Benchmark on the Aerodynamics of a Rectangular 5:1 Cylinder (BARC)

Flows Around Bodies

Underlying Flow Regime 2-15

Description

Introduction

The Benchmark on the Aerodynamics of a Rectangular 5:1 Cylinder, BARC, is aimed at establishing a platform for discussion among scientists working on bluff body aerodynamics, and in particular it concerns the analysis of the turbulent, separated flow around an elongated rectangular cylinder. The characteristics of the flow field around rectangular bodies is of great interest both for fundamental research and for applications. From the fundamental research point of view, {in spite of the simple and nominally two-dimensional geometry, the flow over an elongated rectangular cylinder at high Reynolds numbers is highly complex, being three dimensional, turbulent and characterized by unsteady flow separation and reattachment. On the other hand, thanks to the simple geometry, a detailed analysis of the flow dynamics can be carried out, and different patterns, which can also be found when dealing with more complex geometries, can be identified}. As for applications, this benchmark problem provides useful information on the aerodynamics of a wide range of bluff bodies of interest in civil engineering (e.g. long-span bridge decks or high-rise buildings) as well as in other engineering areas. The 5:1 aspect ratio was chosen because it is characterized by shear-layers detaching at the upstream cylinder corners and reattaching on the cylinder side rather close to the downstream corners. This leads to a complex dynamics and topology of the flow on the cylinder side, which adds to the vortex shedding from the rear corners and to the complex unsteady dynamics of the wake.

The BARC is a blind benchmark, which has not adopted a single set of measurements as a reference at its launching. In this sense, the BARC differs, for instance, from the pioneering benchmark on the aerodynamics of the square cylinder [see for the review of the obtained results ‌49, 50, 68], included since the Nineties in the ERCOFTAC classic collection database [‌15] and now in the ERCOFTAC QNET-CFD Knowledge Base Wiki [‌16]. Indeed, this benchmark used as a reference the measurements by [‌24]. Coherently with the BARC aims, the lack of a reference set of measurement is intended to:

put the benchmark conditions closer to the common situation in many engineering flow problems;
collect as much as possible of experimental and numerical data produced by different groups, in different facilities or by means of different numerical models and codes, but all within a common set-up as a-priori specified;
clearly describe the set-up uncertainties and the measure or modelling errors which affect both wind tunnel tests and computational simulations;
characterize the impact of these uncertainties on the measurements/predictions of the main flow quantities and, when possible, single out and analyse the effects of different set-up parameters;
assess the reliability and the dispersion of the measurements or of the numerical predictions of different quantities of practical interest
make available at its maturity a statistic database, which is expected to be further enriched and updated by new realisations;
give information useful to assess the possibility of developing integrated procedures relying on both wind tunnel and computational outcomes aimed at improving the reliability of the results, and, eventually, to develop Best Practice Advice for wind tunnel tests and computational simulations.

BARC was launched in 2008 with the support of ANIV (Italian National Association for Wind Engineering), IAWE (International Association for Wind Engineering) and ERCOFTAC (European Research Community On Flow, Turbulence And Combustion). The benchmark was announced first at the VI Colloquium on Bluff Body Aerodynamics and Applications (BBAA VI) and on a few international journals. A web site was also made available (http://www.aniv-iawe.org/barc), which provides all the details on the computational and experimental set-ups and on the data to be provided. Contribution to BARC could and can be made on a volunteer basis through registration to the website. During the first five years of activity, thematic sessions have been devoted to BARC at the 5th European and African Conference on Wind Engineering (EACWE, 2009, Florence), at the 5th International Symposium on Computational Wind Engineering (CWE, 2010, Chapel Hill) and at the 13th International Conference on Wind Engineering (ICWE, 2011, Amsterdam). The last thematic session {hosted} 9 contributions and a final synopsis and overview. Besides the contributions to the mentioned conference thematic sessions, five journal papers explicitly referring to BARC have been published up to now [‌8, 11, 12, 26, 27]. In particular, an in-progress overview after the first four years of activity is provided in [‌12].

Review of UFR studies and choice of test case

It is well known that two-dimensional (2D) rectangular cylinders are characterized by one single geometric parameter, i.e. the ratio of the alongwind dimension (Breadth) to the crosswind dimension (Depth), ${B/D}$ , which} governs their aerodynamic behaviour (see e.g. [‌32, 60]). Other geometric (e.g. surface roughness, corner sharpness) and flow parameters (e.g. Reynolds number, turbulence intensity and scale) play a major or minor role, depending on the ${B/D}$ value and on their range of variability. For small ${B/D}$ ratios ( ${<2.5}$ ), the flow separates from the upstream corners and does nor reattach to the side faces of the cylinder, with vortex shedding occurring only from the upstream corners. For larger ${B/D}$ ratios, the shear layer impinges on the side faces of the cylinder. For moderate ${B/D}$ ratios (roughly between 2.5 and 3.5), reattachment is intermittent, and vortex shedding still occurs only from the leading edge. For ${B/D}$ ratios greater than 3.5, reattachment is permanent, and vortex shedding occurs from both the leading and the trailing edges. Under this circumstance, the flow patterns depends on ${B/D}$ in a discontinuous fashion. Trailing edge shedding is {influenced by the dynamics of the shear layer detaching at the upstream corners and impinging on the side face downstream of the separation bubble}; the breadth-based Strouhal number is a multiple of ${0.55\div 0.60}$ , depending on the number of vortices simultaneously attached to each face of the cylinder {(see e.g. \citep{stokes1986, nakamura1991})}. For $B/D < 6$ the Strouhal number is $1 \times(0.55 \div 0.60)$, for $6 < B/D < 9$ the Strouhal number is $2 \times (0.55 \div 0.60)$, for $9 < B/D < 12$ it is $3 \times (0.55 \div 0.60)$. It follows that the choice made for the BARC of a $B/D$ ratio of 5 {brings} a completely reattached flow ($B/D > 3.5$) and simultaneously one single attached vortex on each face of the cylinder ($B/D < 6$).

In the following the studies published prior to the BARC announcement are briefly reviewed, in order to provide the context in which the BARC benchmark was launched. The overview is limited to the aerodynamics of rectangular cylinders with $B/D$ ratio equal to 5 {or close to it, i.e. $3.5 \leq B/D \leq 6$}. Bearing in mind that BARC addresses both the wind tunnel and the computational approaches, they are discussed separately.

Contributed by: Luca Bruno, Maria Vittoria Salvetti — Politecnico di Torino, Università di Pisa