Some swirling-flow challenges for turbulent CFD

The paper examines some of the continuing challenges, within a RANS framework, of computing turbulent swirling flows such as are encountered in industry and the environment. The principal focus is on modelling turbulent transport processes but serious problems also arise in handling numerical issues, too. Recent researches of two of these types of flow by the authors and their colleagues in the Turbulence Mechanics Group at Manchester are examined; namely, the confined flow within a rotor-stator disc cavity and the trailing wing-tip vortex. The former flow, while geometrically axisymmetric, has been found to create multiple rotating vortices necessitating a three-dimensional time-dependent analysis. The wing-tip vortex is extremely sensitive to the choice of turbulence model and only a second-moment closure that complies with the constraints of two-component turbulence has been found capable of handling both the flow over the wing and the wake vortex. Moreover, because of the large distances downstream of the aircraft to which, for practical cases, computations need to be carried, the numerical strategy is brought into question. Finally, arising from these two test cases, outline remarks are made about a swirling flow that poses one of the major computational challenges of the twenty-first century.