Vertical shaft spillways is a kind of spillways which are used in dams. When Dams have valley with deep slope and there is no possibility to construct the spillways into the body of dams, application of vertical shaft spillways will be proposed. Due to the swirling flow which is occurred in vertical shaft spillways, many researches has been developed in order to control and reduce the power of swirling flow. This researches consist of studying on modification of inlet shape and application of complete structure in vertical shaft spillways such as morning-glory spillways with anti vortex plate, piano key spillways and marguerite-shape spillways in order to upgrade the application of these spillways. marguerite-shape inlet increase the effective length of crest, which cause the uniform flow and reduce the head of water and swirling flow strength. According to the few experimental studies over marguerite-shape spillways and limitations and mistakes in experimental studies, operation of marguerite-shape spillways in reduction of swirling flow is numerically analyzed and by changing the height and length of blade of marguerite-shape spillways and hydraulic and hydrodynamic characteristics of these spillways is estimated. Numerical model geometric is based on hydraulic laboratory of civil college in Isfahan University of technology. A reservoir with 2 meters diameter including a dissipater plate has been established 10 centimeters from the reservoir. This numerical study is solved by VOF method and RNG k-? turbulent model and FLUENT software is used. Results show that generally by increase the height and length of blades of marguerite-shape spillways, the barrier area increase against water flow and makes a uniformed flow which reduce the water level and swirling flow power. For example in passed discharge of 23 liter per second, by growth of length of blade from 0.2 to 0.4 meters, the water level reduce in 25 percent and by growth of height of blade from 0.05 to 0.15 meters, the water level reduce in 37 percent. But further growth of length and height of blades and increase the impact between the water flow and spillway structure, rate of water level reduction is diminished and flow rotation will be increased near the vertical shaft. In addition, by growth of water surface level in orifice flow regime, effect of marguerite-shape inlet in improving the hydraulic and hydrodynamic characteristics will be reduced. Key words: Swirling flow, Vertical shaft spillways, Marguerite-shape inlet, Numerical modeling, Orifice flow regime, Discharge coefficient, Rotational flow
