Flow structures around trapezoidal cylinders and their hydrodynamic characteristics: Effects of the base length ratio and attack angle

This paper reports the results of a numerical investigation into the flow around trapezoidal cylinders with various base length ratios and the associated wake structures. Seven base length ratios (d/D, where d and D represent the shorter and longer bases, respectively) ranging from 0 to 1 and three typical attack angles (theta) of 0 degrees (shorter base facing flow), 90 degrees, and 180 degrees are examined at a low Reynolds number of 150. The results indicate that the hydrodynamic forces increase significantly as theta increases from 0 degrees to 180 degrees. Compared with a square cylinder, trapezoidal cylinders with d/D = 0.1 and d/D = 0.3 produce the maximum growth in the time-averaged drag coefficient (up to 54.5%) and the root-mean-square lift coefficient (up to 451.3%), respectively. As d/D increases, the roll-up of the shear layers occurs further downstream, resulting in an increased vortex formation length and subsequent deceleration of vortex shedding. Besides the trailing-edge separation observed in all cylinders at theta = 0 degrees, leading-edge separation in the square cylinder leads to a pair of trapped vortices symmetrically distributed on its lateral sides. At theta = 90 degrees and theta = 180 degrees, the occurrence of trapped and secondary vortices may contribute to the appearance of second and third harmonic frequencies, although the location at which these vortices are formed varies with the trailing edge and attack angle.