Improving the aerodynamic performance of airfoils is a well-known problem in aerodynamics. Many active and passive methods have been used in order to increase the improvement of airfoil's performance. Using flap, cavity, riblet, steps on airfoils with thick blunt trailing edge, are some of these methods. In this thesis, the features of flow field over the Riso airfoil are studied with two methods of improving the aerodynamic performance in low speed wind tunnel. The effects of gurney flap and adding open cavity in a thick blunt trailing edge airfoil has been studied to decrease drag force. Actually, the amount of drag and lift force are measured by using special measurement tools such as force balance and pressure sensor. In this thesis, only the effects of flap and cavity on the features of flow field such as vortex shedding frequency, strouhal number, instantaneous and mean velocity distribution, vorticity distribution, different section velocity profile and etc, has been studied. Experiments were done at different Reynolds number and for three angles (0, 5, 10 degree). Particle Image Velocimetry technique which has high temporal and spatial resolution has been used in this research in order to reach a better insight to the structure and dynamics of this unsteady and complex flow filed. In this technique, many particles have been seeded to the flow and have been detected with laser light. Then the velocity of flow is measured in each point by using image processing of pictures obtained by camera.The experimental method was validated with real results of flow over a circular cylinder. Strouhal number and drag force were compared that shows good agreement. Moreover, the result of image processing software was validated with credible image processing results.For acquire skills to do PIV technique, at first, the phenomenon of free convection around a metal cylindrical with uniform heat flux is experimentally investigated using PIV technique. The experimental tests are performed in a cubic container full of liquid water with free surface. A metal rod horizontally passes through the container. To create a two-dimensional flow, two parallel planes are inserted in the container and then, the metal rod is passed through it. The tests are performed for different heights of water and their results are compared with each other. The results show that the flow field is strongly oscillatory and unsteady. In the second part of thesis, results show that use of gurney flap decreases momentum behind the airfoil and increase the length of wake region, so the drag force increases. Moreover, it decreases the vortex shedding frequency and increases stability. Flow over a thick blunt trailing edge airfoil also shows that use of this method delayed flow separation on the surface of airfoil. When open cavity added to thick blunt trailing edge airfoil, two vortex behind airfoil disappears and the stability of flow increases. Momentum of flow behind the airfoil for small attack angle decreases, so the drag force for small attack angle decreases Keywords: Riso airfoil, flow field, gurney flap, thick blunt trailing edge airfoil, Particle Image Velocimetry