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Analysis of a directional hydraulic valve by a Direct Numerical Simulation using an immersed-boundary method

机译：通过浸入边界法的直接数值模拟对方向性液压阀进行分析

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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 center): 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), which provides 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. The analysis of the discharge coefficient and the flow forces for different openings s and pressure drops Ap is presented in this paper. The behavior 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 Ap 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.
机译：液压阀的改进取决于对驱动工作流体运动的相干结构的仔细分析。过去，已经通过实验测试研究了这些设备。在过去的15年中，还进行了一些数值工作，通过RANS方法解决了人体计算网格上的流动问题。在这项研究中，提出了一种不同的方法来对方向阀（4/3，关闭中心）进行轴对称分析：而RANS技术基于流量的时间平均方程，在当前工作中，非稳态Navier-Stokes使用直接数值模拟（DNS）可以解决方程式，该方程式提供了有关流的瞬时结构，影响阀性能的重要细节。此外，尽管在先前的数值研究中，计算域已通过保形网格离散化，但在这种情况下，流体相互作用已由笛卡尔网格上的浸入边界（IB）方法表示。本文对不同开口s和压降Ap的流量系数和流动力进行了分析。考虑到时间平均和瞬时场，也证明了这些全局参数的行为。对于较小的开口和压降，流量稳定，并在限制部分的侧面附着在排放室的壁上。当s和Ap增加时，射流在受限区域分离，并重新附着在下游（柯恩达效应），保持稳态。最后，对于大的开口和压降，流动变得非常不稳定：流动像自由射流一样，并以大的旋涡为主。

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