203 Vortex Shedding from a Bluff-body Beneath a Free Surface : Effect of the Shape of the Body on the Surface Distortion and Bubble Generation

Overset, or Chimera meshes are used to discretize the governing equations within a computational domain using multiple meshes that overlap in an arbitrary manner. The overset mesh technique is most applicable to problems dealing with multiple or moving bodies. In order to extend existing full craft CFD (RANS) simulations of a surface effect ship (SES) into shallow water and maneuvering cases, an overset mesh is needed. Deep water simulations were carried out using both single and overset grid techniques for evaluation of the overset grid application. The single grid technique applies a hexahedral mesh to the fluid domain and SES geometry. An adequate mesh resolution was determined by performing a grid convergence study on a series of systematically refined meshes. An overset mesh of the same resolution was then constructed and was fixed to the body. Drag and pitch results are compared among the two simulations. Free surface elevations around the craft and under the air cushion for simulations with the single and overset meshes are compared. Steady-state simulations using the overset mesh and the single mesh show general similarities in drag, pitch, and free surface elevations.

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Influence of the hydrofoil inclination angle at free surface flow around junctions

Analele Universităţii "Dunărea de Jos" din Galaţi. Fascicula XI, Construcţii navale/ Annals of "Dunărea de Jos" of Galati, Fascicle XI, Shipbuilding ◽

10.35219/annugalshipbuilding.2020.43.13 ◽

2020 ◽

Vol 43 ◽

pp. 103-108

Author(s):

Costel Ungureanu ◽

Costel Iulian Mocanu

Keyword(s):

Boundary Layer ◽

Free Surface ◽

Bluff Body ◽

Three Dimensional ◽

Free Surface Flow ◽

Breaking Waves ◽

Surface Flow ◽

Vortex Structures ◽

The Body ◽

Flow Topology

"Free surface flow is a hydrodynamic problem with a seemingly simple geometric configuration but with a flow topology complicated by the pressure gradient due to the presence of the obstacle, the interaction between the boundary layer and the free surface, turbulence, breaking waves, surface tension effects between water and air. As the ship appendages become more and more used and larger in size, the general understanding of the flow field around the appendages and the junction between them and the hull is a topical issue for naval hydrodynamics. When flowing with a boundary layer, when the streamlines meet a bluff body mounted on a solid flat or curved surface, detachments appear in front of it due to the blocking effect. As a result, vortex structures develop in the fluid, also called horseshoe vortices, the current being one with a completely three-dimensional character, complicated by the interactions between the boundary layer and the vortex structures thus generated. Despite the importance of the topic, the literature records the lack of coherent methods for investigating free surface flow around junctions, the lack of consistent studies on the influence of the inclination of the profile mounted on the body. As a result, this paper aims to systematically study the influence of profile inclination in respect to the support plate."

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Parametric Numerical Study on the Self-Propulsive Performance of The DARPA Suboff Submarine via the Body-Force Propeller Method

10.1115/omae2021-60488 ◽

2021 ◽

Author(s):

Kenshiro Takahashi ◽

Takayuki Mori

Keyword(s):

Reynolds Number ◽

Free Surface ◽

Body Force ◽

Surface Effect ◽

Numerical Study ◽

The Body ◽

The Self ◽

Navier Stokes ◽

Suction Force ◽

Propulsive Performance

Abstract This study is based on previous works in a series of numerical studies on submarine hydrodynamics, which involved developing a computational fluid dynamics method to estimate the self-propulsive performance of underwater vehicles. Herein, the Defense Advanced Research Projects Agency SUBOFF submarine model was adopted as a benchmark. The computational modeling applied was based on the Reynolds-averaged Navier-Stokes turbulence model. A body-force propeller method was adopted to model the propulsion. The self-propulsive performance was verified via mesh refinement and validated by comparing the computational solutions with the results obtained from the experiments. The effect of the Reynolds number on the self-propulsive performance was investigated by varying the positions of the stern planes, while the free surface effect was determined by varying the Froude number (Fr) via the volume of fluid method. The computed Taylor wake fraction (w) and hull efficiency (ηH) depended on the Reynolds number as it decreased monotonically. The w and thrust deduction fraction (t) for the model of aft-fitted stern planes were approximately 3–7% and 8–10% higher than those of the baseline and fore-fitted stern planes, respectively. The differences in ηH between the models were insignificant. Regarding the free surface effects, the computations of w, t, and ηH generally decreased with Fr, thus exhibiting several humps and hollows. The computed upward suction force and pitching moment varied from negative to positive and vice versa, depending on Fr.

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Part I: Chordwise Distribution of Fluctuating Pressure

Aeronautical Quarterly ◽

10.1017/s0001925900009069 ◽

1981 ◽

Vol 32 (2) ◽

pp. 97-110 ◽

Cited By ~ 12

Author(s):

R.H. Wilkinson

Keyword(s):

Vortex Shedding ◽

Bluff Body ◽

Lift Coefficient ◽

Relative Magnitude ◽

The Body ◽

Front Face ◽

Two Dimensional ◽

Fluctuating Pressure ◽

Pressure Distributions ◽

Dimensional Section

SummaryThe fluctuating loading on a cylindrical bluff body due to vortex shedding increases if the body is capable of vibration. This is a result of amplification of the fluctuating pressures around a two-dimensional section of the body together with an improvement of the spanwise correlation of the vortex shedding. Measurement of the fluctuating forces on the cylinder during this process gives no guide as to the relative magnitude of these effects. In this paper, root mean square fluctuating pressure distributions and pressure correlations across a chord are presented for a square cylinder with front face normal to the approach flow whilst stationary and during forced vibration. The fluctuating lift coefficient for a two-dimensional section of the cylinder and its maximum amplification during vibration are calculated.

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Flow-Induced Vibrations of Single and Multiple Heated Circular Cylinders: A Review

Energies ◽

10.3390/en14248496 ◽

2021 ◽

Vol 14 (24) ◽

pp. 8496

Author(s):

Ussama Ali ◽

Md. Islam ◽

Isam Janajreh ◽

Yap Fatt ◽

Md. Mahbub Alam

Keyword(s):

Heat Transfer ◽

Vortex Shedding ◽

Heat Exchangers ◽

Power Plants ◽

Bluff Body ◽

The Body ◽

Circular Cylinders ◽

Fluid Forces ◽

Spacing Ratio ◽

Flow Induced Vibrations

This study is an effort to encapsulate the fundamentals and major findings in the area of fluid-solid interaction, particularly the flow-induced vibrations (FIV). Periodic flow separation and vortex shedding stretching downstream induce dynamic fluid forces on the bluff body and results in oscillatory motion of the body. The motion is generally referred to as flow-induced vibrations. FIV is a dynamic phenomenon as the motion, or the vibration of the body is subjected to the continuously changing fluid forces. Sometimes FIV is modeled as forced vibrations to mimic the vibration response due to the fluid forces. FIV is a deep concern of engineers for the design of modern heat exchangers, particularly the shell-and-tube type, as it is the major cause for the tube failures. Effect of important parameters such as Reynolds number, spacing ratio, damping coefficient, mass ratio and reduced velocity on the vibration characteristics (such as Strouhal number, vortex shedding, vibration frequency and amplitude, etc.) is summarized. Flow over a bluff body with wakes developed has been studied widely in the past decades. Several review articles are available in the literature on the area of vortex shedding and FIV. None of them, however, discusses the cases of FIV with heat transfer. In particular systems, FIV is often coupled to heat transfer, e.g., in nuclear power plants, FIV causes wear and tear to heat exchangers, which can eventually lead to catastrophic failure. As the circular shape is the most common shape for tubes and pipes encountered in practice, this review will only focus on the FIV of circular cylinders. In this attempt, FIV of single and multiple cylinders in staggered arrangement, including tandem and side-by-side arrangement is summarized for heated and unheated cylinder(s) in the one- and two-degree of freedom. The review also synthesizes the effect of fouling on heat transfer and flow characteristics. Finally, research prospects for heated circular cylinders are also stated.

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Measurements of Bluff Body Wake Flow Around a Conformable Flow Control Surface

Noise Control and Acoustics ◽

10.1115/imece2003-55376 ◽

2003 ◽

Author(s):

David A. Ericson ◽

Michael Jonson ◽

Gary Koopmann

Keyword(s):

Flow Control ◽

Vortex Shedding ◽

Flow Separation ◽

Bluff Body ◽

The Body ◽

Control Surface ◽

Periodic Flow ◽

Two Dimensional ◽

Trailing Edge ◽

Shedding Frequency

The vortex street is a unique type of unsteady flow separation seen commonly in flow over a bluff body with a characteristic periodic wake. A consequence of the periodic flow is that the drag and lift forces acting on the body also oscillate periodically. When the wake shedding frequency is near a structural frequency, flow induced resonance will occur. The continuing interest in the study of vortex street generation is propelled by the ever-present nature of these flows in a variety of applications including aerodynamics, hydrodynamics and underwater acoustics. Recent advances in material science and the development of high power density actuators have led to the study of adaptive structure technology wherein the vorticity of periodic flows can be actively controlled by changing the ‘bluffness’ or shape of the body. In this paper, the development and experimental testing of a two-dimensional shape-variable flow control surface are discussed in relation to the generation and manipulation of periodic flow separation. Two series of wind tunnel tests were designed to evaluate the potential of the morphing structure that replaced a section of the trailing edge of a symmetric airfoil. The test section successfully demonstrated a smooth transition between three prescribed trailing edge profiles ranging from sharp to blunt. Unsteady pressure spectra were measured near the trailing edge for three different shape profiles over a range of speeds between 50 and 110 ft/s. The measured pressure spectra amplitudes were compared to previously-published surface pressure spectra of a similar, two-dimensional, blunt edge foil. A second set of tests was performed to measure the resulting flow field in the direction transverse to the flow and downstream from the airfoil. Velocity measurements were made using a traversing hot-wire probe at three trailing edge configurations and speeds of 50, 70 and 90 ft/s. The corresponding Reynolds number based on wake thickness ranged from 3.9–9.8 × 104. Measured vortex shedding frequencies varied between approximately 50 to 130 Hz at the different trailing edge profiles. This type of change in the vortex shedding frequency can be used to reduce flow-induced vibration and its associated noise generation by avoiding shedding frequencies at operating speeds that coincide with airfoil resonances.

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Water entry of a wedge with rolled-up vortex sheet

Journal of Fluid Mechanics ◽

10.1017/jfm.2017.766 ◽

2017 ◽

Vol 835 ◽

pp. 512-539 ◽

Cited By ~ 5

Author(s):

Yuriy A. Semenov ◽

G. X. Wu

Keyword(s):

Free Surface ◽

Pressure Distribution ◽

Body Surface ◽

Vortex Shedding ◽

Vortex Sheet ◽

Water Entry ◽

The Body ◽

Surface Shape ◽

Force Coefficients ◽

Local Singularity

The problem of asymmetric water entry of a wedge with the vortex sheet shed from its apex is considered within the framework of the ideal and incompressible fluid. The effects due to gravity and surface tension are ignored and the flow therefore can be treated as self-similar, as there is no length scale. The solution for the problem is sought through two mutually dependent parts using two different analytic approaches. The first one is due to water entry, which is obtained through the integral hodograph method for the complex velocity potential, in which the streamline on the body surface remains on the body surface after passing the apex, leading to a non-physical local singularity. The second one is due to a vortex sheet shed from the apex, and the shape of the sheet and the strength distribution of the vortex are obtained through the solution of the Birkhoff–Rott equation. The total circulation of the vortex sheet is obtained by imposing the Kutta condition at the apex, which removes the local singularity. These two solutions are nonlinearly coupled on the unknown free surface and the unknown vortex sheet. This poses a major challenge, which distinguishes the present formulation of the problem from the previous ones on water entry without a vortex sheet and ones on vortex shedding from a wedge apex without a moving free surface. Detailed results in terms of pressure distribution, vortex sheet, velocity and force coefficients are presented for wedges of different inner angles and heel angles, as well as the water-entry direction. It is shown that the vortex shedding from the tip of the wedge has a profound local effect, but only weakly affects the free-surface shape, overall pressure distribution and force coefficients.

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Review—Vortex Shedding Lock-on and Flow Control in Bluff Body Wakes

Journal of Fluids Engineering ◽

10.1115/1.2926511 ◽

1991 ◽

Vol 113 (4) ◽

pp. 526-537 ◽

Cited By ~ 156

Author(s):

O. M. Griffin ◽

M. S. Hall

Keyword(s):

Vortex Shedding ◽

Bluff Body ◽

The Body ◽

Periodic Flow ◽

Cylinder Wake ◽

Mean Flow ◽

Near Wake ◽

Strong Resonance ◽

And Control ◽

Rotational Oscillations

The results of recent experiments demonstrate that the phenomenon of vortex shedding resonance or lock-on is observed also when a bluff body is placed in an incident mean flow with a periodic component superimposed upon it. This form of vortex shedding and lock-on exhibits a particularly strong resonance between the flow perturbations and the vortices, and provides one of several promising means for modification and control of the basic formation and stability mechanisms in the near-wake of a bluff body. Examples are given of recent direct numerical simulations of the vortex lock-on in the periodic flow. These agree well with the results of experiments. A discussion also is given of vortex lock-on due to body oscillations both normal to and in-line with the incident mean flow, rotational oscillations of the body, and of the effect of sound on lock-on. The lock-on phenomenon is discussed in the overall context of active and passive wake control, on the basis of these and other recent and related results, with particular emphasis placed on active control of the circular cylinder wake.

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On the Spacing of Karman Vortices

Journal of Applied Mechanics ◽

10.1115/1.3564657 ◽

1969 ◽

Vol 36 (2) ◽

pp. 370-372 ◽

Cited By ~ 11

Author(s):

D. W. Sallet

Keyword(s):

Strouhal Number ◽

Cross Section ◽

Vortex Shedding ◽

Bluff Body ◽

Slenderness Ratio ◽

The Body ◽

Vortex Street ◽

Circular Cylinders ◽

Flat Plates ◽

Uniform Cross Section

Equations for the absolute dimensions of the Karman vortex street are developed in terms of the coefficient of drag and the Strouhal number of the vortex shedding bluff body. The body is assumed to be of large slenderness ratio and of uniform cross section. The predicted vortex spacings are compared with the experimental results of other investigators for circular cylinders, flat plates, and a wedge.

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Effects of Streamlining a Bluff Body in the Laminar Vortex Shedding Regime

Journal of Fluids Engineering ◽

10.1115/1.4044830 ◽

2019 ◽

Vol 142 (2) ◽

Author(s):

Ritvik Dobriyal ◽

Maneesh Mishra ◽

Markus Bölander ◽

Martin Skote

Keyword(s):

Reynolds Number ◽

Circular Cylinder ◽

Vortex Shedding ◽

Shape Factor ◽

Bluff Body ◽

The Body ◽

Cross Sectional ◽

Shape Factors ◽

Rear Part ◽

Lift And Drag

Abstract Two-dimensional flow over bluff bodies is studied in the unsteady laminar flow regime using numerical simulations. In previous investigations, lift and drag forces have been studied over different cross-sectional shapes like circles, squares, and ellipses. We aim to extend the previous research by studying the variation of hydrodynamic forces as the shape of the body changes from a circular cylinder to a more streamlined or a bluffer body. The different body shapes are created by modifying the downstream circular arc of a circular cylinder into an ellipse, hence elongating or compressing the rear part of the body. The precise geometry of the body is quantified by defining a shape factor. Two distinct ranges of shape factors with fundamentally different behavior of lift and drag are identified. The geometry constituting the limit is where the rear part ellipse has a semi-minor axis of half the radius of the original circle, independent of the Reynolds number. On the other hand, the vortex shedding frequency decreases linearly over the whole range of shape factors. Furthermore, the variation of the forces and frequency with Reynolds number, and how the relations vary with the shape factor are reported.

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