In the fluid power applications where the typical operating conditions are dynamic the knowledge of the unsteady flow forces that act on the spools of the hydraulic valves represents an important issue to be addressed in order to make a correct design of the valve geometry and its driving system.
This paper deals with a rigorous unsteady numerical study of the fluid dynamic behavior of a hydraulic directional control valve.
A theoretical approach based upon the application of the momentum equation in the transient fluid dynamic conditions is presented, while a successive numerical analysis is performed by using the commercial Fluent™ code that provided, in the past, a correct evaluation of the stationary flow forces.
Unsteady simulations have been carried out considering three different conditions: constant pressure boundary conditions during the spool movement, inlet pressure ripple at a constant spool position, damped pressure oscillations during the spool opening phase.
The main objective is to estimate the critical magnitude orders of the pressure ripple frequency and of the axial spool velocity above which the pseudo-steadiness assumption fails; in order to reach this aim, in some cases, the effects of the fluid dynamic phenomena connected with the unsteady flow conditions have been amplified.

Keywords: CFD simulation, unsteady flows, directional control valves