A. Laneville

Turbulence and blockage effects on two dimensional rectangular cylinders

Abstract This paper presents a review of some measurements of turbulence scale effect on the mean drags of rectangular cylinders. In the case of square cylinders exposed to a fixed turbulence intensity, two sets of data indicate a turbulence scale effect on the mean base pressure (∼ 25% reduction) for a turbulence scale twenty fold the cylinder frontal dimension. For smaller scales, the square cylinders mean drag appears influenced principally by the turbulence intensity. This paper also reviews some of the measurements of solid blockage effects on the mean drags and base pressures of two dimensional rectangular cylinders. The different methods are usually empirical and based on slopes of data that differ from one wind tunnel to another. An empirical method taking into account the models aspect ratio is proposed. Its application to smooth flow data (6 different sets) has given good corrected results. The method has been applied to data of models exposed to a turbulent flow and flows with incidence.

Vortex-induced oscillations of two flexible circular cylinders coupled mechanically

Abstract This paper deals with an experimental investigation of the effect of mechanical coupling on the vortex-induced vibration of a twin-cylinder bundle. The cylinders are in tandem and staggered arrangements and vibrate in-phase and out-of-phase. The separation between the cylinders ranges between 10 and 25 diameters while the stagger angle varies between 0° and 20° for a typical separation of 13 diameters. For each relative position between the cylinders the dynamic response of the in-phase vibration is different from the out-of-phase vibration. The response greatly dictated by the results of the aerodynamic coupling investigated previously by the authors. A hysteresis phenomenon with discontinuities is observed in the response of the bundle and is explained in the light of the results of previous studies.

Effect of Ice and Snow on the Dynamics of~Transmission Line Conductors

Transmission line conductors exposed to natural winds are subjected to windinduced vibrations, including aeolian vibrations and wake-induced oscillations (in the latter case, only on bundles). These vibrations may impair the reliability and lifespan of conductors and their accessories, but can be attenuated to non-damaging levels using damping devices and/or spacers or spacer-dampers when required. However, when ice precipitations accrete on the conductors, the severity of the two phenomena can increase dramatically. Moreover, aeolian vibration of conductors coated with ice may then occur in frequency ranges outside damper capabilities. Galloping, another wind-induced instability, also occurs on ice-accreted conductors and may result in spectacular conductor displacements. Galloping is a movement-induced excitation and its mechanism will be described on both single and bundle conductors. This review of the combined effects of ice or wet snow and wind on overhead power lines includes serious loadings and instabilities such as the rebound of conductors following ice drop and the rolling of bundles due to accumulated glaze or rime ice and wind action in exposed spans or in elevated mountain routes. Direct observation of the effects of icing on overhead power lines is an absolutely necessary preliminary to any understanding of the many phenomena that can occur. Without such observations, unrealistic assumptions about the mechanisms in presence could be made and lead to incorrect or incomplete solutions based on analysis. Since the number of variables such as the overhead line designs, the thickness and density of the ice or snow deposits, and the wind loadings to name a few, is large, an overview of the state of the art of these different phenomena will be presented here. Measurements conducted on transmission lines or test lines with natural or artificial ice are included to better understand and quantify those phenomena.

The aerodynamic forces for a Darrieus rotor with straight blades: Wind tunnel measurements

Abstract Wind tunnel measurements of the blade forces for a straight blades Darrieus rotor have indicated that dynamic stalling was present for values of λ less than 3.5. The data obtained are generally in agreement with those of the literature except for some content at a higher frequency.

Phase Angle in Transient Regimes of Vortex-Induced Vibration: An Experimental Determination

This article deals with an experimental determination of the relative phase angle between the shedding of vortices and motion in the case of the Aeolian vibrations of a flexible circular tube. The objective is to compare and determine if the phase value of a transient regime differs from that measured in a steady vibrating state. For each mode of vortex shedding (Von Karman, 2S and 2P), the results show that the value of the phase angle in the unsteady and steady regimes is relatively similar if an appropriate dependency is selected: in the cases of the 2P and 2S modes of vortex shedding, this dependency is either the dimensionless amplitude or the ratio of the velocity of the structure motion to the oncoming flow velocity; in the case of the Von Karman regime, at the onset of the instability, the usual reduced velocity is a better dependent variable. The analysis of jumps occurring in the instruments output reveals a boundary between the Von Karman and the 2P modes of vortex shedding.Copyright

Blockage effects in smooth and turbulent flows: The case of two-dimensional rectangular cylinders

Abstract This paper is concerned with an empirical method that corrects pressure and drag coefficients experienced by 2-D rectangular cylinders exposed to smooth and turbulent flows. The method takes into account the aspect ratio of the cylinder, its solidity, its afterbody length as well as the intensity of turbulence. The method is sufficiently general to correct adequately four different sets of data in smooth flow. The hypothesis of invariance under constraint is not verified by the data.

Vortex-Induced Vibrations of a Flexible Cylinder in a Slowly Varying Flow: Experimental Results

This paper deals with a wind tunnel simulation of the vortex-induced vibrations of a long flexible cylinder in cross-flow when the flow velocity varies periodically with different low frequencies and different flow velocity amplitude. The experimental set-up consists of a flexible tube suspended at the nodes of its first free-free mode of vibration. In order to modulate the wind velocity, the fan rpm of the wind tunnel is controlled: this simulation allows the excursions and incursions in the region of lock-in as well as the periodical crossing of the critical curve separating the 2S and 2P modes of vortex shedding. The additional objective of the simulation is to reproduce more closely the behavior of aerial conductor in the fields and exposed to large scale and low frequency “turbulence”. The results show that the amplitude of vibrations of the flexible tube is modulated with the frequency of the periodic wind fluctuations: it can range from a simple beating to chaotic fluctuations superimposed to a mean. The amplitude of vibrations, when compared to the case of steady wind velocity, may decrease or increase according to the range of the wind mean velocity. Modulation taking different shapes is observed: it may adopt a wave form made of “sharp” peaks or “smooth” periodic oscillations or a combination of the two; sometimes it may be of chaotic form. A link is established between the “sharp” peaks, the occurrence of bifurcations, the presence of the two modes of vortex shedding and the critical curve. The instantaneous amplitude of vibrations is observed to exceed that measured under steady flow conditions.© 2002 ASME

An analytical model of the ovalling oscillations of clamped-free and clamped-clamped cylindrical shells in cross-flow

This paper deals with an analytical model of the ovalling phenomenon or the aeroelastic oscillations of clamped-free and clamped-clamped cylindrical shells. Shells of finite length are exposed to a steady two-dimensional incompressible and irrotational cross-flow, with their wall assumed homogeneous, isotropic and elastic. The governing equations of motion, based on Donnells theory, are coupled to an aerodynamic force taking into account the Reynolds number. The model is tested with 18 shells (34 tests, of which 14 had clamped-free conditions and 20 had clamped-clamped conditions) and its predictions of the ovalling onset velocity are in good agreement with the experimental results.

Galloping Experimental Analysis of Conductors Covered With a D-Section on a High-Voltage Overhead Test Line

This paper presents galloping measurements conducted on a high-voltage overhead test line equipped with three single conductors as well as a second configuration of three conductors interconnected with interphase spacers. The test line consists of three suspension spans and two dead-end spans. To induce galloping amplitudes typical of those encountered as ice is formed on the conductor, a D-section jacketing was added to the conductors in the middle span. Conductor displacement was monitored using two accelerometers on each conductor and two video cameras. Different factors potentially influencing galloping amplitude, such as the mode that is excited, conductor tension, D-section mass per unit length and the wind’s angle of attack have been looked at.Copyright