Hydrodynamic couplings transmit torque by fluid circulation due to a speed differential between the impeller on the drive side and the runner on the driven side without mechanical contact. Detailed studies of the 3D flow in fluid couplings working at steady operating point were carried out in the last few years for laminar and turbulent flows. In this paper a study of fluid couplings working under unsteady operating conditions is reported for the first time. The unsteady Reynolds averaged Navier-Stokes equations together with the k-ϵ model have been solved by a finite-volume method. The calculations were done by using contour-fitted grids with non-staggered variable arrangement in a rotating frame of reference. The results give insight into the flow structure inside a coupling under unsteady working condition. An integration of the flow field for the considered operating points yields the transmitted torque. The time history of the change of the moment of momentum gives further insights into the behaviour of a fluid coupling under unsteady operating conditions.

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