The flow inside a channel with surface mounted ribs is studied by numerical simulation and experimental flow visualization. Both the geometry with one rib and the geometry with two ribs, placed perpendicular to the flow, are studied, with particular emphasis on the effect of the variation of the distance between the ribs on the structure of the flow. Results show that, for Reynolds number flows of 400 and 1000 (based on the uniform incoming velocity and the height of the ribs), the flow reattaches between the ribs when they are separated by distances of 12h (h being the height of the ribs) and 6h, respectively. In this respect, however, the numerical results for the Reynolds number flow of 1000 are in contradiction with published results. The flow inside a channel with surface mounted ribs is studied by numerical simulation and experimental flow visualization. Both of the geometry with one rib and the geometry with two ribs, placed perpendicular to the flow, are studied, with particular emphasis on the effect of the variation of the distance between the ribs on the structure of the flow. Results show that, for Reynolds number flows of 400 and 1000 (based on the uniform incoming velocity and the height of the ribs), the flow reattaches between the ribs when they are separated by distances of 12h (h being the height of the ribs) and 6h, respectively. In this respect, however, the numerical results for the Reynolds number flow of 1000 are in contradiction with published results.