritton (1959) and Berger (1964).

OTEC cold water pipe design for problems caused by vortex-excited oscillations

Abstract: Vortex-excited oscillations of marine structures result in reduced fatigue life, large hydrodynamic forces and induced stresses, and sometimes lead to structural damage and to destructive failures. The cold water pipe of an Ocean Thermal Energy Conversion (OTEC) plant is nominally a bluff, flexible cylinder with a large aspect ratio (L/D = length/diameter), and is likely to be susceptible to resonant vortex-excited oscillations. The objective of this paper is to survey recent results pertaining to the vortex-excited oscillations of structures in general and to consider the application of these findings to the design of the OTEC cold water pipe. Practical design calculations are given as examples throughout the various sections of the report. This paper is limited in scope to the problems of vortex shedding from bluff, flexible structures in steady currents and the resulting vortex-excited oscillations. The effects of flow non-uniformities, surface roughness of the cylinder, and inclination to the incident flow are considered in addition to the case of a smooth cylinder in a uniform stream. Emphasis is placed upon design procedures, hydrodynamic coefficients applicable in practice, and the specification of structural response parameters relevant to the OTEC cold water pipe. There are important problems associated with the shedding of vortices from cylinders in waves and from the combined action of waves and currents, but these complex fluid/structure interactions are not considered in this paper.

Apparatus for measuring fluid flow

Abstract: A fluid flowmeter of the vortex shedding type. The vortex frequency is determined from variations in the internal reflectivity of a prism 16 mounted in the fluid and coupled to a light source 13 and to a detector/frequency meter 14 via an optical waveguide arrangement 15. Passage of a vortex adjacent the prism 16 causes a temporary change in the local fluid refractive index and hence a change in the intensity of light internally reflected by the prism.

Numerical Study on Viscous Shear Flow past a Circular Cylinder

Abstract: Numerical solutions of the unsteady Navier-Stokes equations are presented for a two-dimensional flow past a circular cylinder immersed in a uniform shear flow. Several values of the shear parameter K defined as Ga/U∞ is the oncoming velocity at the center of the cylinder. The Reynolds number Re based on 2a and U∞ is assumed to be 40 and 80. Although a pair of steady standing vortices is formed in a uniform flow when Re=40, it is found that a periodic vortex shedding occurs at the same Reynolds number in the case of a shear flow with a shear parameter K=0.2, however, the vortex shedding is suppressed even in the case of a shear flow revealing a clearly asymmetric flow pattern with respect to the geometrical center line of the cylinder. The same tendency is also observed in the flow of the Reynolds number Re=80. The time-averaged lift force acts in the direction towards the high-velocity side of the shear flow and its magnitude is approximately proportional to the shear parameter.

An Experimental Study on the Dynamic Response of a Vertical Cantilever Pipe Conveying Fluid

Abstract: Transverse vibrations of elastic pipes conveying a fluid have been observed in pipelines and heat exchangers. These fluid-induced vibrations can be a serious problem and in some incidents have caused structural failure resulting in environmental damage and economic loss. For offshore applications such as marine risers, the Ocean Thermal Energy Conversion Plant and deep ocean mining vacuums, the problem is compounded by the existence of vortex shedding, wave excitations, currents, and platform motions. An experimental study was conducted to investigate the effects of internal flow rate and the depth of immersion on the dynamic response of a vertical cantilever pipe discharging a fluid. It was found that the internal flow rate and the surrounding fluid have a significant effect on the natural frequencies of the system. Specifically it was found, that depending on the relative value of the forcing frequency, in comparison to the system natural frequencies, an increase in flow rate may not necessarily result in a larger system response. Conversely, an increase in the length of pipe immersion does not necessarily decrease the response of the system. It is also observed that with increasing flow rate, an auspicious increase in the response of the higher harmonics is noted, indicating an increase fluid coupling of the system. System natural frequencies were observed to decrease with increasing flow rate.

Discrete-Vortex Simulation of Unsteady Separated Flow behind a Nearly Normal Plate

Abstract: A discrete-vortex model is developed for a separated flow past a nearly normal plate immersed in a uniform flow. A reduction in the strength of vortices is introduced as a function of their age to simulate the cancellation of vorticity in near wake. The mean and fluctuating velocities in the near wake, the drag and the convection velocity of rolled-up vortices showed fairly good agreement with experiment. An unifavourable aspect of the reduction in the vortex strength is to yield a higher Strouhal number of vortex shedding than experiment.

The influence of vortex shedding on the diffraction of sound by a perforated screen

Abstract: This paper examines the theory of the interaction of sound with a slit-perforated screen in the presence of a uniform, subsonic tangential mean flow on both sides of the screen. The sound induces vortex shedding from sharp edges of the screen. The coupling of this vorticity with the mean flow leads to a significant modification in the predicted acoustic properties as compared with those predicted by the classical treatments of Rayleigh (1897) and Lamb (1932). In particular a considerable portion of the incident acoustic energy can be lost during the interaction, and is convected away in the mean flow in the form of localized vortical disturbances. The analytical results provide theoretical support for the use of perforated plates to inhibit the onset of cavity resonances in, for example, cross-flow heat exchangers.

Vortex shedding from circular cylinders in turbulent flow

Abstract: SUMMARY Vortex shedding from vibrating circular cylinders in turbulent flow is investigated experimentally in a wind tunnel at subcriticai Reynolds numbers. The cylinders are freely vibrating or driven. Two types of turbulent flow are used: grid generated homogeneous turbulence and turbulent boundary layers. Pressures and their correlation, lift coefficient and response are studied. The response observed is compared with some of the mathematical models for the prediction of vortex induced oscillation

Lower Mode Response of Circular Cylinders in Cross-Flow

Abstract: Transverse amplitude responses of a circular cylinder in cross-flow were determined as a function of reduced velocities for a variety of spring constants and damping coefficients. Maxima were found at reduced velocities of 5 and 16, and were of comparable amplitude. The first resonance, designated the “fundamental mode,” was due to normal vortex street excitation of the spring-mass system. The second resonance, designated the “lower mode,” occurred when the natural frequency was approximately one-third of the normal vortex shedding frequency. By assuming that the driving force was sinusoidal, it was possible to evaluate the lift coefficients at resonance. Lift coefficients for the lower mode behaved similarly with amplitude ratio but were an order of magnitude lower than lift coefficients for the fundamental mode. A mechanism was used to oscillate the cylinder transversely at prescribed frequencies and amplitudes. Dominant wake frequencies were determined from a frequency analysis of the hot-wire signal for a range of velocities and a fixed frequency of oscillation. It was found that synchronization of the shedding frequency to the forcing frequency did not take place for the lower mode. The familiar “lock-in” region, or frequency synchronization over finite bandwidth, was observed for the fundamental mode only. Since the frequency associated with normal vortex shedding was not suppressed when oscillations took place in the lower mode, it would seem that a low frequency vortex street had not replaced the normal one. It is likely, then, that the spring-mounted cylinder responded subharmonically to the exciting force resulting from vortex shedding. In this regard, however, it was curious that subharmonic response was not found at a frequency ratio of 0.5 as it was at 0.33. A conceptual model, which incorporated features of both the low frequency vortex street and subharmonic response, was developed which accounted for lower mode response at a frequency ratio of 0.33 as well as the lack of response at 0.5.

Maximum Side Forces and Associated Yawing Moments on Slender Bodies

Abstract: A method is presented for determining the maximum vortex-induced side force and associated yawing moment on slender bodies at high angles of attack in incompressibl e flow at effective crossflow Reynolds numbers from 104 to 108. An analogy with two-dimensional unsteady flow separation on a cylinder normal to the flow is used to bound the maximum steady vortex-induced side force on slender bodies of revolution. It is shown that the maximum vortex-induced side force occurs in the critical effective Reynolds number range when subcritical separation occurs on one side of the body and supercritical separation on the other. Although the method applies only to bodies dominated by a single asymmetric vortex pair, it could be extended to provide a basic building block for bounding the directional stability of long bodies affected by multiple asymmetric vortex pairs.

Vortex-shedding flowmeter having two bluff bodies

Abstract: A vortex-shedding flowmeter comprising two stationary vortex-shedding cross-members (i.e., bluff bodies) disposed transversely to a stream of fluid. Each cross-member has a bluff face with sharp Karman vortex generating edges with independently selected edge to edge widths d of from 10 to 40% of the inside width of the conduit through which the fluid flows and lengths 1 of from 0.3 to 2.0d. The second cross-member is disposed downstream of the first cross-member a distance of from 4 to 15 times the width d of the first cross-member. Any sensor may be used in any location provided it or they sense the Karman vortices generated by co-action of the two cross-members. This flowmeter provides strong signals with a high signal to noise ratio and high Strouhal number.

The effectiveness of vortex spoilers on a circular cylinder in smooth and turbulent flow

Abstract: As part of the aerodynamic design for a 12' diameter tubular gallery conveyor bridge, wind tunnel investigations were conducted to determine the effectiveness of various vortex spoiler devices. The investigations were carried out on sectional models of a bare circular cylinder, and on cylinders fitted with different configurations of fins. All cases were tested in smooth flow and in grid turbulence having a relative intensity of 14%. The results of the tests indicate that a helical system of projecting fins, more commonly known as “strakes”, was the most effective device for suppressing vortex induced excitations of the cylinder. The effectiveness of this arrangement however, was markedly influenced by the presence of the turbulence in the windstream. In the case of resonant vortex shedding with smooth windstream conditions, the strakes reduced the maximum amplitude of oscillation by a factor of about 20. In the case of resonant vortex shedding coupled with turbulent windstream conditions, the strakes reduced the maximum amplitude of oscillation only by a factor of about 2. In both cases, the strakes caused an upward shift of about 50% in the wind velocity necessary to attain resonance.

On the diffraction of sound by a screen with circular apertures in the presence of a low Mach number grazing flow

Abstract: Acoustic energy is dissipated when sound generates vorticity at a sharp edge. This paper re-examines the classical problem of the diffraction of sound by a thin screen with circular apertures when account is taken of vorticity production at the aperture rims. A uniform grazing flow is assumed to exist on both sides of the screen which, by convecting away the shed vorticity, greatly enhances the level of attenuation. Perforated screens have been used to suppress structural resonances which are coupled with the aerodynamic sound produced by unsteady vortex shedding from the tube-banks in a cross-flow heat exchanger, and the theory of this paper goes some way to predicting the attenuation available from a screen under prescribed mean flow conditions. In particular, one can predict how the relative magnitudes of the open-area ratio of the screen and the grazing flow velocity should be chosen in order to achieve optimal attenuation.

Dual output vortex-shedding flowmeter having drag-actuated torsional sensor

Abstract: A dual output vortex-shedding flowmeter in which a liquid or gas to be metered is conducted through a flow tube having a shedding body transversely supported thereon, the flowmeter being usable in a custody transfer system requiring two independent output signals. Torsionally-supported behind the body and spaced therefrom by a gap is a drag-actuated sensor which includes a pair of parallel legs symmetrically disposed with respect to a spindle forming the fulcrum axis of the sensor, this axis being normal to the longitudinal axis of the tube. In operation, torques are developed alternately in the clockwise and counterclockwise directions, causing the torsionally-supported sensor to oscillate at a frequency proportional to the flow rate of the fluid. These oscillations are converted into corresponding electrical signals by a pair of transducers coupled to opposing ends of the spindle to provide two independent outputs.

Karman vortex type flow measuring apparatus

Abstract: A Karman vortex type flow measuring apparatus comprises a conduit (2) for a fluid whose flow rate or flow velocity is to be measured, and a columnar vortex shedding member (3) extending within the conduit transversely of the fluid flow direction. Frequency of the vortex shedding is detected by a detector (6) whose output electrical signal is compared by a comparator (20) with a slice level signal, to generate a pulse train which is synchronous with the vortex shedding frequency. The slide level is variable in response to fluctuation in the output electrical signal of the detector (6). A highly accurate measurement can be achieved even when the vortex shedding is unstable.

Influence of rotor blade tip shape on tip vortex shedding - An unsteady, inviscid analysis

Abstract: An unsteady potential flow panel method is described based on a time-stepping procedure and using planar quadrilateral panels to represent the surface of thick blades. Each panel has a constant source and doublet distribution and a central control point where an internal Dirichlet boundary condition is applied. The method includes detail paneling around the tip edge and offers the facility of prescribing a tip-edge separation. Preliminary results are shown for four blade tip shapes which are treated as semi-span wings oscillating in pitch about a mean angle of attack of 6 deg. Further work is planned for the wake model before proceeding to higher angles of attack.

Vortex-shedding flowmeter with torsional sensor mounted on torque tube

Abstract: In a vortex-shedding flowmeter the fluid is conducted through a pipe having a shedder (11) fixedly mounted therein. A sensor (12) is torsionally supported downstream behind the shedder (11) on a pivot axis (Y') normal to the flow axis. The torsional support includes a torque tube (14) whose base (14A) is received within a bore in the pipe and is welded thereto and whose tip (14B) is welded to one end of the sensor. As the incoming fluid stream flows past the shedder (11), vortices are successively detached therefrom and appear alternately on either side of the sensor (12) to develop alternating torques about pivot axis (Y'), causing the sensor (12) to oscillate at a rate proportional to the fluid flow rate. These oscillations are converted into corresponding electrical signals by an external torque transducer (15) operatively coupled to the sensor by a sensor link assembly including a rod (18) which extends coaxially into torque tube (14) and is welded to the tip (14B) thereof.

Gaseous fluid flow meter utilizing karman vortex street

Abstract: A gaseous fluid flow meter utilizing a Karman vortex street has a conduit having opposed flat walls and through which a gaseous fluid to be measured flows, a vortex generating member disposed perpendicularly to the direction of flow of the fluid to generate the Karman vortex street downstream thereof, a vortex detector disposed on the conduit and having a transmitter in one flat wall for transmitting a continuous ultrasonic wave across the Karman vortex street and a receiver in the other flat wall positioned opposite the ultrasonic wave transmitter in a direction perpendicular to the direction of the flow of the gaseous fluid through the conduit for receiving the continuous ultrasonic wave to detect the changes in phase of the ultrasonic wave indicating the number of vortices of the Karman vortex street generated in a unit time, and a sound absorbing material on only the portion of the inner surfaces of the flat walls of the conduit around the transmitter and around the receiver and extending sufficiently far along the walls from the transmitter and receiver for preventing the generation of standing waves in front of the walls due to the reflection of the ultrasonic wave.