Vortex Shedding Frequency for a 2D Hydrofoil with a Truncated Trailing Edge

Turbulent boundary layers separating from a blunt trailing edge give rise to organized vortical structures in the downstream wake. The perturbation of this inherent flow-instability at f0 by controlled oscillations of the edge at fe produces corresponding, organized components of unsteady surface pressure along the edge. For edge excitation near the ‘natural’ vortex shedding frequency f0, the phase between the local pressure fluctuations and the edge displacement shows large changes for small changes in excitation frequency. Moreover, in this range of excitation, there is quenching (or attenuation) of the surface pressure component at f0 and resonant peaking of the component at fe. These phenomena are related to the change in sign of the energy transfer between the fluid and the body. Integration of the instantaneous pressure distributions along the surfaces of the edge leads to the instantaneous lift at fe and f0 acting upon the oscillating trailing edge. The location of the lift varies as the cotangent of the dimensionless time during an oscillation cycle. When the edge is excited near, or at, the natural vortex shedding frequency, there is a resonant peak in the amplitude of oscillation of the lift location at fe; that at f0 is invariant. Moreover, the mean location of the lift at fe undergoes abrupt changes in this region of excitation. Flow visualization allows determination of the phasing of the organized vortical structures shed from the trailing edge relative to the edge displacement. Modulation of the flow structure at the frequencies f0 and fe, as well as interaction of small-scale vortices at high excitation frequencies, was observed. These aspects of the near-wake structure are related to the instantaneous pressure field.

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Cavitation Influence on von Kármán Vortex Shedding and Induced Hydrofoil Vibrations

Journal of Fluids Engineering ◽

10.1115/1.2746907 ◽

2007 ◽

Vol 129 (8) ◽

pp. 966-973 ◽

Cited By ~ 61

Author(s):

Philippe Ausoni ◽

Mohamed Farhat ◽

Xavier Escaler ◽

Eduard Egusquiza ◽

François Avellan

Keyword(s):

Reynolds Number ◽

Vortex Shedding ◽

Trailing Edge ◽

Cavitation Inception ◽

Structure Interaction ◽

New Correlation ◽

Von Karman ◽

Vortex Shedding Frequency ◽

Shedding Frequency ◽

Von Kármán

The present study deals with the shedding process of the von Kármán vortices at the trailing edge of a 2D hydrofoil at high Reynolds number Reh=25×103–65×103. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Kármán vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, for fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation.

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Numerical Simulation on Vortex Shedding from a Hydrofoil in Steady Flow

Journal of Marine Science and Engineering ◽

10.3390/jmse8030195 ◽

2020 ◽

Vol 8 (3) ◽

pp. 195

Author(s):

Jian Hu ◽

Zibin Wang ◽

Wang Zhao ◽

Shili Sun ◽

Cong Sun ◽

...

Keyword(s):

Strouhal Number ◽

Vortex Shedding ◽

Angle Of Attack ◽

Wake Flow ◽

Trailing Edge ◽

Inflow Velocity ◽

Lift Curve ◽

Vortex Shedding Frequency ◽

Shedding Frequency ◽

Lift And Drag

This paper presents a numerical modeling procedure for the idealization of vortex shedding effects in the wake flow field of a NACA0009 hydrofoil. During the simulation, the lift and drag acting on the hydrofoil were monitored, and the vortex-shedding frequency of the hydrofoil was analyzed. The effects of inflow velocity, trailing-edge thickness, angle of attack, and maximum hydrofoil thickness on vortex shedding were investigated. The results indicate that an increase in the inflow velocity led to an increase in the vortex-shedding frequency and a negligible change in the Strouhal number. Furthermore, as the thickness of the trailing edge increased, the vortex-shedding frequency decreased gradually, whereas the Strouhal number first increased and then decreased. Vortex shedding and lift curve oscillations ceased altogether after the angle of attack of the hydrofoil increased beyond a certain threshold. When the maximum hydrofoil thickness was increased while keeping the thickness and chord length of the trailing edge constant, the vortex-shedding frequency decreased.

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Base Pressure on a Blunt Base in Transonic Flow: Some Effects of Base Geometry and Bleed Air

Volume 1: Turbomachinery ◽

10.1115/82-gt-317 ◽

1982 ◽

Author(s):

F. Motallebi ◽

S. J. Edwards ◽

J. F. Norbury

Keyword(s):

Experimental Investigation ◽

Vortex Shedding ◽

Transonic Flow ◽

Base Pressure ◽

Two Dimensional ◽

Trailing Edge ◽

Maximum Value ◽

Narrow Slot ◽

Vortex Shedding Frequency ◽

Shedding Frequency

An experimental investigation has been carried out on an aerofoil-like body having a thick square-cut trailing edge. Measurements of base pressure have been made for a range of mainstream Mach numbers from 0.6 to 1.3. The results also include measurements of vortex shedding frequency and schlieren photographs. Bleed air was discharged through the blunt base using three different configurations: (i) A wide two-dimensional slot; (ii) A narrow two-dimensional slot; (iii) A series of accurately bored discrete holes, equal in total area to the narrow slot. As the rate of discharge of bleed air was increased from zero the base pressure was found to rise to a maximum value before falling again at higher rates of discharge. At zero incidence the three configurations gave similar results but when incidence was applied the results were markedly different for the wide and narrow slots.

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The Influence of Boundary Layer Velocity Gradients and Bed Proximity on Vortex Shedding From Free Spanning Pipelines

Journal of Energy Resources Technology ◽

10.1115/1.3231028 ◽

1984 ◽

Vol 106 (1) ◽

pp. 70-78 ◽

Cited By ~ 78

Author(s):

A. J. Grass ◽

P. W. J. Raven ◽

R. J. Stuart ◽

J. A. Bray

Keyword(s):

Boundary Layer ◽

Strouhal Number ◽

Vortex Shedding ◽

Combined Effects ◽

Maximum Increase ◽

Velocity Gradients ◽

Large Velocity ◽

Layer Velocity ◽

Vortex Shedding Frequency ◽

Shedding Frequency

The paper summarizes the results of a laboratory study of the separate and combined effects of bed proximity and large velocity gradients on the frequency of vortex shedding from pipeline spans immersed in the thick boundary layers of tidal currents. This investigation forms part of a wider project concerned with the assessment of span stability. The measurements show that in the case of both sheared and uniform approach flows, with and without velocity gradients, respectively, the Strouhal number defining the vortex shedding frequency progressively increases as the gap between the pipe base and the bed is reduced below two pipe diameters. The maximum increase in vortex shedding Strouhal number, recorded close to the bed in an approach flow with large velocity gradients, was of the order of 25 percent.

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An Experimental Study on the Vertical Flow Past a Finite-Length Horizontal Cylinder at Low Reynolds Numbers

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.493.68 ◽

2014 ◽

Vol 493 ◽

pp. 68-73 ◽

Cited By ~ 2

Author(s):

Willy Stevanus ◽

Yi Jiun Peter Lin

Keyword(s):

Finite Length ◽

Vortex Shedding ◽

Vortex Formation ◽

Reynolds Numbers ◽

Horizontal Cylinder ◽

Vortex Street ◽

Vertical Flow ◽

Low Reynolds Numbers ◽

Vortex Shedding Frequency ◽

Shedding Frequency

The research studies the characteristics of the vertical flow past a finite-length horizontal cylinder at low Reynolds numbers (ReD) from 250 to 1080. The experiments were performed in a vertical closed-loop water tunnel. Flow fields were observed by the particle tracer approach for flow visualization and measured by the Particle Image Velocimetry (P.I.V.) approach for velocity fields. The characteristics of vortex formation in the wake of the finite-length cylinder change at different regions from the tip to the base of it. Near the tip, a pair of vortices in the wake was observed and the size of the vortex increased as the observed section was away from the tip. Around a distance of 3 diameters of the cylinder from its tip, the vortex street in the wake was observed. The characteristics of vortex formation also change with increasing Reynolds numbers. At X/D = -3, a pair of vortices was observed in the wake for ReD = 250, but as the ReD increases the vortex street was observed at the same section. The vortex shedding frequency is analyzed by Fast Fourier Transform (FFT). Experimental results show that the downwash flow affects the vortex shedding frequency even to 5 diameters of the cylinder from its tip. The interaction between the downwash flow and the Von Kármán vortex street in the wake of the cylinder is presented in this paper.

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Numerical investigation of boundary layer effects on vortex shedding frequency and forces acting upon marine pipeline

Applied Ocean Research ◽

10.1016/j.apor.2010.10.002 ◽

2010 ◽

Vol 32 (4) ◽

pp. 460-470 ◽

Cited By ~ 8

Author(s):

M.H. Kazeminezhad ◽

A. Yeganeh-Bakhtiary ◽

A. Etemad-Shahidi

Keyword(s):

Boundary Layer ◽

Numerical Investigation ◽

Vortex Shedding ◽

Vortex Shedding Frequency ◽

Shedding Frequency

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Vortex-Induced Vibration for Heat Transfer Enhancement

Volume 8A: Heat Transfer and Thermal Engineering ◽

10.1115/imece2014-37594 ◽

2014 ◽

Author(s):

Junxiang Shi ◽

Steven R. Schafer ◽

Chung-Lung (C. L. ) Chen

Keyword(s):

Heat Transfer ◽

Boundary Layer ◽

Heat Transfer Enhancement ◽

Natural Frequency ◽

Vortex Shedding ◽

Pressure Loss ◽

Thermal Boundary Layer ◽

Transfer Enhancement ◽

Vortex Shedding Frequency ◽

Shedding Frequency

A passive, self-agitating method which takes advantage of vortex-induced vibration (VIV) is presented to disrupt the thermal boundary layer and thereby enhance the convective heat transfer performance of a channel. A flexible cylinder is placed at centerline of a channel. The vortex shedding due to the presence of the cylinder generates a periodic lift force and the consequent vibration of the cylinder. The fluid-structure-interaction (FSI) due to the vibration strengthens the disruption of the thermal boundary layer by reinforcing vortex interaction with the walls, and improves the mixing process. This novel concept is demonstrated by a three-dimensional modeling study in different channels. The fluid dynamics and thermal performance are discussed in terms of the vortex dynamics, disruption of the thermal boundary layer, local and average Nusselt numbers (Nu), and pressure loss. At different conditions (Reynolds numbers, channel geometries, material properties), the channel with the VIV is seen to significantly increase the convective heat transfer coefficient. When the Reynolds number is 168, the channel with the VIV improves the average Nu by 234.8% and 51.4% in comparison with a clean channel and a channel with a stationary cylinder, respectively. The cylinder with the natural frequency close to the vortex shedding frequency is proved to have the maximum heat transfer enhancement. When the natural frequency is different from the vortex shedding frequency, the lower natural frequency shows a higher heat transfer rate and lower pressure loss than the larger one.

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Applicability of The Equivalent Diameter Approach to Estimate Vortex Shedding Frequency and Acoustic Resonance Excitation From Different Finned Cylinders In Cross-Flow

Journal of Pressure Vessel Technology ◽

10.1115/1.4053015 ◽

2021 ◽

Author(s):

Mohammed Alziadeh ◽

Atef Mohany

Keyword(s):

Strouhal Number ◽

Vortex Shedding ◽

Interaction Mechanism ◽

Acoustic Resonance ◽

Resonance Excitation ◽

Equivalent Diameter ◽

Effective Diameter ◽

Finned Tubes ◽

Vortex Shedding Frequency ◽

Shedding Frequency

Abstract This article explores the applicability of utilizing different equivalent diameter (Deq) equations to estimate the vortex shedding frequency and onset of self-excited acoustic resonance for various types of finned cylinders. The focus is on three finned cylinder types that are commonly used in industrial heat exchangers: straight, twist-serrated, and crimped spirally finned cylinders. Within each type of fins, at least three different finned cylinders are investigated. The results indicate that at off-resonance conditions, utilizing the appropriate equivalent diameter collapses the Strouhal number data within the typical Strouhal number variations of an equivalent diameter circular, bare cylinder. However, when acoustic resonance is initiated, the onset and the peak of resonance excitation in all of the finned cylinder cases generally occurred at a reduced flow velocity earlier than that observed from their equivalent diameter bare cylinders. This suggests that although utilizing the appropriate equivalent diameter can reasonably estimate the vortex shedding frequency away from acoustic resonance excitation, it cannot be used to predict the onset of acoustic resonance in finned tubes. The findings of this study indicate that the effective diameter approach is not sufficient to capture the intrinsic changes in the flow-sound interaction mechanism as a result of adding fins to a bare cylinder. Thus, a revision of the acoustic Strouhal number charts is required for finned tubes of different types and arrangements.

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Flow past a rotationally oscillating cylinder

Journal of Fluid Mechanics ◽

10.1017/jfm.2013.469 ◽

2013 ◽

Vol 735 ◽

pp. 307-346 ◽

Cited By ~ 25

Author(s):

S. Kumar ◽

C. Lopez ◽

O. Probst ◽

G. Francisco ◽

D. Askari ◽

...

Keyword(s):

Reynolds Number ◽

Vortex Shedding ◽

Three Dimensional ◽

Far Field ◽

Two Dimensional ◽

Flow Past ◽

Vortex Shedding Frequency ◽

Shedding Frequency ◽

Flow Visualizations ◽

Hydrogen Bubble Technique

AbstractFlow past a circular cylinder executing sinusoidal rotary oscillations about its own axis is studied experimentally. The experiments are carried out at a Reynolds number of 185, oscillation amplitudes varying from $\mathrm{\pi} / 8$ to $\mathrm{\pi} $, and at non-dimensional forcing frequencies (ratio of the cylinder oscillation frequency to the vortex-shedding frequency from a stationary cylinder) varying from 0 to 5. The diagnostic is performed by extensive flow visualization using the hydrogen bubble technique, hot-wire anemometry and particle-image velocimetry. The wake structures are related to the velocity spectra at various forcing parameters and downstream distances. It is found that the phenomenon of lock-on occurs in a forcing frequency range which depends not only on the amplitude of oscillation but also the downstream location from the cylinder. The experimentally measured lock-on diagram in the forcing amplitude and frequency plane at various downstream locations ranging from 2 to 23 diameters is presented. The far-field wake decouples, after the lock-on at higher forcing frequencies and behaves more like a regular Bénard–von Kármán vortex street from a stationary cylinder with vortex-shedding frequency mostly lower than that from a stationary cylinder. The dependence of circulation values of the shed vortices on the forcing frequency reveals a decay character independent of forcing amplitude beyond forcing frequency of ${\sim }1. 0$ and a scaling behaviour with forcing amplitude at forcing frequencies ${\leq }1. 0$. The flow visualizations reveal that the far-field wake becomes two-dimensional (planar) near the forcing frequencies where the circulation of the shed vortices becomes maximum and strong three-dimensional flow is generated as mode shape changes in certain forcing parameter conditions. It is also found from flow visualizations that even at higher Reynolds number of 400, forcing the cylinder at forcing amplitudes of $\mathrm{\pi} / 4$ and $\mathrm{\pi} / 2$ can make the flow field two-dimensional at forcing frequencies greater than ${\sim }2. 5$.

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