Angle of attack dependence of flow past cactus-inspired cylinders with a low number of ribs

Abstract

The aerodynamic coefficients and the Strouhal number of cylinders with three and four ribs, inspired by succulents Euphorbia trigona and Euphorbia Abyssinica are investigated using 2D Unsteady Reynolds-Averaged Navier-Stokes simulations at Reynolds number 20,000. Both configurations show a significant dependence of the studied characteristics on the angle of attack. The obtained results are compared to the smooth circular cylinder, previous results for cylinders with 24 ribs based on the Saguaro cactus, and cylinders with triangular and square cross-sections. Relative to the circular cylinder, the mean drag coefficient is lowered only for the four-rib case at high angles of attack. However, at some angular positions, the ability to reduce unsteady force fluctuations exceeds Saguaro-inspired cylinders. For both shapes studied, the Strouhal number at most angles of attack is lower compared to both the circular cylinder and cylinders with 24 ribs at the same Reynolds number. The minimum values of the aerodynamic coefficients for both configurations are related to the angular orientation. For the four-rib case a critical angle of α cr ≈ 40 • is observed, at which the mean drag coefficient and the fluctuating lift coefficient attain their minima. The mean lift coefficient reaches at this angle its maximum value before a sudden drop for higher angles of attack. Therefore, for cactus-shaped cylinders with four ribs high angles of attack give the optimum orientation relative to prevailing winds.

Experimental investigation of the angle of attack dependence of the flow past a cactus-shaped cylinder with four ribs

“…Similar behaviour was previously observed in a URANS-based investigation of a cactus-shaped cylinder with four ribs (Zhdanov and Busse, 2019).…”

“…Taking into account that these succulents have developed their plant features, including ribs, independently from cacti through the process of convergent evolution (McGhee, 2011), their shape may provide aerodynamic benefits similar to those of cactus-shaped cylinders with many ribs. In a 2D URANS study it was found that a succulent-inspired cylinder with four ribs has superior aerodynamic behaviour compared to the regular square cylinder at certain angles of attack (Zhdanov and Busse, 2019).…”

LES and PIV Investigation of the Flow Past a Cactus-Shaped Cylinder with Four Ribs

“…The studied shape is identical to the shape investigated in our earlier studies using 2D URANS (Zhdanov and Busse 2019) and experimental (Zhdanov et al 2021) approaches. The cross-section of the cylinder (Fig.…”

“…The variation of these quantities with angle of attack from the present large eddy simulations are shown in Figs. 3 and 4, where they are also compared to experimental (Zhdanov et al 2021) and 2D URANS (Zhdanov and Busse 2019) data for the same shape.…”

“…While the previous biomimetic examples are derived from the circular cylinder crosssection, a number of succulents from the Euphorbiaceae plant family have only four ribs and resemble a square cross-section with concave sides and rounded corners and can thus be viewed from an aerodynamic perspective as biomimetic modifications to the square cylinder. This shape has already been investigated experimentally (Zhdanov et al 2021) and using 2D unsteady Reynolds-averaged Navier-Stokes (URANS) simulations (Zhdanov and Busse 2019). While the 2D URANS simulations gave useful insight into key features of the flow through qualitative prediction of the trends of aerodynamic properties with the angle of attack, a quantitative mismatch with the experimental data was observed in the values of the aerodynamic coefficients and in the critical angle of attack.…”