Analysis of a directional hydraulic valve by a direct numerical simulation using an immersed-boundary method

The improvement of the hydraulic valves depends on the careful analysis of the coherent structures driving the motion of the working fluid. In the past those devices have been studied by experimental tests; during the last 15 years also several numerical works have been presented, solving the flow on body-fitted computational grids by RANS methods. In this study a different approach is proposed for the axisymmetric analysis of a directional valve (4/3, closed centre): whereas the RANS techniques are based on the time-averaged equations of the flow, in the present work the unsteady Navier-Stokes equations have been solved using the Direct Numerical Simulation (DNS); the time evolution of the physics is simulated, providing important details on the instantaneous structures of the flow, affecting the valve performance. Furthermore, while in the previous numerical studies the computational domain has been discretized by conformal grids, in this case the fluid-body interaction has been represented by an immersed-boundary (IB) method on a Cartesian grid, more suitable for unsteady eddy-resolving simulations, as DNS. The analysis of the discharge coefficient and the flow forces for different openings s and pressure drops ∆p is presented in this paper. The behaviour of those global parameters is justified also considering the time-averaged and the instantaneous fields. For small openings and pressure drops the flow is steady and attached to the wall of the discharge chamber on the side of the restricted section. When s and ∆p are increased the jet separates at the restricted section and it re-attaches downstream (Coanda effect), keeping the steady state. Finally, for large openings and pressure drops the flow becomes strongly unsteady: it is organized like a free jet and is dominated by large vortices.