Showing papers on "Vortex shedding published in 1999"

Flow past a sphere up to a Reynolds number of 300

Abstract: The flow of an incompressible viscous fluid past a sphere is investigated numerically and experimentally over flow regimes including steady and unsteady laminar flow at Reynolds numbers of up to 300. Flow-visualization experiments are used to validate the numerical results and to provide additional insight into the behaviour of the flow. Near-wake visualizations are presented for both steady and unsteady flows. Calculations for Reynolds numbers of up to 200 show steady axisymmetric flow and compare well with previous experimental and numerical observations. For Reynolds numbers of 210 to 270, a steady non-axisymmetric regime is found, also in agreement with previous work. To advance the basic understanding of this transition, a symmetry breaking mechanism is proposed based on a detailed analysis of the calculated flow field.Unsteady flow is calculated at Reynolds numbers greater than 270. The results at a Reynolds number of 300 show a highly organized periodic flow dominated by vortex shedding. An analysis of the calculated vortical structure of the wake reveals a sequence of shed hairpin vortices in combination with a sequence of previously unidentified induced hairpin vortices. The numerical results compare favourably with experimental flow visualizations which, interestingly, fail to reveal the induced vortices. Based on the deduced symmetry-breaking mechanism, an analysis of the unsteady kinematics, and the experimental results, a mechanism driving the transition to unsteady flow is proposed.

A study of two-dimensional flow past an oscillating cylinder

Abstract: In this paper we describe a detailed study of the wake structures and flow dynamics associated with simulated two-dimensional flows past a circular cylinder that is either stationary or in simple harmonic cross-flow oscillation. Results are examined for Re = 500 and a fixed motion amplitude of y(max)/D = 0.25. The study concentrates on a domain of oscillation frequencies near the natural shedding frequency of the fixed cylinder. In addition to the change in phase of vortex shedding with respect to cylinder motion observed in previous experimental studies, we note a central band of frequencies for which the wake exhibits long-time-scale relaxation oscillator behaviour. Time-periodic states with asymmetric wake structures and non-zero mean lift were also observed for oscillation frequencies near the lower edge of the relaxation oscillator band. In this regime we compute a number of bifurcations between different wake configurations and show that the flow state is not a unique function of the oscillation frequency. Results are interpreted using an analysis of vorticity generation and transport in the base region of the cylinder. We suggest that the dynamics of the change in phase of shedding arise from a competition between two different mechanisms of vorticity production.

Jet characteristics of a plunging airfoil

Abstract: Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowered downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3

Laminar flow past a rotating circular cylinder

Abstract: The present study numerically investigates two-dimensional laminar flow past a circular cylinder rotating with a constant angular velocity, for the purpose of controlling vortex shedding and understanding the underlying flow mechanism. Numerical simulations are performed for flows with Re=60, 100, and 160 in the range of 0⩽α⩽2.5, where α is the circumferential speed at the cylinder surface normalized by the free-stream velocity. Results show that the rotation of a cylinder can suppress vortex shedding effectively. Vortex shedding exists at low rotational speeds and completely disappears at α>αL, where αL is the critical rotational speed which shows a logarithmic dependence on Re. The Strouhal number remains nearly constant regardless of α while vortex shedding exists. With increasing α, the mean lift increases linearly and the mean drag decreases, which differ significantly from those predicted by the potential flow theory. On the other hand, the amplitude of lift fluctuation stays nearly constant with in...

Helical vortices in swirl flow

Abstract: Helical vortices in swirl flow are studied theoretically and experimentally.A theory of helical vortices has been developed. It includes the following results: an analytical solution describing an elementary helical vortex structure – an infinitely thin filament; a solution for axisymmetrical vortices accounting for the helical shape of vortex lines and different laws of vorticity distribution; a formula for calculation of the self-induced velocity of helical vortex rotation (precession) in a cylindrical tube; an explanation of the zone with reverse flow (recirculation zone) arising in swirl flows; and the classification of vortex structures.The experimental study of helical vortices was carried out in a vertical hydrodynamical vortex chamber with a tangential supply of liquid through turning nozzles. Various vortex structures were formed owing to changing boundary conditions on the bottom and at the exit section of the chamber. The hypothesis of helical symmetry is confirmed for various types of swirl flow. The stationary helical vortex structures are described (most of them for the first time) the features of which agree with the results and predictions of the theoretical model developed. They are the following: a rectilinear vortex; a composite columnar vortex; helical vortices screwed on the right or on the left; a vortex with changing helical symmetry; a double helix – two entangled vortex filaments of the same sign.

Numerical Modeling of Flow and Scour at Pipelines

Abstract: A numerical model for the description of fluid flow and suspended and bed-load sediment transport is presented. Density effects are included in the momentum (Reynolds) equations and in the turbulence (kappa and epsilon) equations. Changes in bed levels are calculated from sediment continuity, and the finite-element grid is adapted to the geometry. The Reynolds equations and the transport equation for suspended sediment are solved numerically using a Taylor-Galerkin finite-element method. The flow at a surface mounted cylinder in a steady flow is predicted in good agreement with experiments. Periodic vortex shedding from a cylinder placed above a rigid bed is predicted in good agreement with laboratory experiments, provided that sufficiently detailed grids (approximately 5,000 nodes) are used. Scour calculations are performed for a cylinder in a steady flow with its underside placed at the level of the original flat bed. Predicted scour at a pipeline in steady flow is in good agreement with laboratory measurements reported in the literature.

Dynamics and flow structures in the turbulent wake of rigid and flexible cylinders subject to vortex-induced vibrations

Abstract: We present simulation results of vortex-induced vibrations of an infinitely long flexible cylinder at Reynolds number Re = 1000, corresponding to a ‘young’ turbulent wake (i.e. exhibiting a small inertial subrange). The simulations are based on a new class of spectral methods suitable for unstructured and hybrid grids. To obtain different responses of the coupled flow–structure system we vary the structure's bending stiffness to model the behaviour of a vibrating inflexible (rigid) cylinder, a cable, and a beam. We have found that unlike the laminar flow previously studied, the amplitude of the cross-flow oscillation is about one diameter for the cable and the beam, close to experimental measurements, but is lower for the rigid cylinder. We have also found that for the latter case the flow response corresponds to parallel shedding, but for the beam and cable with free endpoints a mixed response consisting of oblique and parallel shedding is obtained, caused by the modulated travelling wave motion of the structure. This mixed shedding pattern which alternates periodically along the span can be directly related to periodic spatial variation of the lift force. In the case of structures with pinned endpoints a standing wave response is obtained for the cylinder; lace-like flow structures are observed similar to the ones seen in the laminar regime. Examination of the frequency spectra in the near wake shows that at Re = 1000 all cases follow a −5/3 law in the inertial range, which extends about half a decade in wavenumber. However, these spectra are different in all three cases both in low and high frequencies, with the exception of the beam and cable, for which the high-frequency portion is identical despite the differences in the displacement time history and the large-scale features of the corresponding flow.

Computational Aeroacoustic Analysis of Slat Trailing-Edge Flow

Abstract: An acoustic analysis based on the Ffowcs Williams and Hawkings equation was performed for a high-lift system. As input, the acoustic analysis used unsteady flow data obtained from a highly resolved, time-dependent, Reynolds-averaged Navier-Stokes caclulation. The analysis strongly suggests that vortex shedding from the trailing edge of the slat results in a high-amplitude, high-frequency acoustic signal, similar to that which was observed in a corresponding experimental study of the high-lift system.

Flow control of vortex shedding by a short splitter plate asymmetrically arranged downstream of a cylinder

Abstract: The flow around a circular cylinder (diameter d) was investigated, behind which a short thin splitter plate with a chord of 1.0d was inserted horizontally as an interference element. The plate was traversed upstream along the wake; hence its relative position is defined by gap G from the cylinder base to the plate tip and by level Z from the wake centerline. The variation in both base suction coefficient and Strouhal number with G/d significantly depends on Z/d. In the circular cylinder case with Z/d=0.5–1.3, as the plate approaches the cylinder, the base suction coefficient exhibits a critical fall in a similar fashion to Roshko’s experiment where Z/d=0. Interestingly, however, unlike his experiment, the Strouhal number exceeds the natural one for some range of G/d beyond the critical gap. To further examine the flow mechanism, a similar situation was investigated by using a rectangular cylinder (height h, depth 0.1h) in place of the circular cylinder. A rise in the Strouhal number is observed for Z/h=1....

Suboptimal feedback control of vortex shedding at low Reynolds numbers

Abstract: The objective of this study is to develop a method of controlling vortex shedding behind a bluff body using control theory. A suboptimal feedback control procedure for local sensing and local actuation is developed and applied to the flow behind a circular cylinder. The location of sensors for feedback is limited to the cylinder surface and the control input from actuators is the blowing and suction on the cylinder surface. Three different cost functionals to be minimized (J1 and J2) or maximized (J3) are investigated: J1 is proportional to the pressure drag of the cylinder, J2 is the square of the difference between the target pressure (inviscid flow pressure) and real flow pressure on the cylinder surface, and J3 is the square of the pressure gradient on the cylinder surface, respectively. Given the cost functionals, the flow variable to be measured by the sensors and the control input from the actuators are determined from the suboptimal feedback control procedure. Several cases for each cost functional have been numerically simulated at Re = 100 and 160 to investigate the performance of the control algorithm. For all actuations, vortex shedding becomes weak or disappears, and the mean drag and drag/lift fluctuations significantly decrease. For a given magnitude of the blowing/suction, reducing J2 provides the largest drag reduction among the three cost functionals.

Finite element study of vortex-induced cross-flow and in-line oscillations of a circular cylinder at low Reynolds numbers

Abstract: Vortex-induced vibrations of a circular cylinder placed in a uniform flow at Reynolds number 325 are investigated using a stabilized space–time finite element formulation. The Navier–Stokes equations for incompressible fluid flow are solved for a two-dimensional case along with the equations of motion of the cylinder that is mounted on lightly damped spring supports. The cylinder is allowed to vibrate, both in the in-line and in the cross-flow directions. Results of the computations are presented for various values of the structural frequency of the oscillator, including those that are sub and superharmonics of the vortex-shedding frequency for a stationary cylinder. In most of the cases, the trajectory of the cylinder corresponds to a Lissajou figure of 8. Lock-in is observed for a range of values of the structural frequency. Over a certain range of structural frequency (Fs), the vortex-shedding frequency of the oscillating cylinder does not match Fs exactly; there is a slight detuning. This phenomenon is referred to as soft-lock-in. Computations show that this detuning disappears when the mass of the cylinder is significantly larger than the mass of the surrounding fluid it displaces. A self-limiting nature of the oscillator with respect to cross-flow vibration amplitude is observed. It is believed that the detuning of the vortex-shedding frequency from the structural frequency is a mechanism of the oscillator to self-limit its vibration amplitude. The dependence of the unsteady solution on the spatial resolution of the finite element mesh is also investigated. Copyright © 1999 John Wiley & Sons, Ltd.

Vortex shedding in steady flow through a model of an arterial stenosis and its relevance to mural platelet deposition.

Abstract: In this study, the development of unsteady vortical formations in the separated flow region distal to a stenosis throat is presented and compared with the platelet deposition measurements, to enhance our understanding of the mecha- nisms involved in platelet kinetics in flowing blood Qualitative and quantitative flow visualization and numerical simulations were performed in a model of a streamlined axisymmetric stenosis with an area reduction of 84% at the throat of the stenosis Measurements were performed at Reynolds numbers ~Re!, based on upstream diameter and average velocity, ranging from 300 to 1800 Both the digital particle image visualization method employed and the numerical simulations were able to capture the motion of the vortices through the separated flow region Periodic shedding of vortices began at approximately Re5375 and continued for the full range of Re studied The locales at which these vortices are initiated, their size, and their life span, were a function of Re The numerical simulations of turbulent flow through the stenosis model entailed a detailed depiction of the process of vortex shedding in the separated flow region downstream of the stenosis These flow patterns were used to elucidate the mechanisms involved in blood plate- let kinetics and deposition in the area in and around an arterial stenosis The unsteady flow development in the recirculation region is hypothesized as the mechanism for observed changes in the distribution of mural platelet deposition between Re 5300, 900, and 1800, despite only a marginal variation in the size and shape of the recirculation zone under these flow con- ditions © 1999 Biomedical Engineering Society @S0090-6964~99!00306-9#

Optimal control of vortex shedding using low-order models. part ii-model-based control

Abstract: In Part I, an approach to deriving low order models suitable for use in the development of active control strategies for separated ows was presented. The methodology proposed was applied to a numerical simulation of the incompressible, unsteady wake ow behind a circular cylinder at Re = 100, with control action achieved via cylinder rotation. The resulting low order models were found to predict the ows in four open-loop test cases with suucient delity to justify their application in model-based control of the vortex-shedding ow. In Part II, optimal control theory is used to implement the model-based control, and data from numerical simulations of the controlled ow are presented. It is found that the level of wake unsteadiness can be reduced, even when the low order model is reset on the basis of limited ow eld information. The degree of reduction is dependent on the accuracy of the low order model, and ways of reening it in the light of control simulations are considered. However, results from two straightforward approaches to this problem suggest that it is easy to \\over-tune\" the model, resulting in less successful control.

Numerical investigation of the flow past a cavity

Abstract: Numerical simulations are used to investigate the resonant instabilities in the flow past an open cavity. The compressible Navier-Stokes equations are solved directly (no turbulence model) for two-dimensional cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field. The results show a transition from a shear layer mode, for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear layer mode is well characterized by Rossiter modes. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of the Mach number. The vortex shedding causes the boundary layer to periodically separate upstream of the cavity. The wake mode oscillation is similar to that reported by Gharib and Roshko (J. Fluid Mech.,177, 1987) for incompressible flow with a laminar upstream boundary layer. The results suggest that laminar separation upstream of the cavity edge is the cause of the transition to wake mode.

On the relationship of Effective Reynolds number and Strouhal number for the laminar vortex shedding of a heated circular cylinder

Abstract: The laminar vortex shedding of airflow behind a circular cylinder with different heating temperatures was experimentally investigated with emphasis on the relationship of wake frequency and the Reynolds number. A new method to generate the two-dimensional parallel vortex shedding for the heated cylinder was developed and tested. An “effective Reynolds number” that employs a kinematic viscosity computed from an “effective temperature” is used to account for the temperature effects on the vortex shedding frequency. The present result shows that the frequency data could be successfully collapsed with the effective temperature computed by Teff=T∞+0.28(TW−T∞) for a wide range of cylinder temperatures, T∞ and TW being the free-stream temperature and cylinder surface temperature, respectively. Moreover, the relationship between Strouhal number and effective Reynolds number was found to be “universal.” The physical interpretation of Teff and the applicable region of the St–Reeff curve are discussed.

Method for Accelerating the Destruction of Aircraft Wake Vortices

Abstract: The wake vortices shed by large aircraft and their associated hazard to following aircraft remain an important issue in commercial aviation. Extensive research into methods for alleviating this wake vortex hazard has been continuing for many years. This paper presents results from numerical simulations detailing a potentially useful mechanism for accelerating the destruction of the aircraft vortex wake and reducing the wake vortex hazard. The term destruction is used to refer to the progressive annihilation of the wake caused by mixing of vorticity of opposite sign and the associated elimination of large coherent structures. The emphasis here is on a description of the mechanism and its connection to previous work.

A prediction model for separated-flow transition

Abstract: The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers that are laminar at separation and those that are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation/short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation/long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms ofmore » distance Reynolds number Re{sub x} can be correlated better than momentum or displacement thickness Reynolds number. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.« less

Energy Balanced Double Oscillator Model for Vortex-Induced Vibrations

Abstract: Vortex-induced oscillations may play an important role in the design of slender structures and piping systems exposed to fluid flow An improved theoretical basis is developed for representation of vortex-induced oscillations of structural elements by models consisting of two coupled oscillators The mutual forcing terms are based on exact transfer of energy from the fluid to the structural oscillator This leads to a system with distinct frequency locking and resonance when fluid and structural natural frequencies are similar This work implements a simple quadratic fluid damping, but more general fluid damping can be introduced without changing the basic features of the model The response curve for stationary oscillations consists of two distinct branches, separated by jumps in frequency, amplitude, and phase closely resembling experimentally observed free vortex-induced vibrations A stability analysis shows that the final part of each branch is unstable, and a transient analysis illustrates the transition between the two modes of oscillation for slowly increasing or decreasing fluid velocity through the lock-in region Model calibration is discussed and parameters are identified representing available experimental data for lock-in frequencies and resonance amplitudes for systems with relative structural damping in the interval 00015-0003 and different values of the mass ratio

Three-dimensional linear stability analysis of incompressible viscous flows using the finite element method

Abstract: The linear stability of incompressible flows is investigated on the basis of the finite element method. The two-dimensional base flows computed numerically over a range of Reynolds numbers are perturbed with three-dimensional disturbances. The three-dimensionality in the flow associated with the secondary instability is identified precisely. First, by using linear stability theory and normal mode analysis, the partial differential equations governing the evolution of perturbation are derived from the linearized Navier–Stokes equation with slight compressibility. In terms of the mixed finite element discretization, in which six-node quadratic Lagrange triangular elements with quadratic interpolation for velocities (P2) and three-node linear Lagrange triangular elements for pressure (P1) are employed, a non-singular generalized eigenproblem is formulated from these equations, whose solution gives the dispersion relation between complex growth rate and wave number. Then, the stabilities of two cases, i.e. the lid-driven cavity flow and flow past a circular cylinder, are examined. These studies determine accurately stability curves to identify the critical Reynolds number and the critical wavelength of the neutral mode by means of the Krylov subspace method and discuss the mechanism of instability. For the cavity flow, the estimated critical results are Rec=920.277±0.010 for the Reynolds number and kc=7.40±0.02 for the wave number. These results are in good agreement with the observation of Aidun et al. and are more accurate than those by the finite difference method. This instability in the cavity is associated with absolute instability [Huerre and Monkewitz, Annu. Rev. Fluid Mech., 22, 473–537 (1990)]. The Taylor–Goertler-like vortices in the cavity are verified by means of the reconstruction of three-dimensional flows. As for the flow past a circular cylinder, the primary instability result shows that the flow has only two-dimensional characteristics at the onset of the von Karman vortex street, when Re<49. The estimated critical values of primary instability are Rec=46.389±0.010 and Stc=0.126 for the Strouhal number. These values are very close to the observation data [Williamson, J. Fluid Mech., 206, 579–627 (1989)] and other stability results [Morzynski and Thiele, Z. Agnew. Math. Mech., 71, T424–T428 (1991); Jackson, J. Fluid Mech., 182, 23–45 (1987)]. This onset of vortex shedding is associated with the symmetry-breaking bifurcation at the Hopf point. Copyright © 1999 John Wiley & Sons, Ltd.