Turbulence is the instability generated by shear. Stronger the shear, stronger the turbulence. Lot of factors can give rise to turbulence in a flow.
Laminar flow to Transition (Chaotic flow) to Turbulent flow
Transition from laminar to turbulent flow typically occurs at the local transition Reynolds number, which varies for different types of flows.
Open channel flow
Reynolds number < 500 ( Laminar flow)
Reynolds number > 1000 ( Turbulent flow)
Reynolds number < 2300 ( Laminar flow)
Reynolds number > 4000 ( Turbulent flow)
External flow over cylinder/sphere
Reynolds number < 200000 ( Laminar flow)
Reynolds number > 200000 ( Turbulent flow)
Flow over flat surface
Reynolds number < 500000 ( Laminar flow)
Reynolds number > 500000 ( Turbulent flow)