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

Flows Around Bodies

Underlying Flow Regime 2-15

Best Practice Advice

Key physics and summary of the results

The flow past a rectangular cylinder of breadth to depth ratio equal to 5 is characterized by shear-layers detaching at the upstream cylinder corners and reattaching on the cylinder side rather close 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. Wind tunnel measurements and computational simulations provided by 10 research teams within the BARC benchmark and obtained from nominally common setups have been collected in a single ensemble of realizations in order to obtain statistics of several {flow quantities}, such as bulk parameters and chordwise pressure distributions. Some bulk parameters (e.g. the $St$ number) show narrow histograms, while others (e.g. the $t-std(C_L)$) are significantly dispersed. The $t-std(C_L)$ dispersion is recognized to be due to the high sensitivity of the flow along the side surface to small differences in the wind tunnel setup and in the simulation parameters. Consequently, the statistics of the pressure distribution on the cylinder lateral surfaces also show significant dispersion, both in wind tunnel measurements and in numerical simulations. {Therefore, although it is not yet clear which results may be considered as the "most accurate", we assume as reasonable those results which do not deviate too much from the ensemble average of both the experimental and numerical contributions.} Conversely, the wind tunnel measurements and the numerical predictions of base pressure, and, hence, of the drag have been found to be in overall very good agreement.

Finally, an asymmetry of the time-averaged flow has been recognized in both preliminary wind tunnel tests and in computational simulations. {This may be again explained by the extreme sensitivity of the flow to small uncontrolled uncertainties which can, in some cases, trigger the asymmetry of the mean flow.}

Numerical Modelling

Discretisation method
Grids and grid resolution

Physical Modelling

Turbulence modelling
Transition modelling
Near-wall modelling
Other modelling

Application Uncertainties

Summarise any aspects of the UFR model set-up which are subject to uncertainty and to which the assessment parameters are particularly sensitive (e.g location and nature of transition to turbulence; specification of turbulence quantities at inlet; flow leakage through gaps etc.)

Recommendations for Future Work

Propose further studies which will improve the quality or scope of the BPA and perhaps bring it up to date. For example, perhaps further calculations of the test-case should be performed employing more recent, highly promising models of turbulence (e.g Spalart and Allmaras, Durbin's v2f, etc.). Or perhaps new experiments should be undertaken for which the values of key parameters (e.g. pressure gradient or streamline curvature) are much closer to those encountered in real application challenges.

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