drag of flat plate with cavity
 

I have strange problem in solving cavity flows.

My goemtry is Flat plate of 5 m and caity of 1.5 m in center of flat plate. the depth of caivty is .1 and .5 m.

I used 2d density based solver with sst model. Y plus is 1 and mesh is hyperbolically refined in vicinity of wall and at leading and trailing edge of cavity. Mesh is also refined in shear layer and floor of cavity. succesive Mesh refinment was also done but results do not vary as the first mesh is already very fine.

Now my problem is that my drag coefficient has supecious behaviour as given here:

For flat plate 0.0044
for .1 m cavity depth 0.00445
for 0.5 m cavity depth 0.0019

why drag is reducing for 0.5 m cavity depth case. I am stuck in this case. Your help is highly appreciated.

Best Regards
Far