Drag coefficient problems

hi everyone,

im currently studying about drag coefficient for savonius turbine for my thesis.im trying to simulate the drag coefficient of half bucket design using flow simulation,but the result that i got from the simulation is not accurate compare to the text book drag.In my simulation result i got 0.8463  and the text book drag is 1.2.

I am very new to this flow simulation thing and just learned it bit by bit from the internet.I dont know where the problem is.here i attach some of the photo that maybe can help you guys find my mistake.

btw,i used water,0.4m/s external flow velocity,reynolds number is about 40000

please help me!

SolidworksFlow Simulation