Efstathios Konstantinidis

Bimodal vortex shedding in a perturbed cylinder wake

Cylinder wakes display distinct modes of vortex shedding when perturbed by appropriate means. By investigating experimentally the wake of a circular cylinder perturbed by a periodic fluctuation imposed on the inflow velocity, it is shown that bimodal behavior is possible. During a given experiment, the wake switches back and forth between two different vortex shedding modes, more specifically, a 2S↔2P transition is observed. No discernible change in the timing of vortex formation is found to accompany the transition. Modal decomposition of the velocity field is employed to exemplify the interaction of the imposed symmetrical perturbation and the intrinsic antisymmetrical instability of the near wake.

The timing of vortex shedding in a cylinder wake imposed by periodic inflow perturbations

The interaction of vortex shedding from a circular cylinder with an inflow which has low-amplitude periodic velocity oscillations (perturbations) superimposed upon it, was investigated experimentally by means of particle image velocimetry. The experiments were made at three perturbation frequencies across the lock-on range in which the vortex shedding frequency is synchronized with the subharmonic of the imposed frequency. The basic wake pattern in this range is antisymmetric vortex shedding, i.e. the familiar 2S mode. The timing of vortex shedding is defined with respect to the cross-flow oscillation of the wake which is found to play a critical role. Quantitative analysis of the phase-referenced patterns of vorticity distribution in the wake shows that a vortex is actually shed from the cylinder when the cross-flow oscillation of the wake is strongest, marked by a sudden drop in the computed vortex strength. At the middle of the lock-on range, shedding occurs near the minimum inflow velocity in the cycle or, equivalently, during the forward stroke of a cylinder oscillating in-line with the flow. It is argued that the imposed timing of vortex shedding relative to the cylinder motion induces a negative excitation from the fluid, which might explain why the in-line response of a freely vibrating cylinder exhibits two positive excitation regions separated by the lock-on region found in forced oscillations.

Dynamic response of a turbulent cylinder wake to sinusoidal inflow perturbations across the vortex lock-on range

Large-eddy simulations are employed to investigate the dynamic response of the turbulent wake of a circular cylinder to sinusoidal perturbations in the inflow velocity superposed on a mean component. The perturbation frequency is varied across the vortex lock-on range at a constant amplitude of 5% of the mean velocity corresponding to a Reynolds number of 2580. The effect on the instantaneous, time-averaged and phase-averaged characteristics of the near-wake flow and fluid forces on the cylinder is reported. Comparisons of the present simulations to experimental realizations show that the physics of the unsteady three-dimensional separated flow are well reproduced. The simulations capture the modification of the wake structure including the shrinking of the recirculation bubble and vortex-formation region and the enhancement of the wake fluctuations and vortex strength in the lock-on regime. These wake effects are accompanied by an increase in the steady and unsteady drag and the unsteady lift acting on t...

On the flow and vortex shedding characteristics of an in-line tube bundle in steady and pulsating crossflow

Laser Doppler anemometry and flow visualization were employed to provide a detailed description of the flowfield around an in-line tube bundle in steady and pulsating crossflow of water. The steady flow results indicate that the amplitude of velocity fluctuations associated with vortex shedding and the turbulence levels in the bundle increase with increasing Re . A single mode of alternate vortex shedding with a constant St of 0.14, was found in all rows but the first one in steady flow. Pulsating flow incited vortex shedding from the first row and lock-on. As a result, vortex shedding was more pronounced and turbulence levels increased considerably downstream. It is envisaged that heat transfer in the tube bundle might be augmented by pulsating flow.

A Study of Vortex Shedding in a Staggered Tube Array for Steady and Pulsating Cross-Flow

This paper describes an experimental investigation of the vortex shedding phenomena in a staggered tube array with streamwise and transverse spacing to diameter ratios of 2.1 and 3.6, respectively. LDA measurements were employed to monitor the flow fluctuations and a visualization technique was implemented to reveal the underlying flow patterns in the array for steady and pulsating cross-flow, The results obtained in steady flow are in general agreement with results from previous investigations and show that vortex shedding occurs at two distinct frequencies in the front and inner rows. A lower frequency component was detected at the exit of the array, which has not been previously identified. Pulsating flow caused the frequency of vortex shedding to lock-on at the subharmonic of the imposed frequency. In the lock-on range, vortex shedding from all the tubes was synchronized and in-phase and velocity fluctuations at the shedding frequency increased considerably compared to their counterparts in steady flow.

Relationship between vortex shedding lock-on and heat transfer - Implications for tube bundles in cross-flow

The purpose of this shorter communication is twofold: (a) to establish a relationship between vortex shedding lock-on and heat transfer enhancement from a tube in cross-flow in the light of new findings in the published literature; and (b) to report additional results on the lock-on behaviour of an in-line tube bundle in pulsating cross-flow not previously published. The discussion of the results demonstrates the potential use of flow pulsations as a means of heat transfer enhancement in shell-and-tube heat exchangers together with useful guidelines for further research. Given the large proportion of energy use of heat exchanger equipment in the process and power generation industries, even small benefits in the efficiency of such equipment can lead to significant energy savings.

Added mass of a circular cylinder oscillating in a free stream

The fundamental understanding of the added mass phenomenon associated with the motion of a solid body relative to a fluid is revisited. This paper focuses on the two-dimensional flow around a circular cylinder oscillating transversely in a free stream. A virtual experiment reveals that the classical approach to this problem leads to a paradox. The inertial force is derived afresh based on analysis of the motion in a frame of reference attached to the cylinder centroid, which overcomes the paradox in the classical formulation. It is shown that the inertial force depends not only on the acceleration of the cylinder per se, but also on the relative motion between body and fluid embodied in a parameter called alpha, α, which represents the ratio of the maximum transverse velocity of the cylinder to the free-stream velocity; the induced inertial force is directionally varying and non-harmonic in time depended on the alpha parameter. It is further shown that the component of the inertial force in the transverse direction is negligible for α<0.1, increases quadratically for α<0.5, and tends asymptotically to the classical result as , i.e. in still fluid.

An experimental study of steady and pulsating cross-flow over a semi-staggered tube bundle

Abstract This paper describes an experimental study of the cross-flow characteristics in a semi-staggered tube bundle for Reynolds numbers in the range 1100-12 900. It is shown that by displacing transversely the tubes in the even rows of an in-line bundle by one diameter the vortex-shedding mechanism is suppressed. Vortex shedding is re-established and reinforced by pulsations superimposed on to the approaching flow and a considerable increase in the power of the associated velocity fluctuations is observed in the bundle. Two cases of pulsating flow are examined with different effects on the flow structure of the bundle. Detailed measurements of the mean and fluctuating velocity fields in the semi-staggered tube bundle together with flow visualization images are also reported in the paper in order to examine in depth the effects of tube displacement and flow pulsations. Comparisons with in-line and staggered configurations having the same spacing-to-diameter ratios are made.

Comment on “Lock-in in forced vibration of a circular cylinder” [Phys. Fluids 28, 113605 (2016)]

A recent study by Kumar et al. investigated synchronization in uniform flow past a transversely oscillating cylinder. They showed that free vibration response for a system without structural damping is in excellent agreement with the contour of zero energy transfer on the map of normalized amplitude and frequency corresponding to forced sinusoidal oscillation. However, a careful investigation of their data challenges the full analogy between free and forced sinusoidal vibrations. It is further shown here that free response generally is neither sinusoidal nor corresponds to the contour of zero energy transfer even if the structural damping is zero.

Dynamic Response of a Turbulent Cylinder Wake to Forced Excitation

Three-dimensional large-eddy simulations were carried out to determine the dynamic response of a turbulent cylinder wake to forced excitation at a subcritical Reynolds number of 2580. The excitation frequency was varied across the primary lock-on range while the amplitude of sinusoidal velocity perturbation and the mean velocity imposed at the inflow boundary were kept constant. The velocity fluctuations in the wake and the fluctuating forces on the cylinder are analyzed employing spectral, time-domain and phase-portrait methods. The results show that the dynamic response of the inline force is different than that of the transverse one on the border of the lock-on range; while the inline force exhibits a phase-locked response, the transverse force indicates an intermittent response. This behavior is linked to the wake dynamics which is similar to that of the transverse force. This result is explained on the basis of the Morison equation which shows that the inline force is biased by the inertial components associated with the added mass and pressure waves in unsteady flows. It is further shown that the existence of a mean velocity component alters radically the dynamics of the inline force and appropriate ranges of a dimensionless parameter are proposed to describe the response.Copyright