Flow separation in flow past a solid object is caused by

If $$'x'$$ is the distance measured from the leading edge of a flat plate, then laminar boundary layer thickness varies as

The necessary and sufficient condition which brings about separation of boundary layer is
$${{dP} \over {dx}} > 0$$

As the transition from laminar to turbulent flow is induced in a cross flow past a circular cylinder the value of the drag coefficient drops.