Characteristics coupling


Development and implementation of a calculation method to consider undissolved air for the propagation of pressure waves for a more precise evaluation of resonance frequencies in fluid technical systems



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Simulative pressure vibration analysis is a proven tool for determining the resonant frequencies in hydraulic piping systems. In practical application today, however, there are still significant deviations between simulation and real system with low operating pressures.
The aim of the project is the development of a numerical method for a more precise consideration of the undissolved gas contained in the pressure medium when calculating the pressure wave propagation velocity in fluid-technical line systems.
In particular, the dynamics of the bubbles dissolved in the fluid are to be taken into account, which, depending on their size, distribution and excitation, have a decisive influence on the compressibility of the mixture.

Benefit Schedule
Improvement of the simulative pressure vibration analysis Redefinition of the compression module

Consideration of the bladder dynamics

Connection of the bubble dynamics to the characteristic procedure (MOC)

Simplification of the preliminary design of fluid technology systems

Implementation of a calculation routine in DSHplus

Validation on the test bench


Influence of Undissolved Air on the System Dynamics

Copyright: © ifas

Exact knowledge of the resonant frequencies of a fluid technical line system is of decisive importance for both the technical functioning and the fatigue strength of fluid technical systems. In this context, the simulative pressure vibration analysis is a proven tool to determine the resonance frequencies in pipe systems. In practical application of pressure vibration analysis, however, there are still significant deviations between simulation and real system in systems with operating pressures below 10 bar. At low operating pressures, especially the proportion of undissolved gas contained in the fluid has a great influence on the mixture compressibility, which is described by the compression module. While the pressure-dependent change of the static compression module has been taken into account for a long time, the dynamic situation due to bubbles contained in the fluid has not been considered so far. For this purpose, the bubble dynamics will be considered for the first time in the project by first developing the theoretical basis for the description of the substitute compression module. Subsequently, an adequate model is to be implemented and validated in a test rig developed for this purpose.


Simulative Modelling

The challenges in simulative modelling consist in the coupling of the known fundamentals on the one hand and the (highly dynamic) bubble dynamics on the other hand, which can be described, for example, with the help of the Rayleigh-Plesset equation - a non-linear partial differential equation of second order. The Method of Characteristics (MOC), which is also used in the project, has proved to be a proven solution method.
First results illustrate the numerical challenges due to the strongly varying time scales and confirm a large influence of bubble size and bubble distribution on the compression modulus. The results also show the dependence of the pressure propagation velocity on the excitation of the fluid column, which in turn depends on the choice of displacement units, so that more accurate models can contribute to a safe and efficient design of pipeline systems in the future.


Test Bench Development for Validation

Copyright: © ifas

A test bench is to be designed and put into operation for the validation of the previously developed fundamentals. This test stand is used to determine the speed of sound and compressibility with the introduction of bubbles.

The main challenges here are the targeted generation of bubbles of similar size and the evaluation with regard to bubble size and distribution. These parameters are essential for the parameterization of the already described simulation model. The capillary concept with simultaneous liquid supply pursued here serves the accurate generation of bubble sizes, whereby even bubbles of smaller diameter can be generated. The double shock pressure pulser allows for the first time statements about the dynamic situation of already excited bubbles. Finally, an optical detection of the rising bubbles should provide information about the bubble size distribution.



This research project is funded by the Federal Ministry of Economics and Energy (BMWi) within the framework of the Central Innovation Programme for Medium-Sized Enterprises (ZIM) as a cooperation project between the Institute for Fluid Power Drives and Systems (ifas) at RWTH Aachen University and FLUIDON GmbH. We would like to thank the sponsor and the cooperation partner for their support.