Transient Flow Forces Research Articles

A Review of the CFD Method in the Modeling of Flow Forces

Hydraulic valves are key components of fluid power systems. They control the flow rate and pressure in hydraulic lines, actuator motion, and direction. Valves that control flow rate or pressure can be divided into two main categories: spool-type valves, where control components are similar to the piston inside a sleeve with control orifices; and seat-type valves, in which a poppet inside a seat opens and closes the flow. Forces induced on valve components during oil flow are crucial to the valve’s operational capabilities. They can be calculated using a formula originating from the momentum conservation equation for a two-dimensional control volume. Increasing demands for flow rate and pressure control accuracy cause flow forces to be calculated much more accurately than when using the analytical formula. Therefore, computational fluid dynamics (CFD) simulations are the only effective tool for their calculation. This paper reviews the CFD approaches used for calculating flow forces inside hydraulic valves. It presents typical approaches used for evaluating flow forces inside hydraulic valves. The oldest and most common are conducted for a fixed position of valve components for defined flow conditions, which do not cover all components of flow forces. The dynamic flow forces can be calculated using more complex CFD models using fluid–structure interaction (FSI) techniques. This paper presents available FSI techniques for the simulation of transient flow forces, mainly for valves whose component position is determined by the forces occurring during oil flow.

Effect of transient flow and pseudo-static forces on the lateral earth pressures developed in retained unsaturated backfills

Effect of transient flow and pseudo-static forces on the lateral earth pressures developed in retained unsaturated backfills

Review of the Research on and Optimization of the Flow Force of Hydraulic Spool Valves

As one of the important factors affecting the stability of slide valves, the analysis and research of flow force are of great significance. In recent years, more and more experts and scholars have conducted research in this field, attempting to find methods to reduce or utilize the flow force of hydraulic spool valves. Flow force includes steady-state flow force and transient flow force, with steady-state flow force having the most significant impact on spool valves. The influencing factors of flow force are complex and diverse, including the cavitation phenomenon, shape of the throttling groove, and jet angle. At present, the main ways to reduce flow force are to design the structure of the spool valve, the structure of the valve sleeve, and the flow channel of the valve body. This article mainly reviews the definition, calculation methods, influencing factors, and methods for reducing the flow force of slide valves. This provides a new approach to reducing the flow force in hydraulic spool valves.

Flow-reduced double valve actuation for improving the dynamic performance of electro-hydraulic servo drive

Due to the negative correlation between the frequency response and the rated flow of the servo valve, there remains challenge to achieve high dynamic response of large-flow servo valve-controlled hydraulic systems under extreme operating conditions. Although the effective bandwidth of such system can be extended via various control methods, the inherent performance limit of hardware limits further improvement of the system dynamic response. In this paper, a flow-reduced double valve actuation strategy is proposed to improve the performance of electro-hydraulic servo drives. By reducing the actually used flow, the frequency response of the servo valve is enhanced due to the decreased hydraulic spring force and transient flow force acting on the spool. Two flow-reduced servo valves are adopted in parallel to meet large flow along with the improved dynamic performance. Simulation research is conducted and a servo valve-controlled shaking table with a specially designed valve block is used for experimental validation. Simulation and experimental results prove that the proposed strategy possesses better dynamic performance than the conventional single valve actuation.

Large-eddy simulation of a full-scale underwater energy storage accumulator

Underwater energy storage provides an alternative to conventional underground, tank, and floating storage. This study presents an underwater energy storage accumulator concept and investigates the hydrodynamic characteristics of a full-scale 1000 m3 accumulator under different flow conditions. Numerical simulations are carried out using an LES turbulence model. Time-averaged and transient flow structures and force characteristics are analyzed. The results show that the vortex structures are complex and scale-rich, but periodic vortex shedding can still be identified. The shedding frequency is consistent with the fluctuation of the lift force in the cross-flow direction. A dominant Strouhal number of 0.18 is found. The mean drag and lift coefficients stabilize at 0.45 and 0.60, respectively, and are insensitive to Reynolds number variation. Modal analysis shows that the natural frequency of the accumulator falls between 27 and 48 Hz and is much higher than the vortex shedding frequency. Thus, for the accumulator model investigated in this study, the risk of vortex-induced vibration (VIV) fatigue damage is very low.

Influence and Improvement of Hydraulic Power on Spool Valve Reversing

This paper analysed the effect of steady-state flow force and transient flow force to sliding direction valve, and two examples were given to illustrate adverse consequences caused by excessive fluid power, put forward the compensation measures. The effect of flow force should be considered when designing the hydraulic system in order to make the hydraulic system work more stable.

Numerical Simulation and Experimental Research on Flow Force and Pressure Stability in a Nozzle-Flapper Servo Valve

In the nozzle flapper servo valve, the transient flow force on the flapper is the fundamental reason that affects the pressure stability. The pressure pulsation in the pilot stage causes forced vibration of the flapper, and its deviation will directly influence the control pressure difference, which will make the pressure appear unstable. In order to grasp the principle and characteristics of transient flow force and its influence on pressure stability, a mathematical model of flapper displacement and control pressure is derived. For collecting the dynamic changes of the transient flow force and recording the motion behavior of the flapper, a three-dimensional model of the pilot-stage is established. Numerical simulations of turbulence phenomenon analysis are conducted with a variation of flapper displacement ranging from 5 μm to 20 μm. It can be concluded that the change trend of the flapper displacement is similar to the steady-state flow force and the transient flow force pulsation amplitude. Under the same structural parameters, the pulsating frequency of the flow force remains basically constant. The fluctuation of the flow force of the pilot stage will cause the pressure of the servo valve control cavity to vibrate to a certain extent, which is a factor that cannot be ignored that affects the output stability of the servo valve.

Dynamic Characteristics and Stability Analysis of Conical Relief Valve

The overflow rate of poppet relief valve is established, the dynamics characteristics of poppet relief valve which contains transient flow force and steady flow force is established. Based on this point, the stability conditions of poppet relief valve system are derived using Routh-Hurmitz method. The effect of the design parameters of relief valve on the dynamic characteristics of poppet valve system is analyzed. Using sensitivity analysis, the effects of parameters of the most significant is obtained, that is diameter D of liquid flow cylinder fluid. The results showed that, the established model can largely capture the dynamic characteristics of the relief valve system, the established stability condition can be used as a favorable basis for the design of valve. DOI: http://dx.doi.org/10.5755/j01.mech.25.1.22881

Analysis on the Main Design Parameters Influencing the Impact Efficiency of Dual-Chamber-Controlled Hydraulic Drifter

Previous investigators demonstrate that the impact performance of the drifter is affected by the forces such as viscous friction resistance and axial thrust of liquid. In this research, some other forces including hydraulic sticking force, sealing resistance, steady flow force and transient flow force are also taken into the consideration to develop a non-linear mathematical model of the dualchamber-controlled hydraulic drifter. We take piston mass, supply flow rate, supply pressure and the front and back work areas of the piston as the main design parameters to predict the impact efficiency of the drifter using the controlling variable method. The result shows that the impact efficiency decrease as the increasing of the piston mass or supply flow rate. The impact efficiency reaches a peak value 0.48 when the supply pressure P ranges from 150 to 230 bars. The decrease of the front work area A1 of the piston, as well as the increase of the back work area A2, would improve the impact efficiency. With the combination of the design parameters P, A1 and A2, there is a high efficiency area 162.5 < P < 230 bars and 0.48 < A1/A2 < 0.86 with the impact efficiency value 0.46–0.495.

Numerical study on the hydrodynamic forces on a ship berthing to quay by taking free-surface effect into account

In the present study, numerical simulation of the berthing maneuver of a ship in the prescribed translational motion is performed. The transient viscous flow and hydrodynamic forces on the hull are calculated by solving the unsteady Reynolds-Averaged Navier–Stokes equations in overset grid system, and the free surface is captured using volume-of-fluid (VOF) approach. The present numerical results have been compared with previous computational results by Toda and available experimental data respectively. Since the effects of the quaywall and free surface are taken into consideration in the present study, it is found that the agreement is significantly better than that resulting from Toda’s 3D CFD based approach. Then the effects of various standoff distances between the ship and quaywall on the lateral forces are investigated. Meanwhile, the detailed transient flow features around the berthing ship are obtained, which are helpful to understand the interactional effects between the ship and quaywall. The present results may provide helpful guidance for vessels’ safe maneuvering in berthing motion and the design of fender system in the quay.

Pressure Transient Flow Forces for Hydraulic Spool Valves

The objective of this paper is to experimentally investigate the significance of the pressure transient flow force acting on hydraulic spool valves. In the past, this flow force effect has been routinely neglected due to its assumed small size. Through analytical and experimental methods, this research shows that flow forces due to pressure transient effects can be comparable in magnitude to the steady flow forces acting on the valve and that the past tradition of neglecting this effect may not always be justified. The paper also shows that the traditional steady flow force model does a fairly good job predicting the steady flow forces on the valve, but more research must be done to develop a good model for pressure transient flow forces.

Robust Optimal Design of Unstable Valves

This paper is concerned with the application of robust control design concepts for the physical geometric design of electrohydraulic valves. Limitations of solenoid actuators have prevented single stage electrohydraulic valves which are simpler and more cost effective from being utilized in high flow rate and high bandwidth applications. Fluid flow force induced instability has been proposed as a means to alleviate the demand on the solenoid actuators. Previous research has demonstrated that simple changes in the valve geometry can be used to manipulate both the transient flow force as well as the steady flow force for this purpose. This paper considers the dimensional design of such ldquounstablerdquo valves to minimize the net steady flow force. The robust optimal design method, in which the design must be robust to uncertainties such as variations in operating pressure ranges and dynamic viscosity, etc., is proposed. By representing the original problem as a linear fractional transformation interconnection, the robust design problem is formulated into one of synthesizing an optimal controller for an appropriate static plant with a structured uncertainty. An algorithm for solving this design synthesis problem is proposed. A case study is conducted to compare the nominal optimal (without considering uncertainty) and the robust optimal designs. It is shown that viscosity effect is exclusively utilized in the nominal optimal design, whereas both the viscosity effect and the nonorifice flux effect are needed in the robust optimal design. The robust optimal design imposes smaller steady flow force on the spool than the nominal optimal design under perturbed situations. Based on the robust design method, an actual prototype design of the unstable valve has been developed.

Numerical and Experimental Investigation on the Flow Induced Oscillations of a Single-Blade Pump Impeller

This contribution is addressed to the periodically unsteady flow forces of a single-blade sewage water pump, which affect the impeller and produce radial deflections of the pump shaft. The hydrodynamic excitation forces were calculated from the time dependent flow field, which was computed by numerical simulation of the three-dimensional, viscous, time-dependent flow in the pump. A commercial computer code was used to determine the time accurate Reynolds averaged Navier-Stokes equations. The transient radial flow forces at all time steps for a complete impeller revolution affect the rotor of the single-blade pump and stimulate it to strong oscillations. To determine the influence of the vibration stimulation forces on the dynamic behavior of the pump rotor, an investigation of the rotor’s structural dynamics was accomplished. A dynamic time analysis for the pump rotor provided the dynamic answer from the structural model of the rotor under the influence of the flow forces. The hydrodynamic forces, which were calculated before, were defined as external forces and applied as the load on the rotor. The resulting impeller deflections were calculated by a transient analysis of the pump rotor system using the commercial finite element method software PROMECHANICA. To verify the results obtained by standard numerical methods, the radial deflections of the impeller of a commercial sewage water pump, which has been investigated numerical in advance, were measured for the horizontal and for the vertical coordinate direction by proximity sensors. The measured data were compared to the computed amounts for a wide range of pump operation. The results show a good agreement for a strong part of an impeller revolution for all investigated operating points. The simultaneous measurement of vibration accelerations at the outer side of the pump casing showed the effects of the time-dependent flow, which produce hydrodynamic forces acting at the impeller of the pump and stimulating it to strong oscillations.

Using Steady Flow Force for Unstable Valve Design: Modeling and Experiments

Abstract In single stage electrohydraulic valves, solenoid actuators are usually used to stroke the main spools directly. They are cheaper and more reliable than multistage valves. Their use, however, is restricted to low bandwidth and low flow rate applications due to the limitation of the solenoid actuators. Our research focuses on alleviating the need for large and expensive solenoids in single stage valves by advantageously using fluid flow forces. For example, in a previous paper, we proposed to improve spool agility by inducing unstable transient flow forces by the use of negative damping lengths. In the present paper, how steady flow forces can be manipulated to improve spool agility is examined through fundamental momentum analysis, CFD analysis, and experimental studies. Particularly, it is found that two often ignored components—viscosity effect and non-metering momentum flux, have strong influence on steady flow forces. For positive damping lengths, viscosity increases the steady flow force, whereas for negative damping lengths, viscosity has the tendency to reduce steady flow forces. Also, by slightly modifying the non-metering port geometry, the non-metering flux can also be manipulated to reduce steady flow force. Therefore, both transient and steady flow forces can be used to improve the agility of single stage electrohydraulic valves. Experimental results confirm the contributions of both transient and steady flow force in improving spool agility.

On Using Unstable Electrohydraulic Valves for Control

High bandwidth, high flow rate electrohydraulic valves typically have two or more stages. Most multi-stage valves are expensive, require meticulously clean fluid, and introduce higher order dynamics. On the other hand, single-stage spool valves are cheaper and more reliable. However, a majority of them are not suitable for high bandwidth, high flow rate applications due to limitations of the electromechanical/solenoid spool-stroking actuators. In this paper, we investigate the feasibility of reducing this limitation by exploiting the transient flow forces in the valve so as to achieve spool dynamics that are intrinsically open-loop unstable. While conventional valves are designed to be open-loop stable, the unstable valve design has to be stabilized via closed-loop feedback. Simulation case studies are conducted to examine the potential dynamic and energetic advantages that an unstable valve may offer. These studies indicate that unstable valves provide faster response than the stable counterparts when stroking forces are limited. Moreover, unstable valves tend to require less positive power and energy to operate.