The present thesis provides an overview of the application, development and problems of rotary valves, which are considered as an alternative concept for controlling of steam mass flow rate in the extraction steam turbines. The objective of the current work is to perform numerical fluid dynamics investigations on a rotary valve with an axial gap between movable front part and non-movable rear part. In order to determine the effect of this gap on fluid field in the stator of an adaptive turbine stage, Reynolds averaged NavierStokes equations (RANS equation) are applied. Furthermore, different turbulence models are compared and subsequently based on the advantages of Shear Stress Transport model (SST) over the other turbulence models ,it is selected in the present study. For this purpose, T100 blade profile is chosen and 2D mesh is generated by ICEM CFD and flow computation is carried out by ANSYS CFX. Thereafter, during the post processing by ANSYS CFD POST the flow characteristics, including flow velocity, flow angle, static and total pressure coefficient, loss coefficient and volume flow rate through the gap are investigated, throughout the study for concerning seven different closing degree of rotary valve. Conclusively, from the computational fluid dynamics (CFD) obtained data are compared with the available experimental data, which were obtained in lab from a rotary valve without gap. Finally a conclusion is drawn in order to indicate the impact of this gap on the flow characteristics.