strange error near boundary layer

CF

Hello,

I'm simulating a flow past cylinder using an O grid, in FVM. I normally intialize solution everywhere to u=1. and do the simulation. As time step increases, the velocity near the boundary will decrease since u=v=0 on its surface. It works ok when 1st grid size is at 5.e-3. The cells just next to the boundary have a small magnitude that's tangential to the boundary.

When I reduce my grid further to 1.e-3 at the boundary, without changing the no. of grids and the domain area,for the grid cells just next to the boundary, velocity suddenly gets larger and points somewhat perepndicular to the boundary. Interestingly the solution did not explode and other parts of the flow looks fine (separation etc). At 5.e-3, there's no such problem.

I then increase the no. of grid pts from 60 to 100 in the radial direction (same domain area) and everything seems fine again, with the correct lift/drag being computed.

Moreover, I've used the code on a C grid simulating flow past an airfoil and there's no such problem before.

So does this strange things happen because of the grid? Is there a possibility that it could be due to the error in the code itself?

Thank you.