It is well-known that the discharge efficiency of an orifice varies with the flow condition: it is very low for laminar flow, it reaches a maximum in “mixed” conditions, and it tends to be constant (i.e. insensitive to flow variations) when turbulence is fully developed. However, the classical approach to the modelling of servo-hydraulic actuators is based on the hypothesis that the flow through the servovalve orifices is turbulent, and this assumption can lead to significant errors if the dynamics of actuators operating in extreme conditions is concerned. This is the case of aerospace applications, since flight actuators can be commanded to move against high counteracting loads or at very low velocities, and a laminar (or rather “mixed”) flow pattern can be established in the servovalve orifices. In the paper, the flow through the Moog D633 four-way servovalve is studied by means of experiments and Computational Fluid Dynamics simulations (developed in the STAR-CD environment). Two are the basic objectives of the investigation: to characterise the laminar-to-turbulent flow transition in the orifices of an aircraft-type hydraulic component, providing an original physical interpretation to the increase of the orifice discharge efficiency in “mixed” flow conditions, and to highlight the necessity of using Reynolds-dependant orifice equations for the modelling of high-performance servo-hydraulic actuators.

Keywords: orifice flow, CFD analysis, experiments on hydraulic fluids, laminar-to-turbulent transition, electro-hydraulic servovalve