The essentials of fluid mechanics in a long orifice with cavitation are first introduced in order to clarify the different results found in the literature. In the process equations useful to calculate the discharge coefficient and the conditions at the onset of cavitation are found from first principles. The second part of the article presents all experimental study of the flow in a relatively large-scale channel, which was used to reproduce the flow pattern in a square-edged diesel injector orifice by dynamic similarity. A fixed cavity was formed at the corner of the orifice inlet by reducing the absolute pressure in the flow. The Reynolds number of the flow was maintained at about 13,800. Both cavitating and noncavitating flows were measured by using a laser Doppler velocimeter. Significant changes in mean flow and turbulence turbulence have been found in the presence of the cavity. Turbulence in the cavitating flow is higher and decays more slowly than that in the noncavitating flow. This could explain the sudden increase in spray angle that is observed in diesel fuel injectors at the onset of cavitation.