The article is aimed at showing motivation to setting computations of three-dimensional unsteady turbulent flow in an annual powerful gas-turbine exhaust diffuser with power struts. Two variants of eddy resolving methods among those implemented in the ANSYS FLUENT package are considered. Results of auxiliary test computations of separated flow around an airfoil performed with the URANS and the IDDES methods are presented. It has been conducted numerical simulation of turbulent flow in the exhaust diffuser under real conditions of its work. Comparing to the test data, it is evaluated capabilities of the URANS and the IDDES methods with respect to accuracy of diffuser efficiency prediction. It is shown evident superiority of the IDDES method.