Aerodynamic loads on cactus-shaped cylinders at low Reynolds numbers

Direct numerical simulations of flow past cactus-shaped cylinders are performed at Reynolds numbers of 20, 100, and 300. The results are contrasted to those from smooth cylinders at the same Reynolds numbers. The cavities in the cactus-shaped cylinders are seen to reduce the forces acting on them. At Reynolds number of 20, the drag is reduced by 22% due to reduction in the viscous forces. At Reynolds number of 100, the unsteady pressure forces increase, while the unsteady viscous forces acting on the cactus-shaped cylinder decrease. The overall reduction in drag force is about 18%. At Reynolds number of 300, onset of three dimensionality is observed together with significant decrease in pressure and viscous forces. Both the mean and fluctuating forces are found to decrease considerably. The Strouhal number is also found to decrease by about 10%. These reductions in force magnitudes and observed wake instabilities are attributed to the presence of large-scale, quiescent, recirculating flow within the cactus cavities. (c) 2008 American Institute of Physics.