Nowadays, scientists use several approaches to increase lift force without dramatic increasing in drag force. One of those methods is multi element airfoil. In this study we investigate the configuration of added element (flap) to original airfoil and changing in aerodynamic forces caused by two element airfoil. In this numerical study, an incompressible, turbulence flow over two element airfoil configuration (consists of an airfoil and a flap with half chord length of airfoil and is placed under airfoil) is simulated. For this aim we use overset grid method. we used low Reynolds linear and nonlinear k-? turbulence models. In this work we use an incompressible finite volume method and SIMPLEC algorithm with QUICK scheme for convection terms and collocated grid arrangement. The overset grid method provides a very powerful means of handling complex stationary and moving geometries in computational fluid dynamics (CFD). This method has been used successfully over the last two decade, primarily to solve problems involving fluid flow in complex, often dynamically moving, geometries. Overset grids have the advantage of avoiding grid re-meshing when dealing with moving boundary flow problems. In this method we classify the configuration into simple elements and model each element seperately. using a powerful search algorithm is so important to model elements properly. components of problem are airfoil and flap. the cord lenght of flap is half of airfoil's cord. we study flow around this two elements and investigate role of flap in ayerodynamic forces. flow has been studied in reynolds number 3e6 and with use of overset grid method. the effect of flap position or the horizental and vertical distances between flap and airfoil on the lift coefficient of airfoil has been studied. also effect of airfoil and flap angles of attack on the lift coefficient of airfoil has been investigated. at first we solve the problem for a single airfoil and validates it's results and next we add the flap to the single airfoil. an H-type grid is used as major grid while two C-type and collocated grids are used as minor grids. the results show that adding flap in aproper position causes increasing lift force. Keywords: Airfoil, Overset, K-epsilon, Finite Volume, Flap, High Lift Devices
