Hydraulic spool valves are found in most hydraulic circuits in which flow is to be modulated. Therefore their dynamic performance is critical to the overall performance of the circuit. Fundamental to this performance is the presence of flow reaction forces which act on the spool. These forces can result in the necessity of using two stage devices to drive the spool and in some cases have been directly linked to valve and circuit instabilities. As such, a great deal of research and design has concentrated on ways to reduce or compensate for flow forces. In one particular series of studies conducted on flow divider valves, it was established that a rim machined into the land of the spool reduced the flow dividing error by approximately 70-80%, and it was deduced that the main contribution to this error was flow forces. Direct verification of the claim regarding flow force reduction was not achieved and hence was the motivation for this particular study. <p> This thesis will consider the reaction (flow) force associated with a conventional spool land and one with a rim machined into it, and a modified form of the rimmed land referred to as a sharp edge tapered rim spool land. The rim and the sharp edge tapered rim were specially designed geometrical changes to the lands of the standard spool in order to reduce the large steady state flow forces (SSFF) inherent in the standard spool valve. In order to analyze the flow field inside the interior passages of the valve, three configurations of the spool were considered for orifice openings of 0.375, 0.5, 0.75 and 1.05 mm. Computational Fluid Dynamics (CFD) analysis was used to describe the fluid mechanics associated with the steady state flow forces as it provided a detailed structure of the flow through the valve, and to identify the flow mechanism whereby flow forces are reduced by the machining of a rim and tapered rim on the land of the spool. For all openings of the spool, the sharp tapered rim valve provides the largest reduction in SSFF. It was also observed that for all cases studied, the inflow SSFFs were smaller than for the outflow conditions. <p>The prediction of the steady state flow force on the rim spool was investigated in a flow divider valve configuration, and the results from the CFD analysis indicated a reduction by approximately 70%.

Hydraulic spool valves are found in most hydraulic circuits in which flow is to be modulated. Therefore their dynamic performance is critical to the overall performance of the circuit. Fundamental to this performance is the presence of flow reaction forces which act on the spool. These forces can result in the necessity of using two stage devices to drive the spool and in some cases have been directly linked to valve and circuit instabilities. As such, a great deal of research and design has concentrated on ways to reduce or compensate for flow forces. In one particular series of studies conducted on flow divider valves, it was established that a rim machined into the land of the spool reduced the flow dividing error by approximately 70-80%, and it was deduced that the main contribution to this error was flow forces. Direct verification of the claim regarding flow force reduction was not achieved and hence was the motivation for this particular study. <p> This thesis will consider the reaction (flow) force associated with a conventional spool land and one with a rim machined into it, and a modified form of the rimmed land referred to as a sharp edge tapered rim spool land. The rim and the sharp edge tapered rim were specially designed geometrical changes to the lands of the standard spool in order to reduce the large steady state flow forces (SSFF) inherent in the standard spool valve. In order to analyze the flow field inside the interior passages of the valve, three configurations of the spool were considered for orifice openings of 0.375, 0.5, 0.75 and 1.05 mm. Computational Fluid Dynamics (CFD) analysis was used to describe the fluid mechanics associated with the steady state flow forces as it provided a detailed structure of the flow through the valve, and to identify the flow mechanism whereby flow forces are reduced by the machining of a rim and tapered rim on the land of the spool. For all openings of the spool, the sharp tapered rim valve provides the largest reduction in SSFF. It was also observed that for all cases studied, the inflow SSFFs were smaller than for the outflow conditions. <p>The prediction of the steady state flow force on the rim spool was investigated in a flow divider valve configuration, and the results from the CFD analysis indicated a reduction by approximately 70%.

Directional control valves are widely used in hydraulic systems to control the flow direction and the flow rate. In order to design an actuator for such a valve a preliminary analysis of forces acting on the spool is necessary. The dominant axial force is the so called steady flow force, which is analysed within this study. For this purpose a 2/2-way spool valve with a sharp control edge was manufactured and investigated. CFD simulations were carried out to visualize the fluid flow inside the valve. The measured and simulated axial forces and pressure drops across the test valve are compared and show good qualitative correlation. However, the simulated values of axial forces are in average by 32 % lower compared with the measured ones. Therefore, the components of the axial force were scrutinized revealing a dominancy of the pressure force acting on ring areas in the spool chamber. Although CFD simulations are preferably used to save resources, the results of this study emphasise the importance of the experiments.

Directional control valves are widely used in hydraulic systems to control the flow direction and the flow rate. In order to design an actuator for such a valve a preliminary analysis of forces acting on the spool is necessary. The dominant axial force is the so called steady flow force, which is analysed within this study. For this purpose a 2/2-way spool valve with a sharp control edge was manufactured and investigated. CFD simulations were carried out to visualize the fluid flow inside the valve. The measured and simulated axial forces and pressure drops across the test valve are compared and show good qualitative correlation. However, the simulated values of axial forces are in average by 32 % lower compared with the measured ones. Therefore, the components of the axial force were scrutinized revealing a dominancy of the pressure force acting on ring areas in the spool chamber. Although CFD simulations are preferably used to save resources, the results of this study emphasise the importance of the experiments.

The contribution compares results of Large Eddy Simulations of the cavitating flow in a model oil hydraulic spool valve using an Euler-Euler and a one-way coupled Euler- Lagrange model. The impact of the choice of the empirical constants in the Kunz cavitation model is demonstrated. Provided these are chosen appropriately the approach can yield reasonable agreement with the corresponding experiment. The one-way Euler-Lagrange model yields less agreement. It is demonstrated that this is due to the lack of realistic volumetric coupling, rarely accounted for in this type of method. First results of such an algorithm are presented featuring substantially more realism.

The contribution compares results of Large Eddy Simulations of the cavitating flow in a model oil hydraulic spool valve using an Euler-Euler and a one-way coupled Euler- Lagrange model. The impact of the choice of the empirical constants in the Kunz cavitation model is demonstrated. Provided these are chosen appropriately the approach can yield reasonable agreement with the corresponding experiment. The one-way Euler-Lagrange model yields less agreement. It is demonstrated that this is due to the lack of realistic volumetric coupling, rarely accounted for in this type of method. First results of such an algorithm are presented featuring substantially more realism.