Bluff Structures Research Articles

The impact of lateral flow contraction on the rock plucking process under sub‐critical flow conditions

AbstractRock plucking is the vertical removal of chunks of rock along intersection joint planes by flowing water. The plucking of durable jointed rock in a natural stream alters stream morphology and potentially damages a bridge substructure situated on a rock foundation by local erosion. Despite its importance, the mechanics of bedrock erosion by plucking are not well understood. In this study, we conduct laboratory experiments to investigate the threshold of block entrainment through plucking near a flow obstruction under sub‐critical conditions. In a laboratory flume, we reproduce a wide range of typical lateral flow contraction scenarios that occur around a large rock or bridge substructure and investigate the process of the vertical entrainment of blocks near the obstruction. Two different modes of block removal process (impulsive plucking and accumulative plucking) are observed near the bluff structure. We find that under severe flow contraction, blocks are more likely to escape quickly via impulsive plucking because higher flow contraction and the resulting strong local turbulence around the structure generate large instantaneous uplift force that removes the block within a short period of time; however, we observe accumulative plucking under weaker flow contraction near the bluff structure. From the experimental results, we develop a formula for the threshold point of block dislocation caused by plucking with respect to the degree of lateral flow contraction. The results indicate that increasing the flow contraction ratio reduces block stability because of higher flow acceleration and the increased turbulence effect near the upstream edge of the obstruction. The results of this study should be useful to those who estimate the rate of rock erosion by plucking in natural open channels around a bluff structure.

Realization of both high-intensity and large-scale turbulence using a multi-fan wind tunnel

When experiments are conducted on the effects of freestream turbulence on the flow around bluff structures, generation of both high-intensity and large-scale turbulence in a wind tunnel is of major importance. We attempt this by developing a new type of wind tunnel named ‘multi-fan wind tunnel’ (MFWT), wherein airflow is driven by multiple fans. We focus on the use of shear layers generated between the outflows from adjacent ducts. Herein, we propose an efficient driving method in which an input signal composed of numerous sinusoidal waves is fed to each fan with quasi-random phases (‘random-phase mode’). It was confirmed that this mode generates approximately homogeneous turbulence within a short downstream distance. We found that pulsatile components were eliminated, which prevented the development of turbulence in our preceding experiments using the MFWT. The initial rectangular spectrum provided by forcing rapidly changes its shape as turbulence convects downstream, and asymptotes to a typical broad spectrum including a few decades of $$-5/3$$ power law. The intensity and the integral scale can be controlled by the amplitude of the input signal, and for an approximately isotropic region at large downstream stations, the intensity reaches 12–16% and the integral scale 0.6–0.7 m.

Determination of critical flow velocity for tube-bundle with application of numerical investigation method

The work is devoted to the creation and application of mathematical models for the most dangerous oscillation excitation mechanisms of tubes and cylindrical form bluff structures in liquid or gas flow, as well as to the creation of efficient computational methods for description of these models. A numerical investigation method of hydrodynamic forces arising from a separated flow and tube-bundle oscillations excited by these forces was developed by the authors. The method is based on the application of created original tube-bundle hydroelastic oscillation excitation in a cross-flow mathematical model. Hydroelastic excitation problem is reduced to the stability analysis of undisturbed state of elastic tubes. Analysis is conducted with the assumption of linearity of the destabilizing forces. On the basis of the mathematical model, the necessary and sufficient condition for the stability, expressed through the dimensionless system parameters (mass, damping, velocity), was obtained. Numerical identification of the linear hydrodynamic connection matrix algorithm for particular tube-bundles was elaborated. Verification of algorithm and programs based on it was performed by results of simulations and available experimental data correlation. A method for determination of a linear hydrodynamic connection matrix for tube-bundles with a regular arrangement of the cross-section was offered. It is based on computation of a relatively small, but sufficient for reliable results, part of the tube-bundle.

Flow reproduction using Vortex Particle Methods for simulating wake buffeting response of bluff structures

The paper presents a novel extension of the two-dimensional Vortex Particle Method which allows complex transient flow features computed by an original simulation to be recreated for use as inflow conditions in other simulations. This is facilitated by recording velocity time signals of the original simulation and computing time traces of vortex particles to be seeded into the secondary simulation near its upstream domain boundary. The proposed Flow Reproduction Method (FRM) thus allows us to re-create the flow field, without the need to simulate the underlying physics responsible for the flow features. A natural field of application is the re-creation of wakes from flows past bluff bodies of arbitrarily complex geometry, the resolution of which is computationally expensive. The recording is performed on a sampling system of the velocity field. Reproduction of the sampled simulation is then performed by inserting vortex particles in defined positions and time intervals into the secondary simulation. This proceeds in a smaller domain with the advantage of significantly reduced computational cost. In the simulations presented here, wake flows of upstream cylinders are reproduced. Convergence studies are performed to validate the FRM. The quality of flow reproduction is assessed and quantified. The computational efficiency of the reproduction simulation is enhanced additionally by using different adaptive numerical techniques. The method is then applied to fluid-structure interaction simulations of a wake buffeting problem. Good agreement is found between the dynamic response quantities when comparing original simulations and those with reproduced flows, whereby the latter are performed up to five times faster.

Numerical simulation with a TVD–FVM method for circular cylinder wake control by a fairing

A water drop-shaped fairing is applied to control the wake behind a circular cylinder and to suppress the formation of Karman vortex street in this paper. The results are evaluated using high resolution CFD technique. A finite-volume total variation diminishing (TVD) approach based upon the recently proposed elemental velocity vector transformation (EVVT) method, which aims at solving the incompressible turbulent flow for irregular boundary conditions with renormalization group (RNG) turbulence model, is used to simulate the flow field around circular cylinder systems. The calculations are carried out with cylinder systems with and without fairings, while the fairings have different top shape angles within the range of 30°~90°. The Reynolds number ranges from 1000 to 50000. It is shown that the simulation results of present numerical method reaches good agreement with the available experimental and numerical simulation data of typical circular cylinder flow and a fixed fairing cylinder system flow. Compared with bare cylinder, the faired bluff structures can obviously reduce the lift and drag forces and alter the vortex shedding frequency. Overall, the mean drag coefficient can be reduced up to about (10–31)% and the RMS lift coefficient can be reduced up to (30–99)% for all faired systems at given Reynolds numbers. The influence of Reynolds number and attack angles on the flow field characters of bare cylinder and faired cylinders is also discussed. The faired structures with shape angles within 30°~45°under zero-attack-angle-inflow case are considered as the optimal structures, with which the mean drag coefficient and the RMS lift coefficient can be reduced up to (26–31)% and (98–99)%, respectively. Considering the influence of attack angles on lift and drag coefficients reduction, 75° shaped faired structure may be taken as a proper option.

Numerical simulation of vortex induced vibrations and its control by suction and blowing

The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the Kármán vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle α=100°. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations.

Time and Archaeological Traditions in the Lower Illinois Valley

The issue of time remains a crucial one in Lower Illinois Valley archaeology, and key problems remain unresolved. In this paper, new radiocarbon assays and published dates are used to test hypotheses concerning intra-site bluff top mound chronologies, timing and structure of valley settlement, and the emergence of regional symbolic communities during the Middle Woodland period (ca. 50 cal B.C.-cal A.D. 400). We show that within sites Middle Woodland mounds were constructed first on prominent, distal bluff ridges and subsequently in less-visible spaces, though additional dates are needed to fully understand intra-site chronology. Our analyses generally support previous studies suggesting a north-to-south settlement trajectory of the valley, though habitation site dates indicate a more complicated pattern of regional occupation that has yet to be fully explicated. In addition, floodplain regional symbolic communities also emerged along a north-to-south pattern, though not as rapidly as bluff crest mounds. Importantly, results indicate future areas of research necessary to elucidate regional chronology, resettlement of the valley, and community interactions.

A mathematical simulation to synchronization of a bluff structure

When the periodic vortex-shedding frequency of a bluff structure “locks-in” with one of the natural frequencies of the structure, the state is termed synchronization. When this lock-in or detuning state persists over a considerable period of time, fatigue stresses become predominant which then may lead to structural damage and even failure. Vortex-excited oscillations of bluff structures are one of the important problems in wind engineering, and wake oscillator models, especially the Van der Pol or Rayleigh type, have been studied profoundly over the last 20 odd years. The approach of most researchers is to couple one form of the Rayleigh oscillator with the conventional equation of motion of a single degree of freedom system. It is emphasized though that, depending on the form of the Rayleigh equation chosen, convergence to the solution may not be guaranteed, and the problem becomes ill-defined. The proposed semi-empirical model is of the coupled Rayleigh wake-oscillator type with the equation of motion containing a series term for the forcing function. The paper highlights the mathematical suitability of the proposed form for the aerodynamic response of an isolated structure.

Simulation of large wind pressures by gusts on a bluff structure

This paper illustrates application of the proper orthogonal decomposition (POD) and the autoregressive (AR) model to simulate large wind pressures due to gusts on a low-rise building. In the POD analysis, the covariance of the ensemble of large wind pressures is employed to calculate the principal modes and coordinates. The POD principal coordinates are modeled using the AR process, and the fitted AR models are employed to generate the principal coordinates. The generated principal coordinates are then used to simulate large wind pressures. The results show that the structure characterizing large wind pressures is well represented by the dominant eigenmodes (up to the first fifteen eigenmodes). Also, wind pressures with large peak values are simulated very well using the dominant eigenmodes along with the principal coordinates generated by the AR models.

A Family of Efficient Sloshing Liquid Dampers for Suppression of Wind-Induced Instabilities

Bluff structures in the form of tall buildings, smokestacks, control towers, bridges, etc., are susceptible to vortex resonance and galloping type of instabilities. One approach to vibration control of such systems is through energy dissipation using sloshing liquid dampers. In this paper we focus on enhancing the energy dissipation efficiency of a rectangular liquid damper through the introduction of two-dimensional obstacles as well as floating particles. The investigation has two phases. To begin with, a parametric free vibration study aimed at the optimization of the obstacle geometry is undertaken to arrive at configurations promising increased damping ratio and hence higher energy dissipation. The study is complemented by an extensive wind tunnel test program, which substantiates the effectiveness of this class of damper in suppressing both vortex resonance and galloping type of instabilities. Simplicity of design, ease of implementation, minimal maintenance, reliability as well as high efficiency make such liquid dampers quite attractive for real-life applications.

An efficient liquid sloshing damper for control of wind-induced instabilities

Bluff structures in the form of tall buildings, smokestacks, control towers, bridges, etc. are susceptible to vortex resonance and galloping type of instabilities. One approach to vibration control of such systems is through energy dissipation using sloshing liquid dampers. This paper focuses on enhancing the energy dissipation efficiency of a rectangular liquid damper through introduction of two-dimensional wedge-shaped obstacles. The investigation has two phases. To begin with, a parametric free vibration study aimed at optimization of the obstacles’ geometry is undertaken to arrive at configurations promising increased damping ratio and hence higher energy dissipation. The study is complemented by a wind tunnel test program, which substantiates the effectiveness of this class of dampers in regulating both vortex resonance and galloping type of instabilities. Simplicity of design, implementation with ease, minimal maintenance, reliability as well as high efficiency make such liquid dampers quite attractive for real-life applications.

Finite element formulation for the analysis of turbulent wind flow passing bluff structures using the RNG k− ε model

This paper presents a stable finite element formulation to predict behaviors of high-speed wind passing bluff structures using the Reynolds averaged Navier–Stokes equation and the k– ε model. To incorporate the k– ε model with the finite element framework, a stable and accurate solution strategy is proposed. The streamline-upwind/Petrov–Galerkin scheme is adopted to stabilize the Reynolds averaged Navier–Stokes equation as well as the k– ε equations. The re-normalization group k– ε model is employed to reduce the turbulence over-production around the stagnation points on the upwind side of structures. Detailed discussions on the flow behaviors around the bluff structures are made through examples of wind flows passing a square cylinder and actual bridge sections to validate the proposed formulation. It is shown that the periodicity and magnitude of unsteady forces acting on the square cylinder are well predicted. Aerodynamic forces acting on bridge girder sections with complex geometry are presented for high-speed wind with the Reynolds number over 10 7, and compared with experimental results.

Mapping and Synthesis of Random Pressure Fields

A treatment of the mapping and synthesis of random pressure fields is presented that highlights the concepts of point, spatially and/or temporally averaged random pressure fluctuations in the context of local or integral aerodynamic loads. Space-time averaging techniques are examined that provide promising practical means to measure local random pressure fields acting on bluff structures exposed to complex flow fields and to synthesize their integral effects. Pneumatic manifolding devices utilizing a discrete or distributed matrix of pressure taps facilitate both uniform and nonuniform local spatial averaging. Alternatively, a continuous spatial averaging device utilizing a piezopolymer strip permits a similar local averaging provision. Uniform local averaging permits the evaluation of normal or lateral loads on prismatic structure, while, nonuniform local averaging facilitates expedient measurement of torsional and lateral aerodynamic loads on prismatic and cylindrical members, respectively. Synthesis of locally averaged random fields permits evaluation of mode-generalized aerodynamic loads. The design of pneumatic manifolding devices and their calibration procedures are presented. An example of torsional aerodynamic loads on prismatic structures exposed to simulated atmospheric flow conditions in boundary layer wind tunnels is presented.

Vortex Shedding From Bluff Bodies in a Shear Flow: A Review

This paper examines the effects of velocity shear on vortex shedding from stationary and vibrating bluff bodies. Experiments with circular cylindrical bodies and other cross sections such as D-section cylinders and rectangular cylinders, which were limited to conditions with length/diameter ratios less than L/D = 15 to 20, have shown that the spanwise cellular structure of the vortex shedding is dependent upon end conditions. The vortex shedding also is influenced strongly by the shear flow steepness parameter β¯ which is based upon the incident flow velocity gradient. Experimental evidence is available to show that moderate shear levels of practical importance (β¯∼0.01 to 0.015) do not appreciably decrease the probability of occurrence of vortex-excited oscillations for flexible structures and cables. The effects of incident shear on vortex shedding from stationary and vibrating bluff structures in both fluid media should be investigated further for long cylinders which have minimal end boundary effects. More definitive bounds for and details of this fluid-structure interaction are needed for applications in the wind engineering design of buildings and structures, and in the design of marine structures and cable systems.

Aerodynamic forces on bluff structures in a wake

An experimental investigation into mean aerodynamic forces on a cylindrical structure in a wake of another parallel cylinder is carried out. Measurements of the aerodynamic response were taken for various configurations of streamwise and transverse spacings over a range of 12 000–73 000 of subcritical Reynolds number. On the basis of the geometric parameters and the flow Reynolds number dealt with in this study, empirical correlations of the normalized symmetric variations of drag coefficients as well as of the anti-symmetric behaviour of lift coefficients are presented. The surface pressure distributions lead to additional information about the roles of the main parameters involved in this complex flow situation. The data presented yield further information about the effects of turbulent shear flows on bluff structures and the formation of associated wakes.

Universal Similarity in the Wakes of Stationary and Vibrating Bluff Structures

A universal wake Strouhal number, St* =fsd′/Ub, has been proposed and is based upon the Strouhal frequency fs of the incident flow, the measured wake width d′ at the end of the vortex formation region, and the mean velocity Ub at the edge of the separated boundary layer. This universal parameter collapses these characteristic parameters for bluff bodies onto a single curve for wake Reynolds numbers between Re* = 100 and 107. The pressure drag, vortex shedding frequency and base pressure are related through an inverse dependence between St* and a wake drag coefficient CD* = CD/(d′/d) K2, where K = (1−Cpb)1/2. The product St* CD* is equal to a constant value of 0.073 ± 0.005 for Re* in this same range of Reynolds numbers.

On the aerodynamic mechanism of torsional flutter of bluff structures

This paper is concerned with theoretical and experimental investigations on the aerodynamic mechanism of torsional flutter of bluff structures. In the experiment, measurements were made of the unsteady aerodynamic lifts and moments acting on two rectangular bar models which were forced to oscillate either in a torsional or in a heaving mode in a uniform wind tunnel flow. It is found that the fluid memory effect is, in marked contrast to the situation for transverse galloping, responsible for the onset of torsional flutter of bluff structures. The effect of the pivotal position on torsional flutter of bluff structures is also investigated.

An analysis of binary flutter of bridge deck sections

This paper is concerned with an analytical and experimental study of binary flutter of bridge deck sections. A set of analytical formulas giving the frequency and rate of growth of oscillation, the position of the equivalent center of rotation and the phase difference between bending and torsion near the critical flutter point is presented. The formulas provide an analytical basis for the previously proposed method of classification of binary flutter of bluff structures. The results of wind tunnel experiments on models with simple geometrical shapes confirm that the present formulas are applicable to a variety of structures ranging from a flat plate to much more bluff bridge deck sections.

Regular drag fluctuations due to air flow normal to a plate-type structure

A model structure resembling a flat plate (or a tall prestige building) was tested in a low-turbulence wind tunnel. With flow normal to the façade the model oscillated vigorously in the flow direction when passing through a “critical” velocity range. A sufficient minimum of experimental circumstances are reported to facilitate future search for this “Big Foot” phenomenon that should interest all those who assume that the dynamic drag forces of bluff structures are soley due to the turbulent wind field.

Binary Flutter of Suspension Bridge Deck Sections

Wind tunnel experiments on models of suspension bridge deck sections with plate girders of various heights are made to show that binary flutter of suspension bridge deck sections can be classified into three types which have different mechanisms of excitation; i.e., the classical type flutter, the single-degree-of-freedom type, and the intermediate type. Analyses on an energy equation of bluff structures with many degrees-of-freedom are also given to establish the theoretical basis of the classification of binary flutter. Although the geometry of the deck section is found to be the most dominant parameter that influences the type of flutter, it may also depend on various other parameters. The uncoupled frequency ratio is chosen, among others, and its effects on the three types of binary flutter are investigated.