Abstract: Aim To investigate the physical mechanisms of randomization of the small scale disturbances in local high shear regions in shear flows. Methods The four wave interaction model equations for small scale disturbances were analyzed numerically. Results As the velocity shear increases to a certain extent, the small scale disturbances will exhibit chaotic behavior, which leads to the production of turbulent spots in local high shear regions. Conclusion The chaotic behavior of the small scale motion originates from a different mechanism from that for the large scale motion. On the basis of the new hydrodynamic equations that include a conservation equation of angular momentum, the laminar turbulent transition of shear flows through secondary instability and the direct transition from laminar to turbulent flow taking place under some conditions can be explained consistently.