Predictions of a turbulent separated flow using commercial CFD codes

Numerical simulations of the turbulent flow in (in asymmetric two-dimensional diffuser are carried out using three commercial CFD codes: CFX, Fluent, and Star-CD. A low-Reynolds number k-epsilon model with damping functions and the four-equation upsilon'(2)-f model are used; the first one is available as a standard feature in all the codes, the upsilon'(2)-f model was implemented using the User Defined Routines. The flow features a large recirculating zone due to the adverse pressure gradient in the diffuser; the upsilon'(2)-f predictions agree very well with the experiments both for the mean velocity and the turbulent kinetic energy. The length of the separation bubble is also computed within 6 percent of the measured value. The k-epsilon calculations do not show any recirculation and the agreement with the measurements is very poor. The three codes employed show very similar characteristics in terms of convergence and accuracy; in particular, the results obtained using the upsilon'(2)-f are consistent in all the codes, while appreciable differences (ire obtained when the k-epsilon is employed.