INTRODUCTION This Data Item gives information on the pressure changes occurring in the flow of fluids along a duct with a sudden enlargement of area. The data for incompressible flow are presented in non-dimensional form as total- and static-pressure loss coefficients in Figures 1 and 2 (uniform flow) and in Figure 3 (non-uniform flow). For uniform compressible flows the downstream Mach number@ total-pressure loss coefficient and static-pressure ratio are given in Figures 4 to 6. The values given represent the overall changes between the plane of the enlargement and the recovery plane (see Sketch 2.1) and are therefore strictly applicable only to sudden enlargements with that length of duct downstream. However@ for values of the area ratio@??@ near to 0 or near to 1@ this restriction may be relaxed and the data applied to shorter ducts. For a fluid flowing along a duct with a sudden enlargement of area a loss of total pressure occurs but the static pressure rises. Sketch 2.1 shows a typical static pressure distribution along a duct with a sudden enlargement of area. The recovery plane occurs at about 4D2 downstream of the enlargement.