D N Johnston

The ‘Secondary Source’ Method for the Measurement of Pump Pressure Ripple Characteristics Part 1: Description of Method

The difficulties involved in measuring a pump fluid-borne noise rating are discussed. A new test method is described for measuring the source flow ripple and source impedance of positive displacement hydraulic pumps. This is called the ‘secondary source’ method, and is based on the analysis of the wave propagation characteristics in a circuit which includes the pump under test and an additional source of fluid-borne noise.

Interaction of vehicle and steering system regarding on-centre handling

For the on-centre handling behaviour of vehicles the steering system is absolutely important. To investigate the interaction of the vehicle and steering system a validated, especially tailored simulation model was developed. Some meaningful vehicle and steering system parameters are altered to show the influence on steering wheel torque, steering feel and understeer. The results underline the importance of an accurate steering system model. Identified measures to improve the centre feel and steering response were a stiffer torsion bar, a higher cornering stiffness or a lower overall steering ratio. The steering response, however, suffers when the centre feel is improved by a higher trail. The steering rack friction reduces mainly the steering response while the steering column friction decreases the centre feel whereas a stiffer torsion bar lessens the understeer tendency.

Numerical simulation of fluid flow in poppet valves

Simulations of flow through poppet valves were performed using a proprietary finite volume computational fluid dynamics program. A range of valve geometries was simulated, and the flow was turbulent, incompressible and steady. Simulations were compared with experimental measurements and visualized flow patterns. Qualitative agreement between simulated and visualized flow patterns was good. However, errors in the prediction of jet separation and reattachment resulted in quantitative inaccuracies. These errors were due to the limitations of the upwind differencing scheme employed and the representation of turbulence by the κ-ɛ model, which is known to be inaccurate when applied to recirculating flow.

The Impedance Characteristics of Fluid Power Components: Relief Valves and Accumulators

This paper describes an experimental and theoretical investigation of the impedance characteristics of various relief valves and an accumulator. The test programme involved the measurement of pressure ripples at three locations in a specially designed test circuit. From these measurements, the impedance of a component was evaluated as a function of frequency. The experimental results are compared with mathematical models. For a single-stage relief valve, a simple orifice model has been found to be adequate. For two-stage valves, a much more complex model was considered. This was found to predict the basic trends but was not accurate. Tests on an accumulator yielded results which correlated well with a relatively simple mathematical model. However, it is difficult to quantify the model parameters. It is concluded that for the more complex components used in hydraulic systems it is necessary to measure the impedance characteristics rather than predict them. There is a need to adopt a standard test procedure for this purpose.

Simulation of the Pressure Ripple Characteristics of Hydraulic Circuits

The pressure ripple in a hydraulic circuit is commonly a major source of noise. A computer program has been written to simulate the pressure ripple characteristics of hydraulic circuits and to aid in the design of low noise circuits. Mathematical models of flexible hose and of pump flow ripple and source impedance have been developed. Validation tests on various circuits show that the program produces good correlation with experimental measurements. A simple silencer consisting of an asymmetric pipe loop, known as a Quincke tube, was analysed. Its performance was shown to be limited, providing attenuation over very narrow bandwidths.

In-Situ Measurement of the Wavespeed and Bulk Modulus in Hydraulic Lines

The dynamic performance of a hydraulic system is dependent on the fluid bulk modulus, which can be affected by several factors including air release and pipe compliance. A technique is described for the in situ measurement of the speed of sound from which the bulk modulus may be evaluated. It entails analysis of pressure ripple recorded at three locations in the hydraulic line. Measured values of the bulk modulus in high-pressure pipes were found to be close to the isentropic tangent bulk modulus predicted from the fluid manufacturers data.

The Impedance Characteristics of Fluid Power Components: Restrictor and Flow Control Valves:

Impedance characteristics of hydraulic components have a significant effect on pressure ripple levels in hydraulic circuits. These pressure ripples lead to vibration of pipework and associated fittings and are a source of unreliability and noise. This paper describes an experimental investigation of restrictor and flow control valves using a specially designed circuit. For a restrictor valve, it was found that the impedance characteristics were influenced by fluid compressibility, fluid inertia, valve vibration and downstream impedance effects. When these effects were taken into account, the valve could be modelled as a simple orifice. The impedance characteristics of a flow control valve were found to be more complex and wave propagation within the valve fluid passageway had a dominant effect on behaviour.

A finite Element Model of Hydraulic Pipelines Using an Optimized Interlacing Grid System

Simulation of flow and pressure variations in fluid pipelines using finite difference and finite element models can give unrealistic results, corresponding to errors in natural frequencies. A novel finite element model of hydraulic pipelines has been developed, using an interlacing grid system. The grid spacing is non-uniform and is optimized, using a genetic algorithm, to make some or all of the undamped natural frequencies of the model as close as possible to exact theoretical ones for a uniform pipe with the extreme boundary conditions of either constant pressure or no flow. Inaccuracies in the highest natural frequencies may be acceptable because of the effect of frequency-dependent friction and limited system frequency response. The optimized model gives accurate results in time domain simulation, and it allows variable properties and a variable integration step to be readily accommodated.

Measurement of Positive Displacement Pump Flow Ripple and Impedance

The secondary source method forms the British Standard for pump fluid-borne noise testing. This is a powerful technique but requires care in order to produce accurate results. This paper describes practical aspects for implementing the method. The requirements for the test rig, data acquisition system and analysis are detailed. The British Standard specifies that either mathematical modelling or linear interpolation is used on the source impedance measurements. A method for smoothing the impedance results is described in this paper, which is shown to give more repeatable results than linear interpolation. Some physically realistic mathematical models of pump impedance are described, and their use in determining the internal flow ripple discussed.

Modelling of transient flow in hydraulic pipelines

Abstract This paper describes a method of modelling time-varying flow in hydraulic pipelines which may be incorporated into time domain simulations of hydraulic systems operating with variable time steps. A previously reported finite element method is extended. New approximations to frequency-dependent friction for laminar and turbulent flow are presented. These are applicable to this finite element method as well as the method of characteristics and finite difference methods. Simulation results are compared against theory and excellent agreement is found.