In a general hydraulic system, a flexible high-pressure rubber hose is used. Until now, various researches have been reported about a viscoelastic pipe like a high-pressure rubber hose. It is necessary for the pressure propagation on a viscoelastic pipe that the viscoelastic characteristic of a pipe is taken into consideration. However, an exclusive use measurement bench is required for measurement of a viscoelastic character, in order to measure the flow of a high-pressure rubber hose by high response.
There, we devised how to calculate a viscoelastic character from measurement of only commercial pressure sensors, and we checked that it can measure satisfactorily. Thereby, measurement of viscoelastic character becomes simple, and to reflect to hydraulic system modeling becomes easier.

It is known that the characteristic of high-pressure rubber hose (Below, we call it a \"hose\") is expressed by viscoelastic characteristic, but the value of viscoelastic character of hose is unknown in many cases.
In general, the static value of bulk modulus of hose is shown by a manufacturer of hoses, which is measured by bulk modulus measurement bench. However, it is insufficient for expressing the dynamic characteristic. Because the value of viscoelastic character of hose is required.
Until now, there were the methods of determining viscoelastic character of hose in several ways, but the special equipment was needed in these ways. So it is generally difficult to get the viscoelastic character of hose. There, we considered how to determine the viscoelastic character of hose by easy method without special equipment.
In this method, it is necessary that the following properties are known.
1. The bulk modulus of hose, usually shown by a manufacturer of hose.
2. The bulk modulus of fluid.
In addition to the above properties, the experiment which measures the pressure response characteristic of a target hose is required. The experiment is the measurement of frequency response characteristic between the different two points on hydraulic line which consists in target hose, and the result is compared with the theoretical characteristic of the hose to determine the unknown property for the viscoelastic character.
The experiment for determining the viscoelastic property by this method is measurement of the pressure response which can become comparatively easy as compared with the method performed by other things.
In this paper, the method of determining viscoelastic character of hose is described, and the measurement results of viscoelastic character in several different kinds of hoses are shown.

A multi-degrees-of-freedom model for hydraulic pipeline systems
The dynamic behaviour of hydraulic pipelines has been modeled by the laminar flow of a compressible Newtonian fluid accounting for frequency-dependent friction [1]. The input-output relationship of this pipeline model is described by transcendental transfer functions, which have been approximated by rational fraction expressions for a single pipeline [2,3,4,5] and compound pipeline systems [6]. These modal approximations are intended for time-domain simulation or controller design; they have to be calculated individually for each transfer function.
In a recent article [7], the transfer functions between flow rate excitation and pressure response are determined for two points at arbitrary positions along a pipeline. Modal approximations are given such that the discretized model represents a damped multi-degrees-of freedom (MDOF) system with a flow rate excitation vector and a pressure response vector. For a system of several pipelines, an overall MDOF model can be assembled. It includes an approximation for each transfer function between flow rate excitation and pressure response. Eigenvalues and eigenvectors can readily be determined, offering an interpretation of the dynamic system behaviour.
In this paper, the MDOF model described in [7] is set up for a practical example. It consists of a pipeline network that connects a pump with two hydraulic cylinders. The network is excited by flow rate pulsations from the pump, and the MDOF model is used to simulate the resulting pressure pulsations in all system nodes. The eigenvalues and eigenvectors of the MDOF model reveal the natural frequencies and pressure mode shapes of the network. The high level of simulated pressure pulsations is explained by the fact that the network operates near a lightly damped resonance. Due to the existence of an equivalent mechanical MDOF model, heuristic methods from structural mechanics can be applied for effective system tuning. The relevant natural frequency is lowered by adding auxiliary pipelines at the pressure mode shape antinodes. Another simulation is made to determine the new pressure pulsations resulting from the original flow rate pulsations. It is demonstrated that the level of pressure pulsations is effectively reduced all over the network. Using the eigenvalue analysis, this can be done in a targeted way and a large number of trial simulations can be saved.
References
[1] A.F. D’Souza, R. Oldenburger, Dynamic response of fluid lines. Transactions of the ASME - Journal of Basic Engineering 86 (1964) 589-598.
[2] C.Y. Hsue, D.A. Hullender, Modal approximations for the fluid dynamics of hydraulic and pneumatic transmission lines, Fluid Transmission Line Dynamics, Special Publication for the ASME Winter Annual Meeting, Boston, Massachusetts, 1983, pp. 51-77.
[3] W.C. Yang, W.E. Tobler, Dissipative modal approximation of fluid transmission lines using linear friction model. Transactions of the ASME - Journal of Dynamic Systems, Measurement, and Control 113 (1991) 152-162.
[4] A. Almondo, M. Sorli, Time domain fluid transmission line modelling using a passivity preserving rational approximation of the frequency dependent transfer matrix. International Journal of Fluid Power 7 (2006) No.1 pp. 41-50.
[5] J. Mäkinen, R. Piché, A. Ellman, Fluid transmission line modeling using a variational method. Transactions of the ASME - Journal of Dynamic Systems, Measurement, and Control 122 (2000) 153-162.
[6] E. Kojima, M. Shinada, J. Yu, Development of accurate and practical simulation technique based on the modal approximations for fluid transients in compound fluid-line systems. International Journal of Fluid Power 3 (2002) No.2 pp. 5-15.
[7] G. Mikota, Modal analysis of hydraulic pipelines. Journal of Sound and Vibration (2013), http://dx.doi.org/10.1016/j.jsv.2013.02.021.

The important problem arising at operation of technological installations at the enterprises of energy, chemical, oil-processing and food industries is ensuring their reliability in conditions of high dynamic loadings of pipelines. The unsteady hydrodynamic processes occurring in pipeline highways at fast opening and closing of valves often lead to loss of sealing of pipelines’ joints, breakage of fittings and can become the reason of emergencies.
Such processes are especially dangerous to the pipelines made of polymeric materials being widely applied today, for example, in power plants. About 90 tanks-filters of chemical water purification with a capacity of 30 m3 with hundred meters of the pipeline 150mm diameter in which unsteady flow is occurred are operated in by-product recovery departments of large combined heat and power plants. Plastic pipes have high corrosion resistance, but smaller durability in comparison with steel pipes. Therefore research of dynamic processes in pipelines and development of approaches and devices which allow reducing intensity of dynamic loads of pipelines and fittings of technological installations is actual one.
In the paper, mathematical model of typical pipe system of processing plant, method and software for calculation of parameters of unsteady flow and dynamic loadings on pipework structure is developed. Methods and devices for reducing dynamic loadings are proposed. The simulation results of the model and efficiency of the devices proposed are experimentally verified.

Reduction of noise emissions of hydraulic systems has become essential during the recent years and nowadays has turned into inseparable part of the development process. A detailed knowledge about the vibrational behaviour of hydraulic hoses is very important in order to describe and simulate the dynamic behaviour of a complete hydraulic system.
Within a project at the Institute for Machine Tools of the University of Stuttgart, funded by the German Research Foundation (DFG), numerous investigations were carried out in order to identify the basic mechanisms of the structure-borne noise transmission through high pressure hydraulic hoses [Hei10].
Furthermore a finite element model of the high pressure hydraulic hose was built using the gained knowledge in the first project phase [Hei12].
In the current step of the project the FE-Model was updated applying the results of the conducted modal analysis. The experimental modal properties were compared with the simulative modal properties of the hydraulic hose using optimization looping. A sensitivity analysis and a mesh fineness study were carried out in order to achieve the optimal simulation performance of the FE-model. Then numerous calculations were carried out under different boundary conditions. Thus the dynamic behaviour of hydraulic hoses with different lengths, nominal diameters and reinforcement types can be simulated more quickly under different operating conditions compared to the laboratory testing time.
In the last step of this project the developed FE-model of the hydraulic hose was adapted to an existing model of a hydraulic aggregate in order to simulate the dynamic vibrational behaviour of a complete hydraulic system. The influence of the implemented FE-hose model on the dynamic behaviour of the complete hydraulic system was shown.

REFERENCES

/Hei10/ Heisel, U., Slavov, V., Investigation of the structure-borne noise transmission behaviour of hydraulic hoses. Workshop Proceedings, 7th International Fluid Power Conference, Aachen 2010, Vol.2, p.243-252

/Hei12/ Heisel, U., Slavov, V., Stehle, T., Structure-borne noise transmission behaviour of hydraulic hoses. Workshop Proceedings, 9th International Fluid Power Conference, Dresden 2012, Vol. 2, pp. 289 - 295.

New system optimization opportunities by simulation based line tuning

Caused by the functional principle of digital hydraulics there are a large number of very fast switching operations causing a high degree of pressure and flow pulsation in these systems. These pulsations can prove to be a problem because nowadays there are steadily rising standards for provided comfort, such as low noise and little vibration emissions, and efficiency besides the basic requirement of a stable system behavior. To meet these demands, system developers are often forced to elaborate active countermeasures in the form of complex control strategies. But there are also possibilities of passive influence, for example by adapting the line system. The problem here, however, is the high experimental effort that is required by these adjustments.

In this paper we discuss, by using the example of a hydraulic hose, how this experimental effort can be significantly reduced by using new simulation models and thus an easy form of system improvement is provided.

The necessary calculation methods have been developed considerably in recent years, however, the experimental verification has been paid little attention due to the costs and the huge expenditure of time. To close this gap, Saarland University of Applied Sciences HTWdS and Fluidon Gesellschaft für Fluidtechnik started the research cooperation „VeriSim“.

The realization takes place in four stages, starting with the validation of the measurement method and system through application of industrial recognized statistical methods, e.g. six-sigma. This measurement method is then used to characterize hydraulic components.
These results help to create a database of simulation models.
The next step is to extend these analyses from separate components to whole fluid line systems.
At the end there should be the possibility to simulate complex fluid line systems by linking the separate components the system consists of.

The possibilities offered by the use of these new simulation models are exemplarily shown at a hose connection. Based on the measurement data obtained, it is possible to identify the parameters determining the hoses transfer behavior and to make a statement about what kind of influence it is.

These influences are integrated in the component model. The user now can specify the hose model in the simulation according to its real component and calculate the expected system behavior.

Furthermore, the models can also be used to optimize the system performance by means of a parameter variation.

As a conclusion it can be derived that with these new models the development effort for new systems or the optimization of already existing ones can significantly be reduced. Especially for digital hydraulic systems this offers the opportunity for a quickly applicable and cost-saving passive form of influence on the system behavior by adjustment of its line system.

In the near future, these possibilities will be further enhanced by a steadily increasing number of available component models and improvement of the already existing models. It will also be possible to adjust models to specific user needs.