Showing papers on "Vortex shedding published in 1981"

On vortex shedding from smooth and rough cylinders in the range of Reynolds numbers 6*10^3 to 5*10^6

Abstract: The influence of surface roughness on the vortex-shedding frequency in the wake of a single cylinder has been investigated. The experiments were carried out in an atmospherical and a high-pressure wind tunnel. The tests were started with a smooth cylinder. Then the wake flow of cylinders with relative roughnesses of ks/d = 75 × 10−5, 300 × 10−5, 900 × 10−5, and 3000 × 10−5 was investigated.For all roughness parameters tested the Strouhal number exhibited an increase in the critical flow regime. With growing roughness parameter the step in the curve became smaller. At transcritical flow conditions the Strouhal number was measured to be in the range of Sr = 0·25 ± 0·018 for all surface roughness tested. No regular vortex shedding could be observed in the critical flow range for the smooth cylinder with l/d = 3·38. When prolonging the test body to l/d = 6·75 the wake fluctuations became periodic.

Turbulent uniform flow around cylinders of finite length

Abstract: Experimental results are presented which demonstrate the effects of the free end of a circular cylinder on the mean and fluctuating pressures, mean drag force, and the phenomenon of vortex shedding when the cylinder was exposed to uniform flow. The work was carried out at Reynolds number of 0.7 x 105, and turbulence level of 0.9%. The results of the pressure measurements indicated the presence of a suppressed two-dimensional region on the lower part of the cylinder. The functional relationship between the absolute maximum fluctuating pressure coefficient and cylinder length/diameter ratio was obtained. The accuracy of the drag measurements was checked by integrating the pressure profiles, and excellent agreement between the integrated pressure data and the direct measurement was obtained. The vortex shedding measurements suggested the possible existence of three distinct sets of vortex rows with different frequencies. In the lower region, vortex loops were shed with a frequency such that it yielded a Strouhal number appropriate to an infinitely long cylinder.

Sound diffraction at a trailing edge

Abstract: The diffraction of externally generated sound in a uniformly moving flow at the trailing edge of a semi-infinite flat plate is studied. In particular, the coupling of the sound field to the hydrodynamic field by way of vortex shedding from the edge is considered in detail, both in inviscid and in viscous flow.In the inviscid model the (two-dimensional) diffracted fields of a cylindrical pulse wave, a plane harmonic wave and a plane pulse wave are calculated. The viscous proess of vortex shedding is represented by an appropriate trailing-edge condition. Two specific cases are compared, in one of which the full Kutta condition is applied, and in the other no vortex shedding is permitted. The results show good agreement with Heavens’ (1978) observations from his schlieren photographs, and confirm his conclusions. It is further demonstrated, by an explicit expression, that the sound power absorbed by the wake may be positive or negative, depending on Mach number and source position. So the process of vortex shedding does not necessarily imply an attenuation of the sound.In the viscous model a high-Reynolds-number approximation is constructed, based on a triple-deck boundary-layer structure, matching the harmonic plane wave outer solution to a known incompressible inner solution near the edge, to obtain the viscous correction to the Kutta condition.

Evolution and breakdown of a vortex street in two dimensions

Abstract: The initial-value problem defined by two parallel vortex sheets of opposite sign is studied. Strictly two-dimensional, incompressible, nearly inviscid dynamics is assumed throughout. The roll-up of the sheets into a vortex street is simulated numerically using 4096 point vortices. Much longer runs than in previous work are performed, and it is found that only for a finite range of values of the ratio, h/λ, of sheet separation to perturbation wavelength, does a long-lived vortex street emerge. For h/λ [gsim ] 0·6 a pairing transition within each row intervenes. For h/λ [lsim ] 0·3 we find oscillatory modes.Using up to 16384 point vortices, we also study the breakdown of the metastable street to a two-dimensional, turbulent shear flow. The vortex blobs that made up the street may merge with others of the same sign after the breakdown, but otherwise persist throughout the turbulent regime. Neither their disintegration nor amalgamation with vortices of opposite sign was observed. Using dimensional arguments we derive the relevant scaling theory, and show that it applies to a flow started from two random vortex sheets. The resulting turbulence is not self-similar. For the turbulent flow that follows from the breakdown of a regular vortex street two length scales with different power-law growth in time appear to be necessary. The important differences in the asymptotic structure of flows initialized from random and regular sheets leads us to question the idea of universality. The influence of the symmetry of the initial perturbation on the subsequent development is also considered.

Universal Similarity in the Wakes of Stationary and Vibrating Bluff Structures

Abstract: A universal wake Strouhal number, St * = f s d′/U b , has been proposed and is based upon the Strouhal frequency f s of the incident flow, the measured wake width d' at the end of the vortex formation region, and the mean velocity U b at the edge of the separated boundary layer. This universal parameter collapses the characteristic wake scales for bluff bodies onto a single curve over the entire range of subcritical and transcritical wake Reynolds numbers Re * . The pressure drag, vortex shedding frequency and base pressure are related through a dependence between St C D , the product of the Strouhal number and drag coefficient, and the base pressure parameter K. There is a general collapse of the data up to K = 2, which is the upper limit thus far for bluff body flows.

On the Point Vortex Method

Abstract: The discretization of the integrodifferential equation governing the evolution of a vortex sheet leads to a representation of the sheet by point vortices. It is shown, by examination of the special case of a uniform circular vortex sheet, that the chaotic motion which often arises when the point vortex representation is used is due to the amplification of numerically introduced disturbances. The mechanism is a discrete form of Helmholtz instability. The linear smoothing method of Longuet-Higgins and Cokelet (1976) and the repositioning method of Fink and Soh (1978) are shown to reduce the instability.

Properties of a Vortex Street of Finite Vortices

Abstract: Steady solutions of the Euler equations are calculated for an infinite array of vortices, consisting of two staggered parallel rows of identical vortices of finite area and uniform vorticity. These models are similar to the “vortex streets” studied theoretically by von Karman and others, except that here vortices of finite rather than infinitesimal area are employed.

The effect of base bleed on vortex shedding and base pressure in compressible flow

Abstract: Experiments have been carried out to investigate the phenomenon of vortex shedding from the blunt trailing edge of an aerodynamic body in transonic and supersonic flow. The effect of a discharge of bleed air from a slot in the trailing edge has been included and the relationship between the vortex formation and base pressure has been considered.In transonic flow a small amount of bleed air was found to produce a rearward shift in the point of origin of the vortices with a consequent substantial increase in base pressure. The effect was less marked in supersonic flow. At higher rates of bleed two different regimes of vortex shedding were identified and increase in bleed rate caused a reduction in base pressure. For bleed rates giving near-maximum base pressure no vortex shedding occurred.

A numerical study of vortex shedding from one and two circular cylinders

Abstract: A discrete-vortex representation of the wake of a circular cylinder, in which vortices are convected in a potential-flow calculation and maintain their identities unless they approach one another or a surface closely, predicts many of the unsteady flow features and is computationally more efficient than other schemes. The mean rate of shedding of vorticity is adjusted to be compatible with experiments at a high subcritical Reynolds number of 3 × 104 and the model gives reasonable predictions of separation, drag, lift, Strouhal number and vorticity loss in the formation region. The method is extended to accommodate a second cylinder and many of the surprising features which have been observed experimentally with two cylinders in a side-by-side arrangement are reproduced.

Vortex shedding fluid flowmeter using optical fibre sensor

Abstract: An optical fibre flowmeter is described which uses a single fibre mounted transversely to the fluid flow within the pipe. The fibre is vibrated by the natural phenomenon of vortex shedding, causing phase modulation of the optical carrier within. The modulation is detected at the fibre exit by the fibredyne technique, and the flow rate determined from the vibration frequency.

Modeling of vortex‐corner interaction using point vortices

Abstract: Using point vortices, the interaction of a single vortex, as well as patterns of vortices, with a corner is examined; comparisons are made with corresponding experiments. Trajectories of a vortex swept past the corner can be well‐approximated, provided that a sufficiently weak strength of the vortex is specified. Calculations show that the dimensionless amplitude of the pressure fluctuation at the corner is very sensitive to small variations of the initial position of the vortex, and relatively insensitive to variations in strength; this finding has important consequences for recently observed amplitude modulation of pressure at, and velocity near, impingement. In fact, by prescribing transversely staggered patterns of vortices upstream of the corner, determined from experimental flow visualization, the form of the time‐averaged pressure and velocity spectra can be approximated. The most critical feature of these spectra, a well‐defined low‐frequency component(s), confirms the hypothesized mechanism associated with low‐frequency modulation observed in experiments. In addition, shortcomings of this method, primarily due to distributed vorticity inherent in laboratory vortices, are pointed out.

On the Acoustical Implications of Vortex Shedding from an Exhaust Pipe

Abstract: Asymptotic approximations for small Strouhal number are derived for the solution of the problem of the interaction between an acoustic wace and a subsonic jet flow issuing from a semi-infinite pipe. Density and sound speed differences between the jet flow and the (slowly moving) ambient medium, and a general edge condition are included. The approximations relate to the field inside the jet flow, to the far field, to the reflection coefficient, end-impedance and end correction for the reflected wave inside the pipe, and to the transmitted and radiated sound power. Within the range of parameters considered, the effect of the density and sound speed ddferences and ambient flow is found to be appreciable, although the character of the solution is not changed. However, the choice of the edge condition does have important implications; specifically, the phase of the reflected wave is most sensitive to only slight deviations from the Kutta condition. Only a few years ago, it was quite generally believed that a Kutta condition at a sharp trailing edge (effecting vortex shedding) in aero-acoustical problems may have some importance in that it changes radiation directivities with a few decibels or so, but never in such a dramatic way that orders of magnitude are involved ([9], p. 449; [23], p. 364). However, the experiments of Bechert, Michel and Pfizenmaier [2], together with Howe's [l21 explanation, have shown this opinion not to be true. In the configuration of a subsonic jet issuing from a pipe perturbed from inside by long sound waves, they found that only a small fraction of the net sound power, transmitted through the pipe, was recaptured in the far field. The rest was transformed into hydrodynamic energy of vortices shed from the pipe edge. These vortices, making only little noise, arise from viscous and nonlinear action, which is, in an inviscid linear problem, modeled by an appropriate edge condition, like the Kutta condition. Reductions up to 25 dB were obtained, so in this case the Kutta condition really provides an important sink of sound. (The opinion that in general the Kutta condition and vortex shedding gives a reduction is not correct; at least in the case of an airfoil trailing edge there is, under some conditions, more sound radiated by the vortices interacting with the edge than was used for their production, and the net result is an amplification [21].) Various technological applications exploiting this mechanism for noise reduction are in use or under development [l], e.g., tube ends consisting of many parallel small nozzles, acoustic liners of Helmholtz resonators with through-flow, and exhaust mufflers for piston engines, so a deeper study of various aspects of this process seems well worth doing.

Method and device for the dynamic and density-independent determination of mass flow

Abstract: Method for the dynamic and density-independent determination of the mass flow of fluids. A single vortex generating bluff body is placed in the flow to generate vortices according to the principle of the Karman vortex path. The dynamic pressure (1/2ρv 2 ) is determined via the effect of the drag directly at this bluff body. The velocity of flow (v) is determined via the effect of the vortex shedding at the bluff body. The mass flow is determined according to the equation: m=2·(1/2ρv.sup.2)·1/2·A wherein A is the cross section of the flow channel. Device for implementing such method. The device consists essentially of a vortex-generating bluff body having one or two fixed ends, which itself serves as a transducer or at which one or two transducers are provided. An electronic system for processing the resultant signal is available.

Part II: Spanwise Correlation and Loading

Abstract: The influence of spanwise correlation of fluctuating pressures due to vortex shedding on the total oscillatory loading of a long bluff cylinder in cross-flow is considered. Experimentally determined pressure correlation lengths on a stationary and vibrating cylinder of square cross section are then combined with previously obtained chordwise pressure distributions to demonstrate the amplification of the oscillatory lift on a long bluff cylinder during cross-flow vibrations.

On the Existence of Atmospheric Vortices Downwind of Hawaii during the HAMEC Project

Abstract: Aircraft observations west of the island of Hawaii in June 1980 during the Hawaii Mesoscale Energy and Climate (HAMEC) Project have provided the first in situ measurements of airflow within atmospheric vortices downwind of a tall island. A band of low-level westerly winds was observed to extend more than 150 km west of the island along the axis line separating cyclonic vortices to the north of that line from anticyclonic vortices to the south. The theoretical downstream propagation speed of those vortices is obtained from the solution to a quadratic equation, and while previous satellite studies of atmospheric vortices used the larger root (i.e., ∼80% of the ambient flow), the present data are consistent with the smaller root ∼20%). The turbulent Reynolds number for the flow is 140, and the corresponding vortex shedding time is 32 h.

Characteristics of Circular Cylinders in Turbulent Flows

Abstract: The effect of a free-stream turbulence of high intensity on the flow past a rigid circular cylinder was experimentally studied in a Reynolds-number range 7.9×103∼5.4×l04. Square-meshed grids were used to produce homogeneous turbulent-flow fields. The intensity and scale of a turbulent flow in which the cylinder was immersed were varied by positioning the cylinder at various locations downstream of the grid. Measurements were made of the time-averaged drag coefficient, Strouhal number of the vortex shedding, spanwise correlation length and length of the vortex formation region in the wake of the cylinder. These properties of flow around the cylinder were found to be considerably different from those measured in a smooth flow.

Electronic circuit using digital techniques for vortex shedding flowmeter signal processing

Abstract: A circuit for processing the vortex shedding frequency signal of a vortex shedding flowmeter comprises a phase detector, switch and low pass filter connected in series. The phase detector receives the vortex shedding frequency and applies it over the normally closed switch to the low pass filter which produces an analog signal corresponding to the frequency of the vortex shedding frequency signal. A voltage controlled oscillator is connected between an output of the low pass filter and an input of the phase detector for tracking the vortex shedding frequency and producing a tracking frequency signal which is maintained when the vortex shedding frequency signal disappears. A range or gain code circuit portion is connected for establishing a set time period during which the frequency signal is accumulated in a counter. The accumulated signal from the counter is utilized as a digital signal corresponding to the vortex shedding frequency signal.

The Effect of Sound on the Vortex-shedding from a Circular Cylinder : Acoustical Vibrations Directed along Axis of Cylinder

Abstract: An experimental investigation has been carried out to study acoustic interferences in vortex-shedding from a circular cylinder in a crossflow subjected to the acoustical vibrations directed along the axis vortex-shedding along the cylinder axis. The results indicate that the effects of the acoustical vibrations directed along the axis of the cylinder are similar to the effects of the cylinder on the vortex-shedding from the cylinder, and both vibrations increase the spanwise correlation of the cylinder wake. The acoustic frequencies which produce powerful effects on vortex-shedding correspond to the frequencies of laminar-turbulent transition wave in a separated shear layer, and there are critical sound pressure levels in these acoustical vibrations.

Experimental observation of stable arrangement of vortex rings

Abstract: A vortex ring version of the Karman vortex street is observed through flow visualization. A remarkably stable configuration of discrete vortex shedding is recognized with a distinction from the two‐dimensional case that the spacing between the alternating vortices is not equal.

Simulation of Viscous Diffusion for Extension of the Surface Vorticity Method to Boundary Layer and Separated Flows

Abstract: A numerical method for two-dimensional incompressible viscous fluid flows is tested on the diffusion of a point vortex. It is then applied to the boundary layer to reconstruct the Blasius profile, to demonstrate flow separation, and to simulate turbulence. The significance of Thwaites' boundary layer parameter for flow separation is explained. The Kelvin-Helmholtz instability, which is responsible for two-dimensional turbulence, is represented by the motion of an array of point vortices after an initial disturbance. The formation of the Von Karman vortex street downstream of a circular cylinder is described by computer simulation, and the influence of viscous diffusion is shown. For two different cylinder Reynolds numbers the vortex shedding frequencies and oscillating lift and drag forces are evaluated.

Experiments on the reduction of base drag of a blunt trailing edge aerofoil in subsonic flow

Abstract: It has been pointed out by a number of investigators that aerofoils with blunt trailing edges have many advantages, both structural and aerodynamic, at transonic and supersonic speeds. However, a profile with a blunt trailing edge unfortunately creates a high base drag at subsonic speeds and this could result in the total drag being considerably higher than that for a profile with a sharp trailing edge. This base drag arises due to the periodic vortex shedding from the blunt base which causes low pressures in the wake thus creating the base drag. In order to reduce the base drag of a blunt trailing edge at subsonic speeds, this periodic vortex shedding must be suppressed. This has been achieved by various methods — a splitter plate behind the base, the base bleed, base cavities — and these have shown varying degrees of drag reduction.