The development of concave surface boundary-layer flows is characterized by the formation of streamwise counterrotating Görtler vortices and has been experimentally investigated for concave surfaces of 1 m and 2 m radius of curvature. For this study, the wavelengths of the vortices were pre-set or "forced" by thin perturbation wires placed upstream and perpendicular to the concave surface leading edge. This method was used to obtain uniform vortex wavelengths. Velocity contours were obtained from velocity measurements using a single hot-wire anemometer probe. The most amplified wavelengths of the vortices can be pre-set by the spanwise spacing of the thin wires and free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on the growth rate of the vortices. The occurrence of a secondary instability mode is also shown in the form of mushroom-like structures as a consequence of the non-linear growth of Görtler vortices. Measurements of wall shear stress on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By presetting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of Görtler vortices can be respectively observed.