Helical Fillets Research Articles

Effects of damping and helical fillets on galloping vibration of stay cable installed with a rectangular lamp

Lamps attached on the stay cable can change the stable circular cross-section of the stay cable, which is easy to cause galloping vibration. Stay cables of the Kuipu bridge in the Fuzhou City of China were equipped with damper and helical fillets. After the installation of rectangular lamps on this bridge, no obvious vibration of the stay cables was observed. This is totally different from another bridge, the Kuimen bridge in the Chongqing City of China, which has been recently reported by the authors. The purpose of this study is to clarify the effects of the helical fillets and the structural damping on galloping vibration. The features of galloping vibration of the stay cable attached to a rectangular lamp were carefully studied, and the effectiveness of damping and helical fillets in reducing the galloping vibration was evaluated. First, the force measurement wind tunnel tests on the segmental test model were carried out, and the lift and drag coefficients of the stay cable attached to the rectangular lamp were measured. The dangerous wind attack angle of galloping vibration was predicted by the galloping force coefficient. Second, 3-dimensional vibration measurement wind tunnel tests were carried out to measure the wind-induced response of the cable-lamp test models. The variation of wind-induced response of stay cable with yaw angle and wind velocity was measured for test models with and without helical fillets. In the test, the features of wind-induced vibration were studied at the damping level of 0.1%, 0.6%, 0.7%, 0.8%, and 1.0%. According to Den Hartog’s theory, the minimum coefficient of galloping force is about −9.8. The experimental results of 3-dimensional vibration measurement wind tunnel tests show that the helical fillets wrapped on the cable surface can enhance the critical wind velocity to a certain extent, but the critical wind velocity is still far lower than the design wind velocity. When the damping ratio of the stay cable is up to 1.0% ( SC = 35.1), the galloping vibration of the stay cable attached to a rectangular lamp on the Kuipu Bridge can be effectively mitigated.

Estimating tension of a prestressed concrete cable-stayed bridge under construction and traffic use conditions using a vision-based system

In cable-stayed bridges, cable is the primary member supporting the load, so cable tension is a very important factor in evaluating the integrity and safety of this type of bridge. The direct method of measuring the force of a cable using a load cell or hydraulic jack, and the indirect method of calculating the tension inversely using the shape condition of the cable and the measured dynamic characteristics are the most frequently used methods of measuring cable tension. In this study, cable tension was estimated using the indirect method. A vision-based system was also applied as a sensor to measure the cable’s dynamic response. In order to improve the recognition of the control point during image analysis, the helical fillet of the cable was distinguished from the cable using mathematical morphology. A distorted image including a vehicle shows inaccurate results in image analysis due to the installation position of the vision-based system. Accordingly, the image including the vehicle was removed by calculating the similarity between the two images. In the cable-stayed bridge under construction, the tension measured in the lift-off test and the tension estimated by applying the vibration method showed an error of less than 3 %. The accuracy of the tension estimated using the vision-based system could be confirmed. In addition, when the tension measured in the existing cable-stayed bridge was compared with the tension measured during construction, the tension of the cable was confirmed to be well-maintained even after about 10 years of use.

Optimization of Multiple Helical Fillets Surface to Suppress Rain-wind Vibration of Stay Cables: A Wind Tunnel Investigation

Aims: To develop a new cable surface for control rain wind induced cable vibration of stay cable. Background: Stay cables are light and vulnerable structures. Therefore, it can be easily excited by wind or rain wind interaction. Stay cables wrapped with a single helical fillet have been proposed so far. However, these countermeasures could prevent cable vibration well, especially in dry conditions. Objective: Therefore, the objective of this research is to develop a new cable surface to control not only RWIV but also dry galloping of stay cable. Methods: A wind tunnel test will be used to investigate the RWIV characteristics and its new countermeasure. Results: First, a rain wind-induced vibration of circular stay cable was reproduced in a wind tunnel environment. The effect of upper and lower water rivulets was examined to understand their role on RWIV better. A rainfall simulator was employed to generate artificial rainfall for two different wind tunnel experiments. Finally, to control the RWIV of stay cables, novel multiple helical surface fillets were used. The upper and lower rain rivulets can play a significant role in energizing the RWIV. It was also found that the multiple helical surface fillets can suppress the cable vibration well both in wet and dry conditions. Conclusion: Multiple helical fillets cable surface could successfully prevent both the RWIV and dry galloping. To fabricate helical fillet cable to control cable vibration, 04 to 12 fillets with sizes ranging from 3mmx7.5mm to 5mmx7.5mm and a pitch of 2.95D-4.78D (D: cable diameter) are the most influential parameters and suggested herewith for practical application.

Effect of cable surface geometry and ice accretion shapes on the aerodynamic behaviour of inclined stay cables

A wind-tunnel study has been conducted to evaluate changes in the aerodynamic behaviour of stay cables of a bridge due to ice accretion from freezing rain and, due to different cable surface geometries. Numerical simulations were performed to predict the three-dimensional ice shapes resulting from freezing rain for a variation of environmental conditions: wind speed, wind direction, air temperature, velocity of precipitation and duration of precipitation. Stay-cable models were fabricated using laser selective sintering for three different surface geometries (double helical fillet, concentric rings, and double helical strakes) with ice formed under the same environmental conditions. Additional models with the helical fillet geometry were fabricated to evaluate the effect of the air temperature at which ice was formed, the effect of wind direction during the precipitation event and the effect of the duration of the precipitation. The aerodynamic drag and lift forces acting on the models were measured for different wind speeds and cable-wind angles. The overall study evaluated the mean aerodynamic force coefficients of ten models in smooth flow, and two of the models were also tested in turbulent flow. The critical Reynolds number regime for the models of stay cable with ice accretion shapes occurred at a lower Reynolds number (below 100,000) than for the un-iced cables and the drag coefficient was less sensitive to increases in Reynolds numbers than the equivalent un-iced cables. These effects were attributed to the combined role of the ice-shape in determining the separation point combined with the higher surface roughness of the ice. An important observation was that for the three surface geometries and the range of test conditions that were considered, the maximum drag coefficients for ice-accreted models ranged from approximately 1.1 to 1.3, and were up to 50% greater than those observed without ice. The model with helical strakes and ice accretion exhibited the highest drag coefficient and exhibited more variation in lift coefficient with changes in Reynolds number than the other two cable surface geometries. The aerodynamic forces of an ice-accreted model followed similar trends in smooth and turbulent flow, but the drag coefficients were greater in turbulent flow. The presence and shape of the ice appeared to have a more dominant effect on the separation behaviour of the models than the presence of turbulence (for an ice-accreted model). The aerodynamic coefficients did not change greatly between the cases where ice was accreted at −0.5°C and −1.5°C due to the similar ice shape and thickness near the separation locations. The ice shape formed at −5°C was thicker but more localized and resulted in drag and lift coefficients that had greater variation with changes in the cable-wind plane. The effect of ice duration from 10 h to 20 h resulted in a increased variation in mean lift coefficient behaviour due to the asymmetric shape that resulted from the ice accretion parameters.

Experimental study on high-mode vortex-induced vibration of stay cable and its aerodynamic countermeasures

In recent years, high-mode vortex-induced vibration (VIV) of stay cables has been observed in some cable-stayed bridges. It can cause undue stress and fatigue in the anchorages at the deck and/or pylons and in the cables themselves. In this study, experimental investigations were performed to characterise and control the high-mode VIV of stay cables by using horizontal- and inclined-rigid-stay-cable models. First, wind tunnel tests using the horizontal-rigid-dimpled-stay-cable model were performed to investigate the VIV characteristics of dimpled stay cables and effective VIV suppression measures. Wind tunnel tests were then conducted on an inclined rigid dimpled cylinder with and without helical fillets to investigate further the VIV characteristics of dimpled stay cables and effective suppression measures. Finally, stay-cable field measurements were made for comparison with the experimental results of the stay-cable model. The results show that stay-cable VIV can occur in cases of low damping, and the VIV of the stay cable can be effectively suppressed by increasing the damping ratio of the stay cable. Moreover, helical fillets with a diameter of d = 2.0 or 3.5 mm, which are usually used to control rain–wind induced vibration of stay cables in cable-stayed bridges, cannot effectively suppress the high-mode VIV responses of stay cables. The inclined-dimpled-stay-cable model with a damping ratio of 0.2 % at inclination angles of 25° and 40° exhibits significant VIV for the cases of wind yaw angles of 0°, 15°, -15°, and -30°. However, helical fillets with d = 10 mm (d = 0.07D, where D is the diameter of the stay cable) and pitch of 12D can effectively suppress the VIV responses of the stay cables.

On the aerodynamic characteristics of inclined stay cable sheaths in the critical flow regime

This paper studies the presence of the bistable flow activity around inclined cable models. It presents results from wind tunnel test on static original High-Density Polyethylene cable covers in a range of Reynolds numbers from the subcritical to the critical regime, inclined at angles of 90°(vertical), 60°and 45°. It has already been shown that, in the critical regime, turbulent transition in the boundary layers around a circular cylinder exhibits bistable behavior at zero inclination, i.e. the boundary layer on either side is intermittently turbulent or laminar, leading to intermittent asymmetry of the flow and resulting aerodynamic loads. The present wind tunnel tests consist in measuring the pressure patterns around inclined circular cylinders using numerous pressure taps. Bifurcation diagrams are created in order to quantify the occurrence of bistability and Proper Orthogonal Decomposition is used to identify the geometrical flow modes that are governed by the bistability. The results show that the bistable phenomenon is only identified in the Reynolds number range corresponding to the first transition between the TrBL0 and TrBL1 regimes. Furthermore, the inclination angle seems to have a significant impact on the energy of the flow mode related to unsteady asymmetric reattachment. This paper also treats the potential sensitivity of the aerodynamic drag and lift forces to the circularity defect of the covers and presents the positive impact of the helical fillet on controlling the bistable activity at a critical inclination angle of 60°.

An experimental study on dynamic ice accretion and its effects on the aerodynamic characteristics of stay cables with and without helical fillets

An experimental investigation was conducted to examine the effects of helical fillets on dynamic ice accretion process over the surfaces of bridge stay cables and evaluate its effects on the aerodynamic characteristics of the stay cables under both dry rime and wet glaze icing conditions. The experimental study was performed in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT). Four bridge stay cable models, including a standard plain cable model and three helical filleted cable models of different helical pitch lengths, were used for the experimental investigation. During the experiment, in addition to using a high-speed imaging system to record the dynamic ice accretion process over the cable surfaces, a Digital Image Projection (DIP) based 3D scanning system was also utilized to quantify the 3D shapes of the ice structures accreted on the test models. While a high-resolution digital Particle Image Velocimetry (PIV) system was used to characterize the wake flows behind the cable models during the ice accreting process, the time variations of the aerodynamic drag forces acting on the test models were also measured by using a pair of force/moment transducers mounted at two ends of the cable models. It was found that, under the rime icing condition, the helical filleted cable models accreted more ice structures than the standard plain cable model. However, the helical filleted cable models were found to have less ice accretion under the wet glaze icing condition. The pitch length of the helical fillets was also found to affect the ice accretion process substantially. Under the rime icing condition, while the aerodynamic drag forces acting on the cable models were found to decrease continuously with more rime ice accreting over the cable surfaces, the drag reduction due to the rime ice accretion was found to be less obvious for the helical filleted cable models, in comparison with that obtained for the standard plain cable model. Under the glaze icing condition, the aerodynamic drag forces acting on the cable models were found to decrease quickly at the initial stage of the glaze icing process, and then increase gradually with the increasing ice accretion time at the later stage of the ice accreting process. PIV flow field measurements were correlated with the force measurement data to elucidate the underlying physics for a better understanding of the variation characteristics of the aerodynamic forces acting on the cable models under different icing conditions.

An experimental study on the dynamic ice accretion processes on bridge cables with different surface modifications

An experimental study was conducted to investigate the dynamic ice accretion processes on bridge cables with different surface modifications (i.e., 1. Standard plain, 2. Pattern-indented surface, and 3. Helical fillets). An Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) was used to generate two typical icing conditions (i.e., dry rime icing vs. wet glaze icing). During the experiments, while a high-speed imaging system was used to capture the transient behaviors of surface water transport and dynamic ice accretion processes over the surfaces of the cable models, a high-accuracy force measurement system was utilized to quantify the dynamic aerodynamic loadings acting on the cable models under different icing conditions. It was found that the addition of surface features (i.e., pattern-indented surface and helical fillets) could dramatically influence the dynamic ice accretion process and the final topology of the ice structures accreted on the cable surfaces. While the ice accretion on the standard plain cable and the cable with helical fillets was found to reduce the drag forces by about 25%, the ice accretion on the pattern-indented cable model would induce about 30% greater aerodynamic drag force acting on the cable model.

An investigation of the mechanisms causing large-amplitude wind-induced vibrations in stay cables using unsteady surface pressure measurements

Dry galloping has been observed on in-service bridges and has been reproduced in several wind tunnel experiments of inclined stay cables. In certain cases, the large-amplitude vibrations caused by dry galloping could not be mitigated with damping levels specified by the Post Tensioning Institute (PTI). Wind tunnel investigations were conducted for IHI Corporation on a 1:1 scale sectional model of an inclined bridge stay cable in the 3 m × 6 m Wind Tunnel at the National Research Council Canada in 2015. The purpose of the investigation was to reproduce large amplitude cable galloping, which had been observed at an existing cable-stayed bridge in Japan. The experiment was designed to investigate the influence of different damping levels and the ability of a 5 mm-diameter helical fillet to mitigate dry cable vibrations. The current work identified several important factors that contributed to dry galloping and large-amplitude cable motions. It was shown that the physical mechanisms leading to the onset of large-amplitude motion in the critical and supercritical Reynolds number regimes are distinct. In the critical Reynolds number regime, the drop in drag, increase in lift, and fluctuations in the laminar separation bubbles along the cable destabilize the low pressure lobes, allowing the changes in pressure along the cable to become synchronized with the cable motion in a highly-correlated manner. This type of galloping could not be mitigated with damping greater than that required by the PTI. At high Reynolds numbers, the boundary layer state changed and was associated with the upstream movement of the separation point and an asymmetric pressure distribution, Kármán vortex shedding re-emerged and appeared to combine with low-frequency variations to induce large-amplitude motion. The large-amplitude motion at high Reynolds numbers could be mitigated with additional damping.

The impact of circularity defects on bridge stay cable dry galloping stability

The present work studies the influence of circularity defects, on the aerodynamic behaviour of stay cables of cable-stayed bridges. It focuses on wind tunnel tests on High-Density Polyethylene cable covers with and without helical fillets, in a range of Reynolds numbers from the sub-critical to the critical regime. The paper considers the impact of circularity defects on the aerodynamic stability of cable sheaths by testing various amplitudes of imposed ovalization. The defects are artificially applied on real sheaths whose original cross-sections are close to circular. The experiment consists in measuring surface pressures to investigate how the amplitude of ovalization influences the flow around the sheaths, especially in the critical Reynolds number regime when transition in the boundary layer occurs. The analysis is based on bifurcation diagrams and Proper Orthogonal Decomposition. The investigation demonstrates that important circularity defects can significantly increase the bi-stable nature of the flow around a sheath at the critical regime. Nevertheless, the introduction of a helical fillet de-correlates the flow around the sheath, causing jumps in lift that have different sign along its length.

Aerodynamics of a stay cable with helical fillets - Part II: Fluctuating load and wake characteristics

This paper presents the fluctuating load and wake characteristics of a bridge stay cable model with helical fillets in smooth flow at high Reynolds numbers. Over the Reynolds numbers tested, the frequency content of the lift coefficient in individual pressure tap rings, was dominated by low frequencies. At lower Reynolds numbers, these were caused by single separation bubble instabilities. At smaller spectral magnitudes, a vortex shedding process was observed, with a local Strouhal number dependent on the angular position of the helical fillet nearly normal to flow. For the average over the four rings, the low frequencies cancelled out, and left a high frequency vortex shedding process with a Strouhal number of 0.19 as the prominent load. The Strouhal number was maintained throughout the Reynolds number range tested. In dynamic tests, the pressure distribution was found to periodically rotate relative to the cable circumference at the natural frequency of the cable. Also, when observing the velocity components in the wake, a periodic motion of the wake synchronized with the cable motion was revealed. In static tests, the wake field instantly displaced towards the same side as the lift force generated by the asymmetric pressure distribution during boundary layer instabilities.

Aerodynamics of a stay cable with helical fillets - Part I: Stability and load characteristics

The aerodynamic behaviour of a bridge stay cable with helical fillets in smooth flow at high Reynolds numbers is presented in this paper. The cable response and related sectional load characteristics were studied experimentally on a 1:1 scale cable section model. The studies showed that a cable with helical fillets inclined 60° to the flow could experience large amplitude wind induced vibrations and that the occurrence of vibrations were highly dependent on cable surface irregularities. The ambition is not to explain fully the excitation mechanism, but to present global and local influences of the helical fillets on the flow field. It was revealed that the flow field around the cable shifted between semi-stable transition states which took place when the transition from laminar to turbulent flow propagated from the free shear layers to the boundary layer. The transitions would form locally and spread along the cable axis. The helical fillet appeared to dominate the local flow structures when located at an angular position between 40° and 130° from the stagnation region. In the stagnation and base regions, the surface irregularities appeared to dominate. Furthermore, the helical fillets displaced the mean stagnation line. The application of quasi-steady theory with the measurement data available appeared not to be able to explain the vibrations.

Wind-Tunnel Investigation of the Aerodynamic Performance of Surface-Modification Cables

Abstract The wind-induced vibration of stay cables of cable-stayed bridges, which includes rain-wind-induced vibration (RWIV) and dry galloping (DG), has been studied for a considerable amount of time. In general, mechanical dampers or surface modification are applied to suppress the vibration. In particular, several types of surface-modification cable, including indentation, longitudinally parallel protuberance, helical fillet, and U-shaped grooving, have been developed. Recently, a new type of aerodynamically stable cable with spiral protuberances was developed. It was confirmed that the cable has a low drag force coefficient, like an indented cable, and that it prevented the formation of water rivulets on the cable surface. In this study, the stability for RWIV of this cable was investigated with various flow angles and protuberance dimensions in a wind-tunnel test. It was found that the spiral protuberance cable is aerodynamically stable against both RWIV and DG for all test wind angles. The effects of the protuberance dimensions were also clarified.

Comparison of the aerodynamics of bridge cables with helical fillets and a pattern-indented surface

In this paper, the aerodynamics of bridge cables with helical fillets and a pattern-indented surface are examined. To this end, an extensive wind-tunnel test campaign was undertaken to measure the static force coefficients about the critical Reynolds number region, with varying relative cable-wind angles. The tests confirmed that the pattern-indented tubing exhibits the lowest drag coefficient, reaching its supercritical flow state for Reynolds numbers lower than the other cables tested. For this cable, vortex-shedding was found to be present throughout the supercritical range. The asymmetry of the surface pattern introduce a wind-angle of attack dependency that leads to a prediction of Den Hartog galloping instability. For yawed positions, flow transition was found to be independent of the relative cable-wind angle and therefore only governed by the along-wind flow velocity. The helically filleted cable was found to have a much slower flow transition for near normal flow and relatively large lift force components for the yawed positions. Flow visualizations confirmed the existence of specific flow structures which are often associated with the presence of lower drag or large lift forces. The visualization tests confirmed the presence of an axial flow that was greatly hindered by the presence of the helical fillets.