Abstract: A study of turbulence modeling of 2D separation flows ispresented in the paper. First, experimental data analysis of Delery'stransonic bump showed the nonlinear development of the eddy viscosity in theseparation zone. Second, it is found that the eddy-viscosity decreases in the separation partand then enlarges during reattachment and relaxation process. Third, anonlinear correlation is extracted from the experiment data and thensuccessfully applied into the Balwin-Lomax model to form a mathematicallynew and simple algebraic turbulence model. The proposed model was evaluatedwith several typical separation flows, including low speed flow over plate,transonic flow, supersonic flow and the hypersonic flow. The numericalresults are compared with the experimental data and the results obtainedfrom other models to show that the nonlinear characteristics ofeddy-viscosity proposed in this paper is correct and suitable for 2Dseparation flows.