Negative Drag in Natural Convection
 

Natural convection problem over a circular cylinder. Due to the symmetry half cylinder was used for solving the problem

For Gr=10 and Pr = 0.72
The values of Nu number and Drag i got are 1.4092 and 10.888 respectively for 20,000 elements grid (No. of divisions: 100 along circumference & 100 along radius)

which matched with available literature, but while refining the mesh for the grid independence study the value of Nusselt number matches to the literature but the drag goes in negative

I am unable to sought out the reason behind it, please help me

Thanks in advance,
Ashwini