The unsteady flow of the steam turbine at design state was studied by the method of numerical simulation of 3-D viscous flow

and the analyses of unsteady force on the blade surface and the mechanism of action were carried out.The result indicates that at typical design operating conditions in the last stage of the steam turbine investigated here

shock and potential field interaction from upstream blade row is an order of magnitude greater than wake interaction.The shock and potential field interaction effects cause a periodic increase in the strength of the unsteady force acting on the downstream rotor blade surface.In addition

the wake interaction of upstream blade row and the expansion wave at rotor exit also cause an unsteady force acting on the downstream rotor blade surface.