Sectional model investigation at high Reynolds number for a super tall building

https://doi.org/10.1016/j.jweia.2012.03.028Get rights and content

Abstract

This paper presents the results of an experimental study of the influence of Reynolds number on the vortex-shedding excitation of a super tall building with a quasi-circular cross-section, uniform with height. Sectional model tests were carried out on a large model in a large wind tunnel at high wind speeds to reach Reynolds number in excess of 3 million based on the model diameter. Stationary model tests and free-to-respond model tests were carried out for various azimuth angles, in smooth and turbulent flow. The study revealed that for this building cross-section, the dynamic excitation due to vortex shedding was equally important at high Reynolds numbers when compared to experiments performed in the sub-critical Reynolds number regime.

Highlights

► Rare study of the aerodynamics of a tall building at Reynolds (Re) numbers higher than 1 million. ► Wind tunnel tests at low and high Re numbers are compared in two different test facilities on a building. ► Vortex-shedding excitation is not necessary lower in the supercritical Re number regime. ► Openings can mitigate across-wind excitation but can also make the excitation stronger for some directions. ► Experiments on a large sectional model suspended on springs in a large wind tunnel are described.

Introduction

To enhance the reliability of predicting the aerodynamic behaviour of a super tall building to vortex-shedding excitation, wind-tunnel tests at high Reynolds number were carried out on a large model. The extreme height of the building would make it the tallest structure in the world by more than 350 m. Its slenderness and the circular features of its cross-section motivated the need to initiate an investigation into the effects of Reynolds number on the aerodynamic characteristics of the structure.

To predict the behaviour of a building to the dynamic actions of wind and to provide wind loads for structural design, wind-tunnel tests are generally carried out in turbulent flow on scaled models of the entire structure at relatively low Reynolds numbers compared to the actual Reynolds numbers of the prototype structure. The choice of testing at low Reynolds number is related to cost consideration and the limited availability of large wind tunnels. The tests are thus generally carried out in the sub-critical Reynolds number regime on small models. For structures with a circular cross-section, it is generally considered conservative to predict the across-wind response of a structure that would be in the super-critical regime at full scale based on model-scale experiments in the sub-critical regime.

As described in detail by Zdravkovich (1997), the fluctuating lift force coefficient (CL in Fig. 1) has been reported to be of lower magnitude in the super-critical regime than in the sub-critical regime for a smooth circular cylinder in smooth flow. The dynamic excitation due to the shedding of vortices in the wake in the super-critical regime is thought to be lower than in the sub-critical regime, hence the generalization that tests on structures with circular cross-section are conservative if done in the sub-critical regime. Note that this generalization is based on a limited number of experiments and has been debated in recent years, e.g. Belloli et al. (2007).

However, for a structure with curved surfaces (but not-entirely circular) does this behaviour also apply? Are the predicted prototype loads based on tests at low Reynolds number overestimated? There is a possibility that considerable savings could be made in structural steel and foundation costs if the dynamic loading due to vortex-shedding excitation could be predicted reliably for the Reynolds number range expected for the prototype structure. The difficulty to predict dynamic loads at large Reynolds number constitutes the largest source of uncertainty of load predictions for the structural design of super-tall buildings. The variations of the vortex-shedding frequency or Strouhal number with the Reynolds number regime is another important source of uncertainty of the predictions based on model scale experiments.

It is with these questions in mind that Rowan Williams Davies and Irwin, Inc. (RWDI) and the National Research Council Canada (NRC) joined forces to study the vortex-shedding excitation on a free-to-respond bi-dimensional (2D) representation of a super tall building for the highest Reynolds number possible in NRC wind tunnels while keeping the Mach number in the incompressible regime, in smooth and turbulent flow. In the same test campaign, a circular cylinder model with similar size and roughness was tested, and the results from the building and cylinder models were compared to results for similar sectional models and models of the entire structure collected at lower Reynolds numbers in RWDI's wind tunnels.

The decision to perform this study of the effects of Reynolds number on a sectional model instead of a full three-dimensional (3D) representation of the building was made with the intention of reaching the highest Reynolds number possible with the wind tunnel test facilities available at the time for this type of experiment. A sectional model study generally allows to reach Reynolds number from two to four time larger than a 3D model study. In the present case, the Reynolds numbers reached during the sectional model tests were from 0.6 to 3.2 million, pushing the experiments well in the super-critical Reynolds number regime and closer to the Reynolds number regime that the full scale structure would experienced, i.e. Reynolds numbers from 13 to 335 million based on the overall building diameter. A 3D model study on a free-to-respond model would not have been able to reach the super-critical Reynolds number regime.

While it is possible to rely on dynamic sectional model tests to predict the response of a super-tall building to across-wind vibrations using, e.g. the methodology described in ESDU 96030 (1998), it was never the intention of the present investigation. The sectional model experiments were carried out to compare the response of the building geometry to vortex-shedding excitation at two distinct Reynolds number regime, the highest regime being identified as the best possible representation of the full-scale conditions. Depending on the outcome of the sectional model study, a complementary study was planned on a large 3D aeroelastic model of the building and would have been used to predict the static and dynamic wind loads on the prototype structure.

This paper presents a summary of the sectional model experiments at high Reynolds numbers and focuses on the variations of the fluctuating drag and lift coefficients for oscillating and stationary sectional models.

Section snippets

Experimental conditions

The super tall building examined in this study is a segmented hollow cylinder, with a uniform diameter of 100 m for the majority of the building height. The cylinder is composed of four quarter-ring buildings, connected at regular intervals by sky-bridges. The building cross-section is shown in Fig. 2. This schematic shows the non-axisymmetric nature of the inner surface of the building, and the 16 triangular protrusions (ribs), equally spaced around the perimeter.

A large sectional model was

Tests on free-to-respond building model

As predicted based on model tests carried out at low Reynolds number by RWDI, the dynamic response of the sectional model showed great sensitivity to wind direction. The weaker excitation was found at a wind direction where the openings of the building were aligned with the flow (0° and 90°). The largest response occurred when the axis of the openings formed an angle of 30° (tested in turbulent flow) or 60° (tested in smooth flow) with the wind direction. Strong vortex-induced oscillations were

Conclusions

Wind-tunnel tests on a large free-to-respond sectional model of a super tall building revealed important vortex-shedding induced vibrations in the post-critical Reynolds-number regime. The across-wind excitation was observed to be larger for the cross-sectional shape of the building for some wind directions than for a smooth or ribbed circular cylinder with similar external diameter at a Reynolds number of 3 million. The across-wind excitation was found to increase as the Reynolds number was

References (6)

  • Belloli, A., Giappino, S., Muggiasca, S., Zasso, A., 2007. Vortex shedding on circular cylinder at critical and...
  • Blackburn, H.M., 2003. Effect of Blockage on Spanwise Correlation in a Circular Cylinder Wake. Technical Note, Monash...
  • R.D. Blevins

    Flow-induced Vibration

    (1977)
There are more references available in the full text version of this article.

Cited by (0)

View full text