Numerical study of flow patterns and force characteristics for square and rectangular cylinders with wavy surfaces

This paper presents a numerical study of three-dimensional flow around wavy rectangular cylinders with sinusoidal variations in cross-section area along the spanwise direction over wide Reynolds number regimes. The detailed near wake vortex structures and force characteristics of the wavy rectangular cylinder are captured and the influence of length-to-thickness ratio (L/Dm=1 to 8) of the wavy rectangular cylinder is discussed and compared with a plain straight rectangular cylinder with the same size and flow conditions. Experimental measurements were also performed for the validation of the present numerical results using the PIV technique. For a square cylinder (L/Dm=1), significant force reduction occurs for a wavy square cylinder compared with a straight square cylinder. The three-dimensional free shear layers behind the wavy square cylinder are more stable than those of the straight square cylinder. For a rectangular cylinder (L/Dm≥2), several vortex shedding patterns can be observed. The free shear layers exhibit a steady flow feature with symmetrical flow patterns behind the wavy cylinder for Re=100. At Re=500, the mean drag coefficient and the fluctuating lift coefficient of the wavy rectangular cylinders are greatly suppressed compared to a plain straight cylinder, while such advantageous features gradually disappear with the increase of the aspect ratio and Reynolds number. At Re=5000, the advantageous characteristics on flow control disappear for wavy cylinders with L/Dm>2 due to breaking down of the periodic repeated additional transverse vortices. Finally, a wavy cylinder L/Dm=8, with an angle of incidence α=15° is studied. Results show that such wavy surface does not possess a significant advantage in flow separation control.