Showing papers on "Vortex shedding published in 1979"

Turbulent flow in a depth-limited boundary layer

Abstract: Turbulent flow was investigated at a constant flow Reynolds number in a depth-limited boundary layer. Relative roughness was 0.12. Roughness density (ratio of plan area of elements to total bed area) was varied to cover the range of flow types described by Morris [1955]. The effect of roughness density on velocity profile shape factors, energy dissipation, and streamwise kinetic energy confirmed the existence of three types of roughness density-flow interaction, namely, ‘skimming,’ ‘wake interaction,’ and ‘isolated roughness’ flows. The velocity profiles and kinetic energy measures identified three layers in the vertical profile: an outer layer (y/D > 0.35), a wake layer (0.35 > y/D > 0.10), and an inner region (y/D < 0.10). The wake layer is produced by strong turbulent action accompanying vortex shedding at intermediate roughness densities. Spectral measurements and calculations of the turbulent macroscale indicated that the spectral shape is affected by the free surface but that below y/D = 0.5 a parametric model using the turbulent Reynolds number in the spectral energy equation adequately describes the spectral shape for all roughness densities. Implications of these results are discussed for the study of shallow, natural flows with high relative roughness.

Axial flow in trailing line vortices

Abstract: Axial flow in the core of a laminar steady trailing vortex from the tip of a semi‐infinite wing is analyzed assuming small departure of the axial velocity from the free‐stream velocity. It is further assumed that the axial pressure gradient is determined by the swirl velocities of an ideal infinite line vortex in which the radial and the associated axial velocity variations are neglected in the equation for the angular momentum. The axial and lateral variations of the axial velocity depend on the strength of the vortex and initial axial velocity distribution which must be specified at some station behind the wing except at the virtual origin of the vortex where a nonintegrable singularity exists. Numerical solutions for the axial velocity are obtained using the axial pressure gradient given by the line vortex and analytical solutions are obtained using an equivalent axial pressure gradient with good agreement between the two sets of axial velocity distributions. Resolution of the previous uncertainties in this field is given which were due to the unrecognized singularity at the virtual origin of the vortex. Using the calculated axial velocity the neglected radial and the associated axial fluxes of angular momentum are determined and the limits of validity of the theory presented here in terms of a suitably defined vortex Reynolds number and a nondimensional distance measured from the virtual origin of the vortex are given.

Inviscid Model of Two-Dimensional Vortex Shedding by a Circular Cylinder

Abstract: A discrete vortex model based on potential flow and boundary-layer interaction, rediscretization of shear layers, and circulation dissipation is developed to determine the characteristics of an impulsively started flow about a circular cylinder. The evolution of the flow from the start to very large times, lift and drag forces, Strouhal number, oscillations of the separation and stagnation points, and the vortex-street characteristics are calculated and compared with experiments. b CD CL Cpb c D fv h / j m N n q Re St s ds t At U u v w z F 7 6 X v p Nomenclature = longitudinal spacing of vortices = drag coefficient = lift coefficient = base pressure coefficient = radius of the cylinder - diameter of the cylinder ,D = 2c = vortex shedding frequency = transverse spacing of vortices = an index = distance to nascent vortex from cylinder = number of vortices on a sheet = an index = total velocity at a point = Reynolds number, UD/v = Strouhal number, fvD/ U = distance along a sheet = point vortex spacing = time or Ut/c for U= 1 and c = 1 - numerical step size = velocity of the ambient flow = x component of velocity -y component of velocity = complex velocity potential = complex variable = circulation = circulation per unit length = angle measured from ( - c,0) = dissipation parameter = kinematic viscosity of fluid = density of fluid = vorticity

Experiments on an aerofoil at high angle of incidence in longitudinal oscillations

Abstract: Details of flow visualization, aerodynamic forces and pitching moment, static pressure and skin friction measurements have been carried out on a symmetrical aerofoil at fixed angle of incidence in longitudinal oscillations parallel to the uniform airstream of a wind-tunnel.This investigation shows weak unsteady effects at incidences below that of static stall. For higher incidences, strong unsteady effects appear and depend on the frequency and amplitude of the oscillations. The measurements indicate an overshoot of the instantaneous lift and drag which is explained by a strong vortex shedding process occurring during the dynamic stall encountered by the aerofoil in decelerated motion, as observed for profiles oscillating in pitch through stall. When the aerofoil is going forward in accelerated motion dynamic reattachment may be observed at very high incidence over a short part of the period of oscillation.Dynamic stall and dynamic reattachment contribute to a favourable effect of unsteadiness on the mean lift coefficient, which increases as compared to the steady state one, and which is expressed through an empirical formula involving incidence, frequency and amplitude of oscillations. At given incidence, optimization of this feature is achieved by matching the frequency and the amplitude of oscillation, respectively with the frequency linked with the highest peak of energy in the wake, and with the distance between two consecutive vortices in the mean wake when modelled as a von Karman's vortex street.

Vortex Shedding from Smooth and Roughened Cylinders in Cross-Flow near a Plane Surface

Abstract: The characteristics of the flow field around a circular cylinder in cross-flow placed at various distances from a plane, parallel both to the flow and to the cylinder axis, were analysed using a hot wire anemometer. Experiments were performed in a wind tunnel with Reynolds numbers ranging from 0.85×105 to 3×105. The spectra of the hot wire signals were obtained using a Fast Fourier Transform technique programmed on a PDP 11/40 computer. As regards a smooth cylinder, the main features of the vortex shedding mechanism in the subcritical regime remained unaltered for distances from the plane greater than approximately 0.4 diameters; in particular the Strouhal frequency did not show any significant variation relative to the typical value for an isolated cylinder. As for lower values of the distance from the plane, the regular vortex shedding disappeared and the hot wire spectra showed typical turbulent features. The possibility of obtaining supercritical conditions by roughening the cylinder surface was confirmed together with the importance of the Reynolds number based on the typical roughness size, Rk, in the evaluation of the flow regime around the cylinder. In the case of roughened cylinders, and with values of Rk below-350, the regular vortex shedding disappeared at a distance from the plane smaller than 0.3 diameters. This fact suggests that, at least in part of the supercritical regime, the influence of the plane can be smaller than in the subcritical regime.

Mutual slip‐through of a pair of vortex rings

Abstract: Mutual slip‐through of two identical vortex rings, traveling along a common axis in the same direction, was experimentally achieved in air at various Reynolds numbers between 800 and 2000 and observed by using the smoke wire technique and cigarette smoke. The motions of a pair of vortices agree very well with an approximate potential theory based on the Biot–Savart law.

Heat transfer in transient and unsteady flows past a heated circular cylinder in the range 1 [double less-than sign] R [double less-than sign] 40

Abstract: The response of a heated circular cylinder to impulsive and sinusoidal variations in the velocity of flow past it has been simulated by numerical integration of the governing equations. The fluid has been treated as viscous and incompressible and as having constant properties. The range of Reynolds number investigated was 1 ≪ R ≪ 40. Since vortex shedding normally does not occur in this range, the flows were treated as symmetrical. The thermal and flow transients are presented for the following cases.: (i)impulsive starts from rest to final steady state Reynolds numbers 1, 5, 10, 26·67; (ii)impulsive increases in velocities of 50% magnitude from steady state Reynolds numbers 1, 10 and 26·67; (iii)sinusoidal variation in velocity with amplitude of 10% impressed on a mean flow at Reynolds number 10. Results are also given for the thermal transients associated with instantaneous changes in cylinder temperature at Reynolds numbers 1, 5 and 40. The results obtained for transient and steady state flow parameters are in agreement with those obtained numerically and experimentally by other workers and the results for steady state heat flux from the cylinder are in agreement with experimental values. The new results obtained for heat transfer in unsteady flows provides information which is relevant to the operation of hot-wire anemometers.

The vortex-street structure of turbulent jets. Part 2

Abstract: This paper is an extension of the work reported earlier as part 1 in a companion paper. It endeavours first to discover how the vortex-street structure in the round jet develops during its passage downstream, and secondly to resolve some of the questions concerning the flow structure in the outer part of the mixing region. It appears that the main vortex street that was reported in part 1 has a trajectory that converges on the jet axis at about two potential core lengths from the nozzle. This brings the vortex cores towards the axis and contributes to the erosion of the potential core. A branch vortex street apparently forms on the outer part of the jet mixing region downstream of x/D = 1.5, and as the main vortex street approaches the axis, the branch vortex street moves outwards. The vortices in the branch are jostled occasionally by those in the main vortex street, and various effects are caused, including the frequency modulation of the signals in the outer part of the mixing region. Measurements at higher Mach numbers confirm the appropriateness of Strouhal number scaling of the jet data for the full range of subsonic Mach numbers and suggest that the structure described in this paper is valid up to the sonic speed.

Hot-Wire, Laser-Anemometer, and Force Measurements of Interacting Trailing Vortices

Abstract: Single and multiple trailing vortices shed from semispan wings and a transport model in a wind tunnel were studied by means of a laser velocimeter, hot-wire anemometer, and trailing model incorporating a sixcomponent force balance. Velocity profile and turbulence data from the laser velocimeter and hot-wire anemometer are presented and shown to compare well with the Betz inviscid circulation model. Lift and rolling moment measurements on the following model are compared with those predicted from the flow field measurements.

Fluid flow measuring apparatus

Abstract: A Karman vortex type flow rate measuring apparatus is provided with a plurality of vortex shedding members each having a different effective width, and a plurality of vortex detecting elements The distance between the vortex detecting elements is so selected that it coincides with the distance between the vortices of some one of the vortex trains induced by respective vortex shedding members The apparatus enables a positive and accurate detection of the vortices, and a compensation for drop out of the vortex shedding

Criterion for Vortex Periodicity in Cylinder Wakes

Abstract: The Strouhal number for the vortex shedding from a circular cylinder mounted normal to the freestream is shown in Fig. 1 as a function of Reynolds number. The experimental results1"9 define a unique Strouhal number (within the data scatter) only for Re 3.5xl06. Until Roshko showed otherwise,1 it was generally believed that the periodic von Karman-type vortex street could not exist at supercritical Reynolds numbers. Roshko showed that the harmonic vortex shedding reappeared at /te>3.5x!0 6, with a shorter wavelength, higher frequency. These results were confirmed later by Jones et al.,2 who also found that the harmonic vortex shedding started at Re>3.5xlQ6. The various nonunique harmonic results in the transcritical region

A discrete-vortex analysis of flow about stationary and transversely oscillating circular cylinders

Abstract: : A comprehensive numerical model has been developed to investigate the characteristics of flow about a circular cylinder undergoing synchronized transverse oscillations. The model is based on the rediscretization of the shear layers, wake-boundary-layer interaction, and the dissipation of vorticity. The forces acting on the cylinder, rate of vorticity flux, Strouhal number, cylinder response, oscillations of the stagnation and separation points, longitudinal and transverse spacing of the vortices, and the base pressure have been calculated and shown to be in conformity with those obtained experimentally. The model has been used to predict the characteristics of hydroelastic oscillations of a cylinder in the range of synchronization. The numerical experiments shed considerable light on the interaction between the fluid motion in the wake and the dynamics of the body. An extensive sensitivity analysis has been carried out to determine the stability of all the parameters and hence the stability of the numerical model itself.

Karman vortex flow meter

Abstract: PURPOSE:To improve instrumentation precision by increasing the resolution of a Karman vortex street by controlling the generation state of a vortex by arranging not less than two Karman vortex generating element plates at a desired interval behind a Karman vortex generating element rod CONSTITUTION:Inside flow-meter main body 2 equipped in duct 1, triangular vortex generating element rod 3 is provided and not less than two plate-shaped vortex generating element boards 5 of the same width as rod 3 are independently arranged at a fixed interval behind it This constitution generates an excellent Karman vortex under broad measurement conditions where the flow rate and flow velocity of a fluid are changeable As a result, the resolution is improved and the shape of vortex generating element rod 3 and the number and arrangement state of vortex generating element plates 5 are adequately set within the range of more than two in number so as to realize accurate instrumentation; and the structure is simple and various fluids can be measured Further, a closed pipe and open channel can be used for the duct

Stabilized vortex-shedding flowmeter

Abstract: A stabilized vortex-shedding flowmeter for accurately measuring the flow rate of a liquid or gas conducted through a flow tube. The meter includes a front obstacle transversely mounted across the tube with its longitudinal axis normal to the flow axis of the tube. Supported behind the front obstacle and spaced therefrom by a gap is a rear obstacle constituted by a pair of parallel beams symmetrically disposed with respect to the flow axis and lying in a plane normal thereto. In operation, as the incoming fluid stream is divided by and flows past the front obstacle, a stagnant zone is created in the gap. This zone is initially aligned with the flow axis; but as vortices are successively detached from the front obstacle and appear alternately on either side of the gap, the low pressure produced by each vortex act to draw the stagnant zone in front of the beam adjacent thereto, the fluid then going around and past the other beam and imposing a drag force thereon. Since the vortices alternate, the drag forces imposed on the beams alternate at a rate proportional to the flow rate of the fluid. A transducer system is provided to sense the alternating activity in the tube and to generate a signal whose frequency is linearly related to the flow rate.

Measurement of Vortex Frequencies in a Lean, Premixed Prevaporized Combustor†

Abstract: A modified laser schlieren and fast Fourier transform analysis system has been developed and applied to the study of vortex frequencies in a lean, premixed prevaporized combustor. The system is sensitive to the passage of vortex structures which dominate the flow in the combustor studied. The frequency spectra of the passage of vortices show distinct frequency peaks which are postulated to be characteristic of the processes triggering the vortex shedding. The method provides additional information which should aid in the characterization and understanding of shear flows.

Torque-transducer for vortex-shedding flowmeter having torsional sensor

Abstract: A 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. Torsionally-supported at a downstream position behind the shedder on a shaft which projects outside of the flow tube to provide a shaft extension is a sensor, the pivot axis of the shaft being normal to the longitudinal axis of the tube. In operation, as the incoming fluid stream is divided by and flows past the shedder, vortices are successively detached therefrom and appear alternately on either side of the sensor, thereby developing a torque about the pivot axis. The torques are developed alternately in the clockwise and counterclockwise directions, causing the torsionally-supported sensor to oscillate at a rate proportional to the flow rate of the fluid. These oscillations are converted by a transducer operatively coupled to the sensor into a corresponding electrical signal. The transducer is constituted by two pairs of piezoelectric elements which engage the shaft extension on opposite sides thereof and are preloaded.

Chicago monostatic acoustic vortex sensing system. volume i. data collection and reduction

Abstract: : A Monostatic Acoustic Vortex Sensing System (MAVSS) was installed at Chicago's O'Hare International Airport to measure the strength and decay of aircraft wake vortices from landing aircraft. The MAVSS consists of an array of acoustic antennas which measure the vertical profile up to 60 m altitude of the vertical component of the wind. The decay in wake vortex strength is measured as the vortex passes over successive antennas in the array. Volume I describes the MAVSS principles of operation, the hardware developed, and data reduction methods employed.

Simplified Unsteady Aerodynamic Concepts with Application to Parameter Estimation

Abstract: A simplified aerodynamic force model based on the physical principle of Prandtl's lifting line theory and trailing vortex concept has been developed to account for unsteadiness in the aircraft dynamics. The wake is assumed to be compressed to a single shed vortex element of appropriate strength moving downstream at a speed sufficient to approximate the Wagner function. Results are presented illustrating the ability of the simplified theory to duplicate exact solutions in unsteady aerodynamics. Further, consideration is given to the utility of the model in a parameter identification application.

Formulation of the Three Dimensional Transonic Unsteady Aerodynamic Problem.

Abstract: : Unsteady transonic flow for a swept wing of moderate sweep is formulated in the small disturbance limit. Boundary conditions at the wing, the trailing vortex sheet, and the outer computatinal boundaries are given including the ventilated wall conditions. Shortcomings of the small disturbance hypothesis is reviewed, suggesting means to compensate for them. Viscous interactions are described together with procedures to incorporate their effects. (Author)

Vortex-shedding flowmeter having drag-actuated torsional sensor

Abstract: A 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. 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 the fulcrum axis of the sensor, this axis being normal to the longitudinal axis of the tube. In operation, as the incoming fluid stream is divided by and flows past the shedding body, it creates a stagnant zone in the gap that is initially aligned with the tube axis. But as vortices are successively detached from the body and appear alternately on either side of the gap, the low pressure region produced by each vortex acts to draw the stagnant zone in front of the adjacent leg of the sensor, the fluid flow then going around and past the other leg, thereby developing a torque about the fulcrum axis. The 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 by a transducer coupled to the sensor into a corresponding electrical signal.

Feasibility study of a reverse flow sensing probe

Abstract: Describes a reverse flow sensing hot-wire probe based on the principle of detection of the Karman vortex street in the near wake of a circular cylinder Tests in a laminar separated shear layer point out the limitations of its use in practical situations due to the dependence of vortex shedding on Reynolds number