Drag Reduction of Non-Circular Cylinder at Subcritical Reynolds Numbers

Experimental and numerical investigation of fore body geometrical effects on drag and flow-field of non-circular cylinder ( D-shaped bluff body) were conducted in the subcritical flow regime at Reynolds number in the range of ۱ x ۱۰۵ ≤ Re ≤ ۱.۸ x ۱۰۵. To shield the non circular cylinder ( D-shaped model ) front surface from the positive pressure of the unsteady vortex generation in the near wake, circular disk of various geometries were attached upstream of the non-circular cylinder base model. The fore body makes the streamlines that separate from its edges to attach smoothly onto the front face shoulders of the main body, thereby converting the bluff body into an equivalent streamlined body to result in low drag. The diameter of the fore body (b۱) ranges from ۰.۲۵ to ۰.۷۵ times the hydraulic diameter of base model ( b۲ ) and the gap ratio ( g/ b۲ ),was in the range from ۰.۲۵ to ۱.۷۵ b۲. The experimental and numerical investigations show that by using a circular disk as fore body with a width ratio b۱/b۲ of ۰.۷۵ and a gap ratio of g/ b۲ = ۰.۷۵ results in a configuration having percentage drag reduction of about ۶۷ % and ۶۵ % respectively.

Keywords:

d , shaped model , Pressure drag , Shielding effect , Drag reduction , CFD , Fore body geometry