A freely yawing axisymmetric bluff body controlled by near-wake flow coupling

2019 ◽  
Vol 863 ◽  
pp. 1123-1156 ◽  
Author(s):  
Thomas J. Lambert ◽  
Bojan Vukasinovic ◽  
Ari Glezer
Keyword(s):  
Shear Layer ◽  
Time Scales ◽  
Attitude Control ◽  
Bluff Body ◽  
Cross Flow ◽  
The Body ◽  
Wake Flow ◽  
Small Scale ◽  
Near Wake ◽  
Yaw Attitude

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

Some features of steady separated flow from low speed to hypersonic

2008 ◽  
Vol 112 (1128) ◽  
pp. 109-113
Author(s):  
S. L. Gai
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Separated Flow ◽  
Base Flow ◽  
The Body ◽  
Wake Flow ◽  
Recirculating Flow ◽  
Flow Features ◽  
Hypersonic Speeds ◽  
Low Speeds

Steady non-vortex shedding base flow behind a bluff body is considered. Such a flow is characterised by the flow separation at the trailing edge of the body with an emerging shear layer which reattaches on the axis with strong recompression and recirculating flow bounded by the base, the shear layer, and the axis. Steady wake flows behind a bluff body at low speeds have been studied for more than a century (for example, Kirchhoff; Riabouchinsky). Recently, research on steady bluff body wake flow at low speeds has been reviewed and reinterpreted by Roshko. Roshko has also commented on some basic aspects of steady supersonic base flow following on from Chapman and Korst analyses. In the present paper, we examine the steady base flow features both at low speeds and supersonic speeds in the light of Roshko’s model and expand on some further aspects of base flows at supersonic and hypersonic speeds, not covered by Roshko.

scholarly journals Kill Line Model Cross Flow Inline Coupled Vortex-Induced Vibration

2017 ◽  
Author(s):  
Baiheng Wu ◽  
Jorlyn Le Garrec ◽  
Dixia Fan ◽  
Michael S. Triantafyllou
Keyword(s):  
Bluff Body ◽  
Oil And Gas ◽  
Cross Flow ◽  
The Body ◽  
Bluff Bodies ◽  
Structural Vibrations ◽  
Vortex Induced Vibrations ◽  
Series Of Experiments ◽  
Central Pipe ◽  
Semi Empirical

Currents and waves cause flow-structure interaction problems in systems installed in the ocean. Particularly for bluff bodies, vortices form in the body wake, which can cause strong structural vibrations (Vortex-Induced Vibrations, VIV). The magnitude and frequency content of VIV is determined by the shape, material properties, and size of the bluff body, and the nature and velocity of the oncoming flow. Riser systems are extensively used in the ocean to drill for oil wells, or produce oil and gas from the bottom of the ocean. Risers often consist of a central pipe, surrounded by several smaller cylinders, including the kill and choke lines. We present a series of experiments involving forced in-line and cross flow motions of short rigid sections of a riser containing 6 symmetrically arranged kill and choke lines. The experiments were carried out at the MIT Towing Tank. We present a systematic database of the hydrodynamic coefficients, consisting of the forces in phase with velocity and the added mass coefficients that are also suitable to be used with semi-empirical VIV predicting codes.

The Interaction of Porous Metal Coating With the Near Wake of Bluff Body

2012 ◽  
Author(s):  
Hanru Liu ◽  
Jinjia Wei ◽  
Zhiguo Qu
Keyword(s):  
Circular Cylinder ◽  
Bluff Body ◽  
Porous Metal ◽  
Metal Coating ◽  
Wake Flow ◽  
Navier Stokes ◽  
Aerodynamic Forces ◽  
Near Wake ◽  
Drag Forces ◽  
Flow Induced Noise

The flow around a circular cylinder with porous metal coating (PMC) is numerically investigated based on an approach of unsteady Reynolds Averaged Navier-Stokes (URANS) at subcritical Reynolds number. The model validation is carried out through comparison with some available experimental results in the literatures. It is found that the simulated results in the present work coincide well with the experimental data. The interaction of PMC with the near wake of circular cylinder such as streamline, vorticity and shear stress are studied in detail. The result reveals that PMC has capability of manipulating the wake flow so that the near wake of PMC cylinder is substantially different from that of smooth one. In addition, the fluctuations of aerodynamic forces are mitigated effectively. Varying the thickness of porous metal coating causes various velocity distributions and aerodynamic performance of bluff body. When the thickness is appropriate, the drag forces can be reduced to a certain extent. It is expected that the modification of flow characteristic and aerodynamic forces also produces the suppression of flow-induced noise generated by bluff body. These studies on wake flow and analysis of its relationship to flow-induced noise will be useful to understand the mechanism of controlling bluff body flow-induced noise by using PMC and to optimize the PMC for controlling flow and flow-induced noise.

Experimental Studies of Hydrodynamic Properties and Screening of Riser Fairing Concepts for Deep Water Applications

2015 ◽  
Author(s):  
Rolf Baarholm ◽  
Kjetil Skaugset ◽  
Halvor Lie ◽  
Henning Braaten
Keyword(s):  
Experimental Studies ◽  
Real Life ◽  
Cross Flow ◽  
Free Vibrations ◽  
The Body ◽  
Scale Model ◽  
Small Scale ◽  
Structural Fatigue ◽  
And Performance ◽  
Set Up

The VIV oscillations of marine risers are known to increase drag, and lead to structural fatigue. One proven method of suppressing this vibration is the use of fairings and strakes. These coverings essentially modify the flow along the cylinder, tripping the production of Karman vortices so that they act less coherently or far enough downstream so they interact less with the body. The Norwegian Deepwater Programme (NDP) has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV mitigation and galloping. Furthermore, emphasis is put on easy handling and installation. This paper describes the work and findings in an early phase of the development. This includes small scale model test campaigns. In addition to the bare riser for reference, the behaviour and performance of a total of 10 different fairing concepts are evaluated. Free oscillation tests are performed in a towing tank, where 2D fairings were tested in a pendulum set-up. The set-up enables free vibrations in up to 3 DOF (in-line and cross-flow vibrations and yaw). Fix tests with the purpose of establishing hydrodynamic coefficients for the various fairings have been performed in a large cavitation tunnel. Clear differences in performance have been noticed; particular for drag and galloping responses. Based on the results from the 2D tests, a screening of the fairing designs has been performed and the findings have set the course for further development of the most promising candidates for real life applications.

scholarly journals The State-of-the-Art Brief Review on Piezoelectric Energy Harvesting from Flow-Induced Vibration

2021 ◽  
Vol 2021 ◽  
pp. 1-19
Author(s):  
Hongjun Zhu ◽  
Tao Tang ◽  
Huohai Yang ◽  
Junlei Wang ◽  
Jinze Song ◽  
...  
Keyword(s):  
Energy Harvesting ◽  
Bluff Body ◽  
Mechanical Energy ◽  
Wake Flow ◽  
Structural Vibration ◽  
Small Scale ◽  
Flow Induced Vibration ◽  
Energy Harvesters ◽  
Piezoelectric Energy ◽  
Self Powered

Flow-induced vibration (FIV) is concerned in a broad range of engineering applications due to its resultant fatigue damage to structures. Nevertheless, such fluid-structure coupling process continuously extracts the kinetic energy from ambient fluid flow, presenting the conversion potential from the mechanical energy to electricity. As the air and water flows are widely encountered in nature, piezoelectric energy harvesters show the advantages in small-scale utilization and self-powered instruments. This paper briefly reviewed the way of energy collection by piezoelectric energy harvesters and the various measures proposed in the literature, which enhance the structural vibration response and hence improve the energy harvesting efficiency. Methods such as irregularity and alteration of cross-section of bluff body, utilization of wake flow and interference, modification and rearrangement of cantilever beams, and introduction of magnetic force are discussed. Finally, some open questions and suggestions are proposed for the future investigation of such renewable energy harvesting mode.

scholarly journals Decomposition of wake dynamics in fluid–structure interaction via low-dimensional models

2019 ◽  
Vol 867 ◽  
pp. 723-764 ◽  
Author(s):  
T. P. Miyanawala ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
High Amplitude ◽  
Wake Flow ◽  
High Dimensional ◽  
Near Wake ◽  
Structure Interaction ◽  
Moderate Reynolds Number ◽  
Low Dimensional ◽  
Lock In

We present a dynamic decomposition analysis of the wake flow in fluid–structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to provide the significance of low-dimensional wake flow features and their interaction dynamics to sustain the free vibration of a square cylinder at a relatively low mass ratio. To obtain the high-dimensional data, we employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model for a moderate Reynolds number ($Re$). The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize each corresponding POD mode to detect features of the organized motions, namely, the vortex street, the shear layer and the near-wake bubble. We find that the vortex shedding modes contribute solely to the lift force, while the near-wake and shear layer modes play a dominant role in the drag force. We further examine the fundamental mechanism of this dynamical behaviour and propose a force decomposition technique via low-dimensional approximation. To elucidate the frequency lock-in, we systematically analyse the decomposed modes and their dynamical contributions to the force fluctuations for a range of reduced velocity at low Reynolds number laminar flow. These quantitative mode energy contributions demonstrate that the shear layer feeds the vorticity flux to the wake vortices and the near-wake bubble during the wake–body synchronization. Based on the decomposition of wake dynamics, we suggest an interaction cycle for the frequency lock-in during the wake–body interaction, which provides the interrelationship between the high-amplitude motion and the dominating wake features. Through our investigation of wake–body synchronization below critical $Re$ range, we discover that the bluff body can undergo a synchronized high-amplitude vibration due to flexibility-induced unsteadiness. Owing to the wake turbulence at a moderate Reynolds number of $Re=22\,000$, a distorted set of POD modes and the broadband energy distribution are observed, while the interaction cycle for the wake synchronization is found to be valid for the turbulent wake flow.

Energetic scales in a bluff body shear layer

2019 ◽  
Vol 875 ◽  
pp. 543-575 ◽  
Author(s):  
D. M. Moore ◽  
C. W. Letchford ◽  
M. Amitay
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Reynolds Numbers ◽  
The Body ◽  
Aspect Ratios ◽  
Separated Shear Layer ◽  
Image Velocimetry ◽  
Highly Nonlinear ◽  
Point Correlation ◽  
Reynolds Number Dependency

A detailed experimental campaign into separated shear layers stemming from rectangular sections (having aspect ratios of 5 : 1, 3 : 1 and 1 : 1) was carried out at Reynolds numbers range between $1.34\times 10^{4}$ and $1.18\times 10^{5}$ based on the body thickness. Particle image velocimetry was used to locate the highest concentration of fluctuations in the velocity field and subsequent hot-wire measurements at those locations provided adequate spectral resolution to follow the evolution of various instabilities that are active within the separated shear layer. Similar to recent findings by this same group, the shear layer behaviour is observed to contain a combination of Reynolds invariant characteristics, including its time-averaged position, while other properties demonstrate clear Reynolds number dependency, including the spatial amplification of turbulent kinetic energy. Additional results here show that the ratio of side lengths of the body is a key parameter in revealing these effects. One reason for this is the level of coupling between modes of instability, which is evaluated using two-point correlation methods. These findings indicate that the separated shear layer on a bluff body is highly nonlinear. A specific set of scales responsible for these unique behaviours is identified and discussed, along with their relationship to other scales in the flow.

Theoretical and Numerical Study of Vortex-Wake Flow Generated From Stack of Rectangular Bodies

2007 ◽  
Author(s):  
Khaled Alhussan
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Numerical Study ◽  
The Body ◽  
Fluid Particle ◽  
Wake Flow ◽  
Free Shear Layer ◽  
Complex Flow ◽  
Discrete Vortex

Flow over external bodies has been studied extensively because of their many practical applications. For example, flow past a rectangular bodies, usually experiences strong flow oscillations and boundary layer separation in the wake region behind the body. As a fluid particle flows toward the leading edge of a rectangular body, the pressure of the fluid particle increases from the free stream pressure to the stagnation pressure. The boundary layer separates from the surface forms a free shear layer and is highly unstable. This shear layer will eventually roll into a discrete vortex and detach from the surface. A periodic flow motion will develop in the wake as a result of boundary layer vortices being shed alternatively from either side of the rectangular shapes. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure. The work to be presented herein is a theoretical and numerical analysis of the complex fluid mechanism that occurs over stack of rectangular bodies for different number of rectangular bodies, specifically with regard to the vortex shedding and generation of wake. A number of important conclusions follow from the current research. First, study of the actual flow configuration over rectangular bodies offers some insight into the complex flow phenomena. Second, the characteristics of the vortex and wakes change considerably with the number of bodies.

Near-wake structure of an oscillating cylinder: effect of controlled shear-layer vortices

1996 ◽  
Vol 322 ◽  
pp. 21-49 ◽  
Author(s):  
C. K. Chyu ◽  
D. Rockwell
Keyword(s):  
Shear Layer ◽  
Vortex Formation ◽  
Small Scale ◽  
Asymptotic State ◽  
Near Wake ◽  
Wake Structure ◽  
Image Velocimetry ◽  
Image Density ◽  
Karman Vortex ◽  
Limiting Case

The instantaneous structure of the near wake of a cylinder subjected to small-amplitude perturbations is characterized using high-image-density particle image velocimetry. Emphasis is on control of the small-scale shear-layer vortices, which feed into the Kármán vortices. Modifications of the Kármán vortex formation are classified according to patterns of modulated and locked-on shear-layer vortices. The formation length of the Kármán vortices can be dramatically shortened and, in the limiting case, occur adjacent to the base of the cylinder when it is perturbed at the inherent instability frequency of the shear layer and its subharmonics. Moreover, the induced shear-layer vortices can lead to large-amplitude transverse undulations of the entire near-wake region during formation of the Kármán vortices.These variations of the near-wake structure are further elucidated by considering the transient response of the wake, induced by abrupt cessation and onset of periodic motion of the cylinder. Distinctive intermediate states of the wake arise during relaxation to its asymptotic state; such relaxation requires a very large number of periods of the inherent instability of the shear layer.

Effect of Base Slant on Flow in the Near Wake of an Axisymmetric Cylinder

1980 ◽  
Vol 31 (2) ◽  
pp. 132-147 ◽  
Author(s):  
Thomas Morel
Keyword(s):  
Drag Coefficient ◽  
Three Dimensional ◽  
Local Maximum ◽  
The Body ◽  
Wake Flow ◽  
Force Measurements ◽  
Near Wake ◽  
Slant Angle ◽  
Cylinder Length ◽  
Axisymmetric Bodies

SummaryThe effects of slanting the base of a slender axisymmetric cylinder (length/diameter ratio of 9), aligned with the flow, was studied experimentally. The body was equipped with interchangeable rear ends covering a range of slant angles between 0° (vertical) and 70°. It was found that the base slant has a very dramatic effect on body drag, particularly in a relatively narrow range of slant angles where the drag coefficient exhibits a large local maximum (over-shoot). Detailed study of the flow showed that the drag overshoot is related to the existence of two very different Separation patterns on the slanted base. One pattern is similar to that found behind axisymmetric bodies with no base slant, and its main feature is the presence of a closed Separation region adjacent to the base. The other pattern is highly three-dimensional with two streamwise vortices forming along the sides of the slanted base. This pattern sets in very abruptly at a “critical” slant angle α ∼ 47°. Drag force measurements showed that, at first, the drag coefficient slowly increases with the slant angle, but then jumps suddenly upwards to more than double its baseline value (from CD = 0.24 to CD = 0.625) at the critical angle. At angles higher than that CD decreases again, and at 70° it is about equal to the baseline value. Further effects of the slant angle are the generation of a large side force and a significant increase in near-wake flow periodicity.

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