flow along a cylinder

marangoni

Hi, I am trying to simulate the flow along the cylinder(flow is parallel to the axis), the ratio of radius to length is very small. Actually cylinder is moving in a free space and gas is rest. If I use radius as length scale, Re is very small (less than 100), but if cylinder length is used, Re is large (about 1e5). I first used laminar model, the heat transfer between cylinder and gas is smaller than experimental data; then I used Spalart Allmaras model, the agreement is better. But I am not fully convinced the length scale used for Re (or which model I should use). Does anyone have any insight on this?

Also, when I used turbulent model, I find the far boundary is farther than that in laminar flow. anyone have any idea about this too?

Thanks in advance for any help