Several bluff bodies in an airflow, such as rectangular cylinders with moderate side ratio, in particular conditions of mass and damping can experience the interference of vortex-induced vibration ...(VIV) and galloping. This promotes a combined instability, which one may call “unsteady galloping”, with peculiar features and possibly large vibration amplitudes in flow speed ranges where no excitation is predicted by classical theories. The mathematical model proposed between the 70's and the 80's by Prof. Y. Tamura to simulate this phenomenon was considered here for the case study of a two-dimensional rectangular cylinder with a side ratio of 1.5, having the shorter section side perpendicular to the smooth airflow. This wake-oscillator model relies on the linear superposition of the unsteady wake force producing VIV excitation and the quasi-steady force that is responsible for galloping. The model formulation was slightly modified, and the way to determine a crucial parameter was changed, revealing a previously unexplored behavior of the equations. In the present form, the model is able to predict the dynamic response of the rectangular cylinder with a satisfactory qualitative and, to a certain extent, quantitative agreement with the experimental data, although the limitations of the present approach are clearly highlighted in the paper. The mathematical modeling of unsteady galloping and the analysis of the results offer a deep insight into this complicated phenomenon and its nonlinear features. The model also represents a useful engineering tool to estimate the vibration of a structure or structural element for which the interference of VIV and galloping is envisaged.
•Mathematical model for VIV-galloping interference applied to a rectangular cylinder.•An existing model was modified, and its dynamical behavior extensively explored.•Satisfactory agreement with the experiments if a crucial parameter is correctly set.•The nonlinear wake-oscillator model offers an interpretation of the phenomenon.•Useful engineering tool to estimate the VIV-galloping response of slender structures.
The phenomenon of interference between vortex-induced vibration (VIV) and galloping in the transverse degree of freedom was studied in the wind tunnel in the case of a spring-mounted slender ...rectangular cylinder with a side ratio of 1.5 having the short side perpendicular to the flow. The tests were carried out in a wide Scruton number range, starting from low values and increasing it in small steps by using eddy-current viscous dampers. This study helped understanding the dynamics of the interaction between the two excitation mechanisms and clearly highlighted the transition through four regimes of VIV-galloping interference. It was found that a high value of the mass-damping parameter is required to decouple the ranges of excitation of vortex-induced vibration and galloping completely, and for the quasi-steady theory to predict the galloping critical wind speed correctly. This conclusion is also relevant from the engineering point of view, as it means that structures and structural elements with ordinary mass-damping properties can exhibit sustained vibrations in flow speed ranges where no excitation is predicted by classical theories of vortex-induced vibration and galloping. Although most of the experimental tests were conducted in smooth flow at zero angle of attack, the paper also discusses the sensitivity of the results to a small variation of the mean flow incidence and to the presence of a low-intensity free-stream turbulence.
•The interference of VIV and galloping was studied in the wind tunnel for a rectangular cylinder.•High Scruton number values are necessary for decoupling the two excitation mechanisms.•The transition with the Scruton number through various regimes of interference is described.•The applicability and the limits of the quasi-steady theory is discussed.•Sensitivity study to low free-stream turbulence and small angle of attack.•Low free-stream turbulence is found to boost the interference of VIV and galloping.
Seismic assessment of existing masonry structures requires a numerical model able to both reproduce their nonlinear behaviour and account for the different sources of uncertainties; the latter have ...to be dealt with since the unavoidable lack of knowledge on the input parameters (material properties, geometry, boundary conditions, etc.) has a relevant effect on the reliability of the seismic response provided by the numerical approaches. The steadily increasing necessity of combining different sources of information/knowledge makes the Bayesian approach an appealing technique, not yet fully investigated for historic masonry constructions. In fact, while the Bayesian paradigm is currently employed to solve inverse problems in several sectors of the structural engineering domain, only a few studies pay attention to its effectiveness for parameter identification on historic masonry structures. This study combines a Bayesian framework with probabilistic structural analyses: starting from the Bayesian finite element model updating by using experimental data it provides the definition of robust seismic fragility curves for non-isolated masonry towers. A comparison between this method and the standard deterministic approach illustrates its benefits. This article is part of the theme issue 'Environmental loading of heritage structures'.
•A design procedure for mdof aeroelastic modelling is presented and assessed.•Second order mode shapes response has been added to CAARC standard tall building.•A novel wind tunnel test campaign is ...presented and critically analyzed.•The comparison between the loads of the aerodynamic and aeoroelastic model is presented.
A novel approach to full-aeroelastic model design is presented, considering continuous structures and coupled modes. Its validity is proven by building a full-aeroelastic CAARC standard tall building model featuring 12-degrees-of-freedom at 1:360 scale. A complete description of its design, manufacturing, tuning, and dynamic identification is reported. The model is tested in a novel wind-tunnel experimental campaign performed at the CRIACIV facility (Prato, Italy). A comparative study of aerodynamic and full-aeroelastic response in terms of base moment and accelerations shows that the aerodynamic model tends to overpredict the response along the broader side of the model. The contribution of second modes is found to be relevant for induced acceleration at the rooftop.
This paper deals with the phenomenon of interference between vortex-induced vibration and transverse galloping, which occurs in the case of low-damped rectangular cylinders with a side ratio ...comprised between about 0.75 and 3. The results of a wide experimental campaign carried out in the wind tunnel are discussed, focusing on the key role played by the mass-damping parameter (Scruton number) of the oscillating body. It was found that a high value of the Scruton number is required to decouple completely the ranges of excitation due to vortex-induced vibration and galloping, and for the quasi-steady theory to predict correctly the galloping critical wind speed. Moreover, the problem is analytically addressed through a nonlinear wake-oscillator model, which is able to reproduce several features of the complicated phenomenon, provided that a crucial parameter is correctly calibrated.
This paper deals with vortex-induced vibrations of a rectangular 4:1 cylinder with a twofold purpose. First, it provides experimental results as a benchmark for checking the predictions of ...mathematical models and computational techniques. In particular, a wide wind tunnel campaign on a sectional model in smooth flow yielded the oscillation amplitudes in the lock-in range for several Scruton numbers. The dominant frequency of the velocity fluctuations in the wake of the oscillating body was measured. The second purpose of the paper is the improvement of a current approach for assessing vortex-induced vibrations of bridge decks. For this reason, prior to starting the analysis an overview of the existing mathematical models is reported. Afterwards, it is shown through wind tunnel tests that the single-degree-of-freedom model developed by Scanlan at the beginning of the 1980s is unsuitable to predict the VIV response of bridge decks for Scruton numbers different from that at which the model parameters were identified. Finally, an improved version of the model is proposed, allowing to approximate accurately the limit-cycle amplitudes for the entire range of Scruton numbers tested. The identification of the aeroelastic parameters of the improved version of the model requires only three decay-to-resonance tests for three different Scruton numbers. From the engineering point of view, this represents an important advantage, especially when the mass and the damping of the structure are uncertain or dampers have to be designed, as it makes the assessment of the sensitivity to vortex-induced vibration of bridge deck cross sections less time-consuming and expensive. In the future, it will be necessary to verify that the functional form postulated for the aeroelastic parameters of the model is valid not only for elongated rectangular cylinders but also for bridge deck cross sections.
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•Experimental results as a benchmark to check the predictions of VIV models and CFD methods.•The Scanlan model is unsuitable to predict the VIV response of bridge decks when the Scruton number is varied.•An engineering model is proposed, allowing us to approximate accurately the Griffin plot.•The calibration of the model requires only three tests for different Scruton numbers.•This model may reduce the cost of VIV assessment of bridge decks.
The aerodynamics of a stationary two-dimensional cylinder with rectangular 5:1 cross section is the object of an international experimental and numerical benchmark study (BARC). The unsteady flow ...past such a bluff body is governed by the impinging shear-layer instability mechanism, which is also responsible for potential flow-induced vibrations. This paper discusses the characteristics of the high-Reynolds-number unsteady flow over the cylinder, investigated in the wind tunnel through pressure and force measurements on a sectional model in smooth and turbulent flows. In addition, hot-wire anemometry measurements of the velocity in the wake of the cylinder were carried out in smooth flow. The study is particularly focused on understanding the influence of free-stream turbulence and angle of attack on the aerodynamic behavior of the body. Key features are highlighted, such as a slight Reynolds-number dependence of force coefficients and Strouhal number in smooth flow, or the turbulence-induced variation of the length of the separation bubbles along the lateral sides of the cylinder. Incoming turbulence is also found to affect the vortex-shedding mechanism of impinging shear-layer instability, promoting the switch from a dominant mode to another. Finally, the wake measurements described some vortex-shedding features, confirming a slight Reynolds-number dependence of the Strouhal frequency.
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•The aerodynamics of a benchmark rectangular 5:1 cylinder was studied in the wind tunnel.•Pressure, force and wake measurements in smooth and various turbulent flows.•Reynolds-number effects were observed, in particular for small angles of attack.•Pressure data revealed the earlier reattachment of the shear layers in turbulent flow.•Free-stream turbulence promotes the switch between vortex-shedding modes.
Slender structures with a bluff non-axisymmetric cross-section are prone to both vortex-induced vibration (VIV) and transverse galloping. When the mass-damping parameter of the system is low, the two ...phenomena can interfere giving rise to a peculiar type of instability, for which the quasi-steady theory does not apply, and therefore which one may call “unsteady galloping”. Since for large structures (such as high-rise towers or bridge pylons) this phenomenon seems to be potentially an issue, rather than the quasi-steady galloping, it is particularly important to verify whether the former also occurs in realistic turbulent wind flows, and to understand its specific features. With this aim in mind, static and dynamic wind tunnel tests have been carried out on a two-dimensional rectangular cylinder with a side ratio of 1.5 (having the short side perpendicular to the flow) immersed in various grid-induced homogeneous isotropic turbulent flows. From a quasi-steady perspective, the static tests showed the proneness to galloping instability of the considered cross-section even in highly turbulent flow, though strongly dependent on the integral length scale of turbulence. In addition, they revealed an attenuation of the strength of vortex shedding, but suggested an increased tendency of VIV and galloping to interfere as compared to the smooth-flow case. The dynamic tests confirmed this tendency and highlighted a complicated behavior of the model in turbulent flow, with some features that still remain unexplained. In particular, even larger values of the mass-damping parameter of the system are necessary for the quasi-steady theory to be able to predict correctly the galloping instability threshold. Another important result is that the integral scale of turbulence was found to play a key role also in the unsteady galloping behavior of the considered rectangular cylinder.
•Static and aeroelastic tests on a rectangular cylinder in various turbulent flows.•High Scruton numbers are required for the quasi-steady theory to apply.•The oscillations always appear slightly after the vortex-resonance flow speed.•Complicated interplay of galloping behavior, unsteady wake and incoming turbulence.•The turbulence length scale plays a key role in the galloping instability.
•Unlike any common structural steelwork section, no data are available in the literature for RSW member sections•The results highlight the need for wind tunnel tests on RSW member ...sections•Conservative values of the aerodynamic coefficients are provided for preliminary wind load estimations or for temporary bracings design•The results in the present study are not affected by the Reynolds number mismatch since the models are tested at real scale
Rack-supported warehouses (RSWs) represent a modern typology of storage racks. In this system, cladding panel weight and corresponding applied loads, such as wind or snow load, are supported by storage racks, in addition to pallet load and seismic action. While this structural system allows for reducing the amount of structural steel, the uprights and beams, composing each rack, are directly exposed to the wind during the earliest erection phases. This load condition may govern the design of the uprights or that of temporary bracings. Wind load estimation requires the knowledge of the aerodynamic coefficients of each structural member section, for any angles of wind incidence. Unlike any common structural steelwork section, no data are available in the literature for RSW member sections. The current work represents a first step to cover this lack in the literature by reporting the results of an extensive wind tunnel campaign carried out on several portions of uprights and beams commonly designed and produced for RSWs. The results highlight the need for wind tunnel tests on RSW member sections when producers can no longer afford an overestimation of the wind load. In addition, conservative values of the aerodynamic coefficients are provided for preliminary wind load estimations or temporary bracings design. Empirical relationships for the aerodynamic coefficients, changing with equivalent side ratio, are also reported. Finally, design recommendations are provided by highlighting a critical structural configuration during the early erection phases of RSWs that govern the design of the uprights or temporary bracings. A worked example is then developed to clarify the application of the present results in the definition of wind loads.
Seismic assessment of existing masonry structures requires a numerical model able to both reproduce their nonlinear behaviour and account for the different sources of uncertainties; the latter have ...to be dealt with since the unavoidable lack of knowledge on the input parameters (material properties, geometry, boundary conditions, etc.) has a relevant effect on the reliability of the seismic response provided by the numerical approaches. The steadily increasing necessity of combining different sources of information/knowledge makes the Bayesian approach an appealing technique, not yet fully investigated for historic masonry constructions. In fact, while the Bayesian paradigm is currently employed to solve inverse problems in several sectors of the structural engineering domain, only a few studies pay attention to its effectiveness for parameter identification on historic masonry structures. This study combines a Bayesian framework with probabilistic structural analyses: starting from the Bayesian finite element model updating by using experimental data it provides the definition of robust seismic fragility curves for non-isolated masonry towers. A comparison between this method and the standard deterministic approach illustrates its benefits.
This article is part of the theme issue ‘Environmental loading of heritage structures’.
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