I. Introduction he flow over shallow cavities is related to several engineering applications. Solar energy collectors, combustion chambers, wheel wells, fuel vents on aircrafts and the flow between train wagons are just a few examples. In some cases, cavities can be a considerable acoustic noise source, and especially for increasing Mach number, they can also lead to significant structural loads. It is therefore necessary to understand such flows and find means to control them1. Despite the simple geometry, cavity flows exhibits a broad range of fluid mechanical phenomena, like an unsteady shear layer developing from the leading edge, vortex shedding, recirculation zones, instability and three-dimensional effects that play a role in the flow1. Cavities flow has been a topic of research since 1950’s. However, there are still a lot of aspects to be investigated since the complexity and broad range of cavity flows offers a great challenge: namely, to enhance the understanding of the flow physics. The present work is a continuation of previous experimental studies conducted in an open test section subsonic wind tunnel where the Particle Image Velocimetry (PIV) technique has been used to visualize the airflow over shallow cavities. The type of cavity under investigation can be found on solar collectors, normally used for water heating purposes. The upper surface of these devices is made of flat glass pieces. If we introduce vertical wind barriers along the perimeter of the collector, the efficiency of the heat absorption improves accordingly, because wind convection is inhibited2, 3. Up to now the Particle Image Velocimetry (PIV) 4, 5 technique has been used to investigate the influence of Reynolds number, of the cavity aspect ratio and of the surface roughness on the flow topology. In the present study, the three-dimensional effects originated at the cavity’s sides and the cavity surface roughness surface have been considered. Results of the flow topology obtained with roughness added (using sand paper) on the whole cavity surface and with roughness just in the entrance region have been compared. The data showed that the flow topologies in these situations are not very similar. In the former work4 just the configuration of roughness on the whole surface was studied. Regarding the three-dimensional effects, PIV measurements have been conducted at several planes, parallel to the main flow direction in order to find out the distance from the middle of the cavity where the three-dimensional effects start to be relevant. These planes were located, Z= 0, 0.18, 0.20, 0.21, and 0.23 far from the cavity’s middle. These configurations were analyzed for two different values of mean wind velocity, and for one value of aspect ratio, AR, of 7.3. It has been considered also the walled and unwalled cavity (without frontal walls, see Fig. 3). Comparisons between the results obtained showed that the flow pattern is similar up to the position Z= 0.21. However, as expected, at the position Z= 0.23 and forward, important changes in flow topology were observed. In order to gain more insight of this complex flow, and with the purpose of investigating the three-dimensional effects, a numerical analysis taking into the constructive aspects of cavity model has been started. In the present study it will be presented a numerical and experimental comparison between the results obtained for the case of two-dimensional cavity with smooth surface. As expected, a good agreement between numerical and experimental results was observed for the two-dimensional case and smooth surface. The numerical investigation of three-dimensional effects will be reported in future works.