The recirculating flow produced by a sudden widening of flow in a shal
low open channel is computed using three turbulence models including (
1) a constant-eddy-viscosity model; (2) a depth-averaged k-epsilon mod
el; and (3) a two-length-scale model. The computational results demons
trate the existence of a friction-dominant regime. In this regime, all
turbulence models give approximately the same recirculating length an
d the same recirculating flow rate despite the different levels of edd
y viscosity determined by the models. The length of the recirculating
zone obtained by the models is approximately equal to one frictional-l
ength scale, and the recirculating flow rate equal to 2.5% of the fric
tional flow rate; these numerical results are in agreement with the ex
perimental observations. The implication on the computation of the tur
bulent flow in natural waterways is discussed in light of these numeri
cal results.