Showing papers on "Vortex shedding published in 1985"

Evolution of a single wake behind a pair of bluff bodies

Abstract: In this paper the flow behind a pair of bluff bodies placed side by side in a stream is studied using a variety of flow-visualization methods. Above a critical gap size between the bodies, vortex-shedding synchronization occurs, either in phase or in antiphase. It has previously been assumed that such synchronization forms a wake comprising two parallel vortex streets in phase and in antiphase respectively. In the present paper we find that two antiphase streets are indeed formed, although in-phase shedding leads to the development of a single large-scale wake. The vortices which are formed simultaneously at the cylinders rotate around one another downstream, each pair forming a ‘binary vortex’. The combined wake comprises a street of such vortices, which we term a binary vortex street. Below a critical gap size between the bluff bodies the flow becomes asymmetric. We observe in this regime certain harmonic modes of vortex shedding whereby the shedding frequency on one side of the wake is a multiple of that on the other. Again, a large-scale wake is formed downstream. The present observations lead to a new interpretation of hot-wire-frequency data from other studies in terms of the harmonic modes.

Sinusoidal flow relative to circular cylinders

Abstract: The motions of vortices around single cylinders and around pairs of cylinders in relative sinusoidal flow are investigated in this paper. Using simultaneous flow visualization and force measurements, the vortex motions are related to the fluid-induced lift and in-line forces. For the single cylinder, several repeatable patterns of vortex shedding are identified within particular ranges of flow amplitude. The process of pairing of vortices from a previous half cycle with those in a present half cycle is fundamental to all the patterns. Visualization is shown to be more effective in a reference frame which is fixed with respect to the undisturbed fluid rather than with respect to the cylinders. For this reason, the examples of vortex motions are taken from a rig in which vertical cylinders are oscillated in a tank of fluid. By oscillating a pair of cylinders over a range of gaps, orientations and amplitudes, it is found that the vortex-shedding patterns identified for a single cylinder can synchronize either in phase or in antiphase between the two cylinders. Such observations help to explain how lift and in-line forces are influenced by cylinder proximity and in some cases these forces are significantly magnified. Force coefficients are evaluated for both the single cylinder and the pair of cylinders.

Forces on cylinders in viscous oscillatory flow at low Keulegan-Carpenter numbers

Abstract: This paper presents a comparison between theory and experiment for the in-line forces on cylinders of general cross-section in planar oscillatory flows of small amplitude. The theoretical analysis evaluates corrections to the standard inviscid inertial force at low Keulegan-Carpenter numbers which arise from the presence of viscous laminar boundary layers and from the development of vortex shedding. The boundary-layer contribution due to both skin friction and displacement effects is calculated to first order in the Stokes parameter β−½. The contribution to the in-line force from separation and vortex shedding, for which the results presented only apply to sharp-edged bodies, is taken from previous work on vortex shedding from isolated edges using the discrete vortex modelling technique. The resulting force has components both in phase with the fluid acceleration (inertia) and in phase with the velocity (drag).The theoretical results are compared to measurements taken in a [xcup ]-tube water channel on a number of cylinders of different cross-section including circular cylinders and sharp-edged sections. The comparisons suggest that the theory is valid for Keulegan–Carpenter numbers below about 3 and for moderately high values of the β parameter.

The effect of sound on vortex shedding from cylinders

Abstract: The influence of a transverse sound wave on vortex shedding from a rigid circular cylinder in a duct has been explored at Reynolds numbers from 20000 to 40000. In the absence of sound, the vortex shedding is found to consist of strings of coherent cyclic events which have frequencies that wander randomly about the nominal vortex-shedding frequency. Application of sound at the vortex-shedding frequency eliminates this wander and correlates the shedding along the cylinder axis. The frequency of vortex shedding can be shifted by sound applied either above or below the nominal vortex-shedding frequency. This entrainment is produced by the velocity induced by the sound wave rather than by the sound pressure. These phenomena are also observed in tube rows.

Influence of rotation on the near-wake development behind an impulsively started circular cylinder

Abstract: The early phase of the establishment of the flow past a circular cylinder started impulsively into rotation and translation is investigated by visualizing the flow patterns with solid tracers and by analysing qualitatively (flow topology) and quantitatively (velocity distributions and singular-point trajectories) the corresponding photographs. The range considered corresponds to moderate Reynolds numbers (Re [les ] 1000). The rotating-to-translating-speed ratio α increases from 0 to 3.25 and the motion covers a period during which the cylinder translates 4.5 or even 7 times its diameter. The details of the mechanisms of the near-wake formation are considered in particular and the increase of the flow asymmetry with increase in rotation is pointed out. Thus the existence of two regimes has been confirmed with the creation or non-creation of alternate eddies after an initial one E1 Furthermore, the new phenomena of saddle-point transposition and intermediate-eddy coalescence have been identified in the formation or shedding of respectively the odd and even subsequent eddies Ei (i = 2,3,…) when they exist. The very good agreement between these experimental data and the numerical results of Badr & Dennis (1985), obtained by solving the Navier-Stokes equations and presented in a parallel paper, confirms their respective validity and permits the determination of the flow characteristics not accessible, or accessible only with difficulty, to the present experiments. These flow properties such as drag and vorticity are capable of providing information on the Magnus effect for the former property and on unsteady separated flows for the latter.

Viscous flow induced by counter‐rotating vortices

Abstract: The nature of the viscous boundary‐layer flow induced near a wall, caused by a pair of counter‐rotating vortices above the wall, is investigated. Solutions for the unsteady flow that develops near the wall are obtained numerically for a variety of cases. The vortices are taken to be of equal strength with cores located at equal distances from the wall; depending upon the assumed sense of rotation, the vortices either move toward the wall or recede from it, creating a region of either inflow or outflow near the wall, respectively. The calculated results show that the adverse pressure gradient near the wall induced by the vortex motion gives rise to explosively growing regions of recirculating flow near the wall. The possible relevance of these results to Gortler vortex instability and turbulent flow near walls is discussed.

Dynamics of Forebody Flow Separation and Associated Vortices

Abstract: It is well established that there is a strong coupling between body motion and boundary layer separation with attendant vortex shedding. In the present paper this coupling is studied for the particular case of a missile or an aircraft fuselage at very high angles of attack. It is shown that the unusual results obtained in recent tests can be explained by considering the so-called "moving-wall effect" on boundary layer transition and/or separation.

Dynamic ground effects on a two-dimensional flat plate

Abstract: The effect of time-variant vortex shedding is simulated by a sequence of discrete vortices convecting downstream in the wake of a two-dimensional flat plate whose lifting condition is modeled by means of the quasi-vortex lattice method. The boundary condition of this problem is specified in such a way that the tangency condition on the surface of the flat plate is satisfied; the boundary condition also takes into account the effect of airfoil motion relative to the ground. Significant lift changes are shown to occur, due to the dynamic ground effect, that are crucial in aircraft takeoff and landing transitions.

The loading on a cylinder in post-critical flow beneath periodic and random waves

Abstract: A major problem encountered in model testing of offshore structures in wave tanks is the difficulty of extrapolating fluid loading on tubular members to full scale conditions. It is well known that in steady flow the drag of a circular cylinder undergoes large changes over the Reynolds number range Re = 10E5. - 10E6. Measurements are presented of the in-line transverse loads on a small element of a vertical and horizontal cylinder in waves at Keulegan Carpenter numbers up to 20 and Reynolds numbers up to 5 x 10E5.. Drag and inertia coefficients are calculated for the vertical cylinder and Morison's equation is found to give a good fit to the measured in-line force. Large transverse forces indicate the presence of strong vortex shedding at post-critical Reynolds numbers. Measurements in regular waves and random waves are found to give similar values of rms force coefficients. Vortex shedding is triggered in random waves when the Keulegan Carpenter number exceeds 7. In both orientations the cylinder loads show little influence of water particle orbit shape. Morison's equation fails to fit the horizontal cylinder data due to the presence of a strong vortex shedding component.

High angle-of-attack calculations of the subsonic vortex flow on slender bodies

Abstract: A nonlinear vortex-lattice method is utilized for the calculation of separated vortex flows over slender bodies at high incidence. Symmetric and asymmetric vortex flow cases are calculated showing good agreement with recently obtained experimental data for an ogive-cyclinder body at high Reynolds numbers. The only input needed for these calculations is the positions of the separation lines on the body. Studies of some of the numerical aspects of this method are described and conclusions are implemented to improve the calculations.

An experimental investigation of the heat-transfer effects of a longitudinal vortex embedded in a turbulent boundary layer

Abstract: Longitudinal vortices occur in numerous flow situations in which heat transfer is of considerable importance, such as on the endwalls and blade surfaces of turbine cascades. Previous research implies that the heat transfer rate is increased in the region of these vortices. However, most of these investigations have involved complex turbulent flows in which the longitudinal vortices are just a small component of the total flow configuration. The purpose of this experimental investigation is to isolate the specific effects of longitudinal vortices on heat transfer by embedding a single longitudinal vortex in an otherwise two-dimensional turbulent boundary layer. By obtaining spatially resolved heat transfer measurements and detailed fluid mechanical data, the local effects of the longitudinal vortices on the transfer rate will be determined, as well as the fluid dynamic mechanisms which cause these changes.

The effect of large-eddy breakup devices on oncoming vorticity

Abstract: Experiments have shown that a large-eddy breakup device consisting of a short splitter plate placed in the turbulent boundary layer over a plane wall can lead to a reduction in drag. We investigate the idealized problem of an incident line vortex convected past such a device. The vorticity shed from the trailing edge of the plate is found to cancel the effect of the incident vortex and to reduce velocity fluctuations significantly in the vicinity of the wall.

Rotor wake characteristics of a transonic axial flow fan

Abstract: State of the art turbomachinery flow analysis codes are not capable of predicting the viscous flow features within turbomachinery blade wakes. Until efficient 3D viscous flow analysis codes become a reality there is therefore a need for models which can describe the generation and transport of blade wakes and the mixing process within the wake. To address the need for experimental data to support the development of such models, high response pressure measurements and laser anemometer velocity measurements were obtained in the wake of a transonic axial flow fan rotor.

The flow past two cylinders having different diameters

Abstract: This paper describes the results of an investigation of the flow over two cylinders in which the diameter of the upstream cylinder is half that of the downstream cylinder. Measurements were made of the surface pressure distribution around the downstream cylinder, and of the base pressures and Strouhal numbers of the two cylinders for separations between 1·32 D and 3·88 D, where D is the diameter of the downstream cylinder, and for angles of stagger between 0° and 45°. With the cylinders in tandem two different flows were found, and it is suggested that there is a step change between the flows at a critical separation. The behaviour of the measured quantities with a change of the angle of stagger is found, in general, to depend on whether the separation is less or greater than the critical separation. For each separation an angle of stagger is found at which the pressure distribution has a strong asymmetry over the rear of die cylinder. These pressure distributions are similar to each other, and they indicate the presence of an enhanced flow over the upper half of the cylinder. Also the lift force, for a given separation, then has its maximum value. It is found that for separations greater than the critical separation the Strouhal numbers of the vortex shedding from the two cylinders are identical over a considerable range in the angle of stagger.

Response behavior of hot-wires and films to flows of different gases

Abstract: Measurements of the voltage output for hot-wire and film anemometers placed in flows of nine different gases have been made as a function of flow velocity. It has been possible to correlate the measurements quite accurately by treating the data in terms of suitably defined Reynolds and Nusselt numbers. In order to obtain these correlations it has been necessary to consider and correct for the effects of probe-end conduction losses, temperature dependencies of gas molecular properties, flow slip at the probe surfaces, and gas accommodation. With the exception of the results for helium (for which accommodation effects are strong), the most important correction is shown to be that for the different temperature dependencies of the gas molecular properties. This finding is contrasted with previous studies which have assumed that the largest effect among different gases was due to variations in the Prandtl number. The importance of the nature of the flow over the cylindrical devices to the heat transfer behaviour is described. A previously unreported hysteresis in the heat transfer behaviour for Re ≈ 44 has been characterized and attributed to the presence or absence of eddy shedding from the heated cylinder.

Flow structures of a basic annular jet

Abstract: This paper describes the modal expansion and conditional sampling results of static pressure fluctuations in the inner and outer mixing regions of a basic annular jet. An understanding of the mechanism of the wake shed- ding process from the internal recirculating region behind the blunt interface is attempted by conditionally sampled pressure signals. The evolution and characteristics of the shedding wake, wake-induced vortices, and jet vortices in both shear layers are measured and presented. The evolution of the wake-induced vortices in the outer shear layer as a result of excitation by the disturbances associated with the shedding wakes is further supported. In the case of the basic annular jet, another type of coherent structure was found in the outer mixing region, in addition to those associated with the outer jet vortices.5 This additional structure, termed the wake-induced vortex, seemed to be the consequence of the presence of the internal recirculating region and the shedding of the standing vortices or the wake vortices downstream of the blunt interface.5-6 Both the outer jet vortices and wake-induced vortices have, besides the axisymmetrical mode constituents, azimuthal constituents. The constituent of the first mode could be as dominant as that of the axisymmetrical.7 The existence and dominance of the wake-induced vortices depended on the diameter ratio D//Z>0. For large diameter ratios, the wake- induced vortices seemed to be more dominant. The investigation of the influence of the shedding wakes and their associated disturbances on the initial outer shear layers and the subsequent evolution of the induced structures may help to further clarify the phenomena of shear layer ex- citation and large-scale structure dynamics which are fun- damental in the study of turbulent free shear flows. Moreover, the inner region resembles that of a near wake behind an axisymmetric blunt body. Similar studies are rare and the ones reported are limited primarily to the case of a disk in cross flow. The present investigation was an attempt to understand the mechanism of the wake shedding process from the inter- nal recirculating region in the near wake of the blunt inter- face and the mode of evolution of the wake-induced vortices in the outer mixing region. This paper documents the results and interpretations based primarily on a mode expansion technique and a time domain analysis on the static pressure fluctuations. Measurements of the velocity fluctuations are being carried out and will be reported separately.

A new approach to flow problems past a porous plate

Abstract: A new method to simulate flows past a two-dimensional porous plate is proposed. The method is a simple ap- plication of a discrete vortex method that has been well established for flows past an inclined flat plate. In this method, a flow past a porous plate is described as superposition of the velocity potential of a uniform flow on that of a flow past a flat plate. The effect of porosity is replaced by that of a mass flow rate of the approaching freestream passing through the plate. Calculated flow features are compared with experiments. It is shown that the present method is effective in the simulation of the flow past a porous plate. an example, Koo and James3 devised a mathematical model for steady two- dimensional flow through a screen. In the model, the screen is replaced by a source distribution and the stream function is adjusted to give the correct mass and momentum flow across the screen. The results predict drag with reasonable accuracy so far as the solidity of the screen is small. As demonstrated by the photographs taken by Inoue et al.,5'6 however, one of the important features of flow past a porous plate is the vortex shedding from the edges of the plate. The vortices shed con- tinuously from the edges tend to dominate the flowfield with the increasing solidity of the plate. Thus, in order to correctly analyze the flowfield past a porous plate, the effect of the vor- tices shed from the edges should be taken into consideration. In this paper, a new method to analyze the flowfield past a porous plate is proposed. It is well known that a discrete vortex method is quite effective in simulating a high Reynolds number flow past an inclined flat plate.7"9 In this method, the effect of vortices shed from the edges of the plate is properly treated. Thus, if we can add the effect of porosity to the flow past a flat plate, then the flow past a porous plate may be ade- quately analyzed. Our idea is that the effect of porosity can be expressed by superposition of the velocity potential of a uniform flow on the velocity potential of a flow past a flat plate. In the next section, the problem is formulated mathematically and the method of solution is described. Mathematical Formulation and Numerical Procedure First, we consider a flow past a flat plate. The fluid is assumed to be set in motion impulsively from rest. This situa- tion can be realized experimentally by moving the plate im- pulsively in an otherwise stationary fluid. At the initial in- stance, the flow is assumed to be irrotational and, therefore, the velocities of the flow are infinite at both edges of the plate. In order to remove this divergence, vortices are generated so that they suppress the divergence at the edges (Kutta condi- tion). The viscosity plays its role only at this moment of vortex generation and, except for this moment, the flow is assumed to be inviscid. As in the previous studies,7"9 the motion of the

Heat-transfer distribution around a cylinder in pulsating crossflow

Abstract: The effects of sinusoidal flow pulsations on the heat transfer from a cylinder to a crossflow at Re = 50,000 were investigated. A range of different pulsation amplitudes of up to 25% and frequencies both above and below the natural shedding frequency were used. The pulsating flow was clean and well organized. It had greater than 95% of the power at the fundamental frequency with a low turbulence level (less than 0.5%). The time-averaged local heat transfer was experimentally measured for a constant-temperature surface-boundary condition using a small heat flux gage in the cylinder wall. Distributions were obtained by rotating the cylinder through 180 deg. The experiments showed no significant increase of heat transfer due to the flow pulsation in either the wake or attached boundary layer region. Small local increases were found near the separation point.

Vortex shedding flow meter with stress concentration signal enhancement

Abstract: This invention discloses a vortex shedding flowmeter comprising a vortex generating member disposed across one cross section of the flow passage and a vortex sensing member including a wing disposed across another cross section of the flow passage parallel to the vortex generating member, wherein one extremity of the wing is secured to a stocky container member housing a piezoelectric element by a flange of a thin diaphragm construction, which stocky container member is rigidly secured to the wall of the flow passage, while the other extremity of the wing is secured to the opposite side wall of the flow passage in a resilient manner.

Vortex shedding flow meter with noise suppressing and signal enhancing means

Abstract: This invention discloses a vortex shedding flowmeter comprising a vortex generating member disposed across one cross section of the flow passage and a pair of vortex sensing wings respectively disposed across two cross section of the flow passage straddling the vortex generating member in a parallel relationship thereto, wherein one extremity of each wing is secured to a stocky container member housing a piezo-electric element by a flange of a thin diaphragm construction, which stocky container member is rigidly secured to the wall of the flow passage, while the other extremity of each wing is secured to the opposite side wall of the flow passage in a resilient manner.

Unsteady Vortex Airfoil Interaction

Abstract: : Unsteady flow in the vicinity of an airfoil in a subsonic flow with strong vortices is investigated for a two-dimensional problem. This investigation concentrates on the interaction of the passing vortex, flow field with the steady flow field of the airfoil. Unsteady flow separation and generation of new vortices of the airfoil occurs depending on vortex strength and core path. Inviscid calculations of vortex path and sound generation are compared with the experimental results. The associated pressure waves have a strong directivity. The vortices used for the interaction experiments are generated by different vortex shedding cylinders in a stationary duct flow or by airfoils in the starting flow of a shock-tube. High speed interferometric flow recording and wall pressure measurement are the main experimental techniques.