Bernhard Manhartsgruber | Johannes Kepler University Linz (original) (raw)

Papers by Bernhard Manhartsgruber

Proceedings in applied mathematics & mechanics, Dec 1, 2009

ABSTRACT This paper deals with the modeling of transients in low pressure transmission lines. Mod... more ABSTRACT This paper deals with the modeling of transients in low pressure transmission lines. Modeling of low pressure lines becomes more and more important for increasing efficiency of fast switching applications and performance of pumps e.g. common rail diesel injection systems and suction pipes of pumps. One simulation method is the lumped parameter model. For example a straight pipe can be modeled as a cascade of inertia, friction and compressibility blocks. In this paper, the idea of cascades is adopted for transient simulation of nonlinear fluids. The model includes a nonlinear fluid law of an oil-air mixture and the balance equations i.e. the compressibility and the inertia of the fluid. Friction is modeled by the frequency dependent friction model of Kagawa et.al. Comparison of simulation results with measurements from a test rig shows good correlation. Finally the scope of this simulation model is discussed and compared with measurements. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mathematical modelling and simulation have become widespread tools in the fluid power industry. T... more Mathematical modelling and simulation have become widespread tools in the fluid power industry. The sheer number of available software packages of commercial and academic nature as well as the abundance of published modeling strategies for certain components gives multiple possibilities for modeling one and the same fluid power system. The mathematical model is required to capture the behavior of the real system with a certain degree of accuracy while using a minimum of computational resources for reaching this goal. This trade-off cannot be assessed by looking on model complexity on the level of differential equations only. The computational efficiency is defined by the combination of mathematical modelling on the high level of differential equations, the choice of a solver, i.e. the time discretization, and the details of software implementation like the optimized compilation of right hand sides for the solvers. In order to compare different modeling strategies on various software platforms, well defined benchmark problems are needed. This paper presents a first attempt towards such a set of benchmark problems targeted at the specific problems of fluid power simulation. In a first benchmark case, a system with reversing flow at an orifice under the additional difficulty of a stiff differential equation system is presented. A second benchmark case uses the coupling between dry friction in a hydraulic cylinder and wave propagation in a transmission line which has been shown to give notorious modelling problems in the literature. The goal of the present publication is to present a proposal for such benchmark systems in order to make the accuracy and computational performance of simulation approaches more comparable.

Linköping electronic conference proceedings, Dec 20, 2017

The Lattice-Boltzmann Method for the approximate solution of the Navier-Stokes equations has beco... more The Lattice-Boltzmann Method for the approximate solution of the Navier-Stokes equations has become an interesting alternative to classical finite volume based discretization methods. Because the flow domain is not meshed in the classical sense but only voxelized and geometrically complex boundaries can be introduced in an easy form by bounce-back or off-Lattice boundary conditions, the method lends itself very well to simulations of channel flows inside hydraulic components. In this paper, a flow problem in a single acting cylinder attached to a 3/2 directional spool valve is used as a benchmark problem. The Lattice-Boltzmann simulation is used to generate a reference solution for the pressure step response of the blocked cylinder with superimposed wave propagation. From this reference data set, a non-parametric frequency domain input-output model is extracted and compared with results from classical lumped parameter modeling.

IFAC-PapersOnLine, 2015

Transmission line dynamics is an important subject for the simulation of fluid power systems. In ... more Transmission line dynamics is an important subject for the simulation of fluid power systems. In the case of moderate amplitudes, linear models are widely applied. While the frequency domain offers a very compact and straightforward description of the inputoutput behaviour, the implementation of transmission line models in standard simulation software using a linear, time-invariant, finite-dimensional state space system description poses an approximation problem that has been treated by a large number of authors in the fluid power literature. This paper presents the solution of the approximation problem in an optimal way and gives a set of reference data.

Bent axis hydraulic pumps and motors are extremely popular due to their high efficiency and large... more Bent axis hydraulic pumps and motors are extremely popular due to their high efficiency and large speed range. A number of different concepts exist with respect to kinematic restraints on the cylinder barrel motion. Some manufacturers rely upon a timing gear for precise synchronization of the shaft and barrel speeds while other companies have successfully introduced bent axis units without such a mechanism. The paper analyses the dynamics of bent axis machines with tapered pistons driving the cylinder barrel. A rotation of the pistons inside the corresponding bores is proposed to result in changing cylinder chamber to case drain leakages. The reported phenomenon is shown to have a significant effect on the low frequency part of pressure and flow pulsations. In this way, frequency components far below the fundamental frequency associated with the shaft revolution are generated.Copyright © 2016 by ASME

Computational Fluid Dynamics (CFD) has become a valuable tool in the development of fluid power c... more Computational Fluid Dynamics (CFD) has become a valuable tool in the development of fluid power components due to its ability to model flows in complex geometries where simple analytical models are difficult to apply. The downside of CFD is the high computational cost of the method which prevents the application to whole fluid power systems. However, large parts of such systems can usually be modelled by more simple approaches like transmission line modelling. A coupled approach for a simple transmission line system is shown in this paper: Cylindrical parts of the flow domain are modelled with a spatially one-dimensional transmission line model augmented with frequency-dependent friction. The geometrically more complex intersections of cylindrical geometries like elbow joints and T-junctions are treated with a CFD solver in the Open-FOAM system. The simulation results from this coupled model are compared against a set of measurements from a test rig for elbow joints where cylindrical transmission line sections are connected at an angle of 90 degrees.Copyright © 2014 by ASME

Simulation methods from simple lumped parameter approaches to complex computational fluid dynamic... more Simulation methods from simple lumped parameter approaches to complex computational fluid dynamics codes have become a widely used tool in the fluid power community. Certain tasks like the predicition of flow forces on the control spools in valves or the design of port plates in axial piston pumps are usually treated by the aid of numerical simulation. Like in many other cases, the underlying principle is the control of flow by orifices. The importance of orifice flow for hydraulic systems is reflected by the vast number of publications on various aspects of orifice flow in the fluid power literature. In lumped parameter simulations, the orifice equation giving the flow rate as a square root of the pressure drop is widely used even in transient cases where it is not clear whether the flow develops fast enough to justify the assumption of stationary flow. On the other end of the model complexity spectrum computational fluid dynamcis codes are used in the fluid power community. These very complex models require a high number of parameters for the tuning of turbulence models, wall models, and the like. The quality of the results heavily dependes on a good choice for these parameters. Additionally, the vast majority of turbulent flow simulations is done with the assumption of an incompressible fluid. Very often, the results from simulations deviate heavily from measurement results and only after parameter tuning a good match between model and simulation is achieved. This paper suggests the use of direct numerical simulations for simple and prototypical geometries in order to gain a better understanding for transient orifice flows lacking the fully developed flow assumed in traditional models.

This paper deals with the position control of a hydraulic servo-drive for translatory motion. The... more This paper deals with the position control of a hydraulic servo-drive for translatory motion. The application of a nonlinear composite feedback control based on singular perturbation methods (Kokotovic, Khalil & O’Reilly 1986) is described. A comparison with linear output and state feedback shows the benefits of nonlinear control. The nonlinear design uses feedback linearization for the reduced system and simple proportional feedback for the control of the pressure difference (fast system). The feedback linearization results in a standard second order behavior of the reduced system with cutoff frequency and damping as decoupled design parameters. The proposed method is especially suitable for oscillation drives. Experimental results are given for step responses and sine tracking.

Proceedings Of The Institution Of Mechanical Engineers, Part I: Journal Of Systems And Control Engineering, May 1, 2006

The Galerkin finite method for the simulation of transient, laminar flow in hydraulic pipelines w... more The Galerkin finite method for the simulation of transient, laminar flow in hydraulic pipelines with an interlacing grid has gained considerable popularity because of the relatively good accuracy achieved with a moderate model order. However, the implementation used in a number of software packages has recently been shown not to guarantee model passivity. This paper reviews the concept of using an interlacing grid. Two modifications of the previously published interlacing grid method are proposed and the resulting model is shown to fulfil the passivity requirement in an example where the previously published method fails. The new model also improves the approximation error in terms of the deviation of the frequency response from the theoretical frequency response given by the transcendental transfer functions of the well-known ‘frequency-dependent friction’ model. This improvement is bought at the expense of a slight increase in model order due to additional nodal values for the friction model at the boundary nodes. Furthermore, the new model is given in the form of a descriptor system in order to maintain the sparse structure of the system matrices.

Journal of Fluids Engineering-transactions of The Asme, Oct 27, 2008

This paper deals with the utilization of the dynamic characteristics of laminar flow in circular ... more This paper deals with the utilization of the dynamic characteristics of laminar flow in circular pipes for the indirect measurement of flow rates. A discrete-time state space realization of the transmission line dynamics is computed via inverse Laplace transform and an identification and model reduction method based on the singular value decomposition. This dynamic system is used for the computation of the flow rate at one end of a pipe section. Special attention is paid to the identification of the speed of sound and the dimensionless dissipation number of the pipe section, since exact knowledge of these parameters is crucial for the reliability of the measurement results. First, experimental validation results are given in a limited range of operating frequencies between 100 Hz and 2000 Hz. Flow rate variations within ±1.2 l/min have been measured with an uncertainty of ±0.07 l/min at the 95% confidence level. The test fluid was mineral oil.

Axial and radial piston pumps are the work horse of the fluid power industry in the medium to hig... more Axial and radial piston pumps are the work horse of the fluid power industry in the medium to high power range. During the maturation of the technology in the last five decades, both the pressure levels and the maximum rotational speeds have been increased significantly to meet the market demands for an increased power to weight or size ratio. The maximum speed of operation is often limited by cavitation occuring in the suction duct of pumps. A well known but expensive solution to the problem is the use of booster pumps to raise the suction pressure at the piston pump inlet. The rationale behind this solution is very simple: The pressure oscillations inevitably caused by the nonuniform operation of the piston pump will occur around an increased mean pressure level, thereby raising also the pressure minima and avoiding cavitation. The present paper looks into the dynamics of the suction process in more detail. A simulation model of an axial piston pump with a detailed model of the wave propagation in the suction line is analysed for the potential of mitigating the pressure oscillations locally at the pump inlet by actuating the pipe wall for instance with piezo ring actuators. In a first simulation study, the power electronics of the actuation system is idealized and a mathematical optimization of the actuation signals for a certain operating point of the pump is set up. A theoretical proof of concept can be achieved in this simulation. A second, and more detailed simulation includes the computation of power budgets for the power electronics operating the piezo actuation. An experimental proof of concept is left to future work at this point.

The International Fluid Power Conference is a two day event, intended for all those professionall... more The International Fluid Power Conference is a two day event, intended for all those professionally-involved with hydraulic or pneumatic power devices and for all those, wishing to be informed about the 'state of the art', new discoveries and innovations within the field of hydraulics and pneumatics. The gathering of experts at this conference in Maribor has been a tradition since 1995, and is organised by the Faculty of Mechanical Engineering at the University of Maribor, in Slovenia. Fluid Power conferences are organised every second year and cover those principal technical events within the field of fluid power technologies in Slovenia, and throughout this region of Europe. This year's conference is taking place on the 14th and 15th September in Maribor. We wish all participants at the International Conference-Fluid Power 2017 continued successful professional work, and hope that we have yet again added another small piece within the mosaic of fluid power.

ABSTRACT Fluid power systems consist of components like pumps, valves and actuators and of the li... more ABSTRACT Fluid power systems consist of components like pumps, valves and actuators and of the lines and hoses interconnecting these systems. Simple interconnections without branch points can be modelled as hydraulic two-port networks. This paper demonstrates the identification of linear state space models describing the input-output behaviour of hydraulic two-port networks in terms of pressure and flow-rate. The linear modelling approach restricts the applicability to the case of laminar flow with negligible influence of convective terms. Special attention is paid to a priori knowledge of certain model properties: The numerical optimization procedure used in the proposed identifcation method guarantees the passivity of the models and allows for instantaneous coupling of collocated pressure and flow variables according to the Joukowsky relation. The method takes experimental frequency response data as an input and generates a series of state-space approximations with increasing system order starting at order one. A hydraulic hose is presented as an example.

IFAC-PapersOnLine, 2015

Due to rising importance of wave propagation effects in fluid power systems, CFD (computational f... more Due to rising importance of wave propagation effects in fluid power systems, CFD (computational fluid dynamics) codes take also on greater significance. These highly sophisticated codes have the capability to calculate the full resolution of a pressure field as well as of a velocity field in arbitrarily complex geometries. But this brilliant development brings also a huge disadvantage, namely enormous computational costs. Because of this reason CFD software often disqualifies itself as first choice application. However, considering a whole computational domain and it's physics the idea comes up, that there are regions which require a higher spatial resolution than others. Mesh refinement is a common practice to get a higher resolution in regions of greater interest while reducing the overall number of cells and hence the simulation time. This paper presents results of simulations with OpenFOAM, where mesh refinement has been done with polyhedral elements. As a simple benchmark system a pipe with a cross-sectional jump has been used. First the optimal CFD result for this system has been determined by doubeling the number of elements in each direction for each additional simulation until the variations in the outcome has vanished. This simuation result has been compared to a set of simulations, where mesh refinement has only adapted to the region after the cross-sectional jump, where the higher number of cells has been considered as a benefit.

ABSTRACT Mineral-oil based hydraulic fluids are used for power transmission in a number of wide-s... more ABSTRACT Mineral-oil based hydraulic fluids are used for power transmission in a number of wide-spread technologies such as hydraulic drives in heavy industrial equipment and construction vehicles as well as automotive systems like power steering and fuel injection. The mathematical modeling of these systems traditionally focuses on the medium to high pressure range where the effects of increased compressiblity due to free air are assumed to be negligible. The goal of the present paper is the precise measurement of the behaviour of a sample of a standard mineral oil based hydraulic fluid under controlled periodic volume change. The fluid is contained in a sealed test chamber equipped with sensors for pressure and temperature as well as a servo-hydraulic actuation system for the controlled volume change. Preliminary results show the expected hysteresis behavior according to the different time scales of the processes of air release and dissolution. Fifty consecutive cycles of compression and decompression are measured in steady state. This collection of measurement data is used for the identification of a dynamic model for the prediction of the amount of free air from the history of pressure change in the system. The fluid used in the tests is an HLP type hydraulic oil with a nominal viscosity of 32 cSt (32· 10−6 m2/s). Absolute pressure values between 0.3 bar and 32 bar (3 · 104 Pa to 3.2 · 106 Pa) are covered by the published experimental results.

ABSTRACT Liquid transmission lines are used in a large number of technologies from water distribu... more ABSTRACT Liquid transmission lines are used in a large number of technologies from water distribution and oil pipelines to the high pressure applications in hydraulic drives and fuel injection. Due to various physical effects the idealization of a weakly compressible fluid with a constant bulk modulus of compressibility resulting in a constant wave speed becomes insufficient in a number of interesting applications. The present paper presents simulation results for a simple case where a single transmission line is excited by a sinusoidal flow rate at one end while the other end is blocked. The flow equations are discretized both in time and space resulting in a huge equation system to be solved for the periodic solution.

Transmission line modeling has played a crucial role in understanding the dynamics of fluid power... more Transmission line modeling has played a crucial role in understanding the dynamics of fluid power systems. A vast body of literature exists from simple lumped parameter approaches to fully coupled three-dimensional fluid structure interaction models. When it comes to computationally efficient, yet physically sound low order models needed for fast computations iteratively called by optimization codes or for the purpose of model based control design, there is still room for improvement. Modal approximations of the input-output behaviour of liquid transmission lines have been around for decades. The basic idea of tuning the parameters of a canonical linear time invariant state space model to fit the transfer functions of a transmission line model in the H2-optimal sense under passivity constraints has been published by the author of the present paper in the past. However, the method so far was barely usable due to numerical difficulties in the underlying optimization process. A new implementation of the method employing quadruple-precision floating point numbers has recently been found to resolve the convergence problems and is reported in the present paper. The new version of the method is based on analytic computation of the cost and constraint functions as well as their gradients in the computer algebra package Maple and automatic code generation for compilation in FORTRAN. Results are very promising because both the entire low frequency behaviour and the first three eigenmodes of a transmission line model can be accurately covered by a model of order eight only.

Figure 1: Typical ILs anions (top) and cations (bottom) it allows a numerous combinations. When c... more Figure 1: Typical ILs anions (top) and cations (bottom) it allows a numerous combinations. When combined with certain specified cation anions you will obtain another saltwhich, if it does not work by trial and error but intentionally, can be synthesised into a completely new material with entirely new properties. However the cations and anions present in ionic liquids are so formulated that the resulting salts hardly crystallise. Therefore the ionic liquid is liquid within a wide temperature range. An important feature of ionic liquids is the possibility of adapting these physical-chemical properties through changing the natures of the anions and cations. The number of possible combinations is extremely high, that is why the best ionic liquid is supposed to be adapted for different usage: 1•10 18. The possibilities for their usages within different areas of technology have been researched because of the numerous good properties of ionic liquids and their advantages over conventional liquids. Within different spheres of industry the ILs they are already used in practice. They are used e.g. during chemical synthesis and separation, cellulose production, crude oil processing, paint production, nuclear and solar energy, hydrogen storage, waste processing and battery production. Also their applicability's regarding heat transfer and storage is at a stage of research and development. The scope of applications is constantly expanding. In addition the arena of functional liquids including lubricants and therefore hydraulic fluids as well are within a the phase of research and development. E.g. [2], [3], [4], [5]. 2 Physico chemical Properties of ILs Due to their excellent properties ILs catches the attention of a broad professional public. The most outstanding properties of ionic liquids are outlined below. ILs have virtually no vapour pressure; pure and degassed ionic liquids show no cavitations. They also do not have boiling point and are non-flammable below the high thermal decomposition point, while their melting points can go down to-60 °C. Due to their excellent thermo-oxidative long term stability up to 300 °C and more they are not prone to ageing. ILs usually have small friction coefficients and good lubrication properties: quite often pure ionic liquids without any EP and AW additives can be found, which show friction coefficients better (and sometimes even much better) than a conventional fully synthetic high performance fluid under identical conditions.

ASME/BATH 2019 Symposium on Fluid Power and Motion Control, 2019

The hydraulic binary counter requires switching valves with a hysteretic response. In this paper ... more The hydraulic binary counter requires switching valves with a hysteretic response. In this paper an elastic snap through element is studied as means for that. The concept is based on a buckling beam which is elastically supported in axial direction in order to adjust its buckling properties with moderate manufacturing precision and to assure a well defined snap through behavior. The elastic support is provided by a cantilever beam. A rigorous optimization is performed heading for a most compact and fatigue durable design which exhibits the required lateral force displacement characteristics. A genetic algorithm is used to find the global design optimum. The stress/displacement properties of each design variant are computed by a compact model of the snap through system. It is derived by a Ritz method to obtain approximate solutions of the nonlinear buckling beam behavior. Its validity is checked by a Finite Element model. A compact design is possible if high strength spring steel is ...

Models for the dynamic behaviour of transmission lines are essential for the understanding of wav... more Models for the dynamic behaviour of transmission lines are essential for the understanding of wave propagation phenomena in hydraulic pipelines and hoses. If the Reynolds numbers are low enough to justify the assumption of laminar flow and if convective terms are negligible, the governing equations are linear and a very compact description of the input-output behaviour of a transmission line exists in the frequency domain. For a coupled simulation of networks of transmission lines interacting with other, possibly nonlinear components such as valves, there are numerous approaches for the approximation of the transcendental transfer functions arising from the transmission line modelling by finite dimensional models in the time domain. Discrete-time approaches such as the method of characteristics with a fixed grid are known to be very accurate but computationally ineffective due to the high number of state variables involved. This paper shows a method for the derivation of reduced order models with a trade-off between the degree of accuracy and the system order and with the additional feature that important properties like the passivity of the transmission line model is guaranteed.

Proceedings in applied mathematics & mechanics, Dec 1, 2009

ABSTRACT This paper deals with the modeling of transients in low pressure transmission lines. Mod... more ABSTRACT This paper deals with the modeling of transients in low pressure transmission lines. Modeling of low pressure lines becomes more and more important for increasing efficiency of fast switching applications and performance of pumps e.g. common rail diesel injection systems and suction pipes of pumps. One simulation method is the lumped parameter model. For example a straight pipe can be modeled as a cascade of inertia, friction and compressibility blocks. In this paper, the idea of cascades is adopted for transient simulation of nonlinear fluids. The model includes a nonlinear fluid law of an oil-air mixture and the balance equations i.e. the compressibility and the inertia of the fluid. Friction is modeled by the frequency dependent friction model of Kagawa et.al. Comparison of simulation results with measurements from a test rig shows good correlation. Finally the scope of this simulation model is discussed and compared with measurements. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mathematical modelling and simulation have become widespread tools in the fluid power industry. T... more Mathematical modelling and simulation have become widespread tools in the fluid power industry. The sheer number of available software packages of commercial and academic nature as well as the abundance of published modeling strategies for certain components gives multiple possibilities for modeling one and the same fluid power system. The mathematical model is required to capture the behavior of the real system with a certain degree of accuracy while using a minimum of computational resources for reaching this goal. This trade-off cannot be assessed by looking on model complexity on the level of differential equations only. The computational efficiency is defined by the combination of mathematical modelling on the high level of differential equations, the choice of a solver, i.e. the time discretization, and the details of software implementation like the optimized compilation of right hand sides for the solvers. In order to compare different modeling strategies on various software platforms, well defined benchmark problems are needed. This paper presents a first attempt towards such a set of benchmark problems targeted at the specific problems of fluid power simulation. In a first benchmark case, a system with reversing flow at an orifice under the additional difficulty of a stiff differential equation system is presented. A second benchmark case uses the coupling between dry friction in a hydraulic cylinder and wave propagation in a transmission line which has been shown to give notorious modelling problems in the literature. The goal of the present publication is to present a proposal for such benchmark systems in order to make the accuracy and computational performance of simulation approaches more comparable.

Linköping electronic conference proceedings, Dec 20, 2017

The Lattice-Boltzmann Method for the approximate solution of the Navier-Stokes equations has beco... more The Lattice-Boltzmann Method for the approximate solution of the Navier-Stokes equations has become an interesting alternative to classical finite volume based discretization methods. Because the flow domain is not meshed in the classical sense but only voxelized and geometrically complex boundaries can be introduced in an easy form by bounce-back or off-Lattice boundary conditions, the method lends itself very well to simulations of channel flows inside hydraulic components. In this paper, a flow problem in a single acting cylinder attached to a 3/2 directional spool valve is used as a benchmark problem. The Lattice-Boltzmann simulation is used to generate a reference solution for the pressure step response of the blocked cylinder with superimposed wave propagation. From this reference data set, a non-parametric frequency domain input-output model is extracted and compared with results from classical lumped parameter modeling.

IFAC-PapersOnLine, 2015

Transmission line dynamics is an important subject for the simulation of fluid power systems. In ... more Transmission line dynamics is an important subject for the simulation of fluid power systems. In the case of moderate amplitudes, linear models are widely applied. While the frequency domain offers a very compact and straightforward description of the inputoutput behaviour, the implementation of transmission line models in standard simulation software using a linear, time-invariant, finite-dimensional state space system description poses an approximation problem that has been treated by a large number of authors in the fluid power literature. This paper presents the solution of the approximation problem in an optimal way and gives a set of reference data.

Bent axis hydraulic pumps and motors are extremely popular due to their high efficiency and large... more Bent axis hydraulic pumps and motors are extremely popular due to their high efficiency and large speed range. A number of different concepts exist with respect to kinematic restraints on the cylinder barrel motion. Some manufacturers rely upon a timing gear for precise synchronization of the shaft and barrel speeds while other companies have successfully introduced bent axis units without such a mechanism. The paper analyses the dynamics of bent axis machines with tapered pistons driving the cylinder barrel. A rotation of the pistons inside the corresponding bores is proposed to result in changing cylinder chamber to case drain leakages. The reported phenomenon is shown to have a significant effect on the low frequency part of pressure and flow pulsations. In this way, frequency components far below the fundamental frequency associated with the shaft revolution are generated.Copyright © 2016 by ASME

Computational Fluid Dynamics (CFD) has become a valuable tool in the development of fluid power c... more Computational Fluid Dynamics (CFD) has become a valuable tool in the development of fluid power components due to its ability to model flows in complex geometries where simple analytical models are difficult to apply. The downside of CFD is the high computational cost of the method which prevents the application to whole fluid power systems. However, large parts of such systems can usually be modelled by more simple approaches like transmission line modelling. A coupled approach for a simple transmission line system is shown in this paper: Cylindrical parts of the flow domain are modelled with a spatially one-dimensional transmission line model augmented with frequency-dependent friction. The geometrically more complex intersections of cylindrical geometries like elbow joints and T-junctions are treated with a CFD solver in the Open-FOAM system. The simulation results from this coupled model are compared against a set of measurements from a test rig for elbow joints where cylindrical transmission line sections are connected at an angle of 90 degrees.Copyright © 2014 by ASME

Simulation methods from simple lumped parameter approaches to complex computational fluid dynamic... more Simulation methods from simple lumped parameter approaches to complex computational fluid dynamics codes have become a widely used tool in the fluid power community. Certain tasks like the predicition of flow forces on the control spools in valves or the design of port plates in axial piston pumps are usually treated by the aid of numerical simulation. Like in many other cases, the underlying principle is the control of flow by orifices. The importance of orifice flow for hydraulic systems is reflected by the vast number of publications on various aspects of orifice flow in the fluid power literature. In lumped parameter simulations, the orifice equation giving the flow rate as a square root of the pressure drop is widely used even in transient cases where it is not clear whether the flow develops fast enough to justify the assumption of stationary flow. On the other end of the model complexity spectrum computational fluid dynamcis codes are used in the fluid power community. These very complex models require a high number of parameters for the tuning of turbulence models, wall models, and the like. The quality of the results heavily dependes on a good choice for these parameters. Additionally, the vast majority of turbulent flow simulations is done with the assumption of an incompressible fluid. Very often, the results from simulations deviate heavily from measurement results and only after parameter tuning a good match between model and simulation is achieved. This paper suggests the use of direct numerical simulations for simple and prototypical geometries in order to gain a better understanding for transient orifice flows lacking the fully developed flow assumed in traditional models.

This paper deals with the position control of a hydraulic servo-drive for translatory motion. The... more This paper deals with the position control of a hydraulic servo-drive for translatory motion. The application of a nonlinear composite feedback control based on singular perturbation methods (Kokotovic, Khalil & O’Reilly 1986) is described. A comparison with linear output and state feedback shows the benefits of nonlinear control. The nonlinear design uses feedback linearization for the reduced system and simple proportional feedback for the control of the pressure difference (fast system). The feedback linearization results in a standard second order behavior of the reduced system with cutoff frequency and damping as decoupled design parameters. The proposed method is especially suitable for oscillation drives. Experimental results are given for step responses and sine tracking.

Proceedings Of The Institution Of Mechanical Engineers, Part I: Journal Of Systems And Control Engineering, May 1, 2006

The Galerkin finite method for the simulation of transient, laminar flow in hydraulic pipelines w... more The Galerkin finite method for the simulation of transient, laminar flow in hydraulic pipelines with an interlacing grid has gained considerable popularity because of the relatively good accuracy achieved with a moderate model order. However, the implementation used in a number of software packages has recently been shown not to guarantee model passivity. This paper reviews the concept of using an interlacing grid. Two modifications of the previously published interlacing grid method are proposed and the resulting model is shown to fulfil the passivity requirement in an example where the previously published method fails. The new model also improves the approximation error in terms of the deviation of the frequency response from the theoretical frequency response given by the transcendental transfer functions of the well-known ‘frequency-dependent friction’ model. This improvement is bought at the expense of a slight increase in model order due to additional nodal values for the friction model at the boundary nodes. Furthermore, the new model is given in the form of a descriptor system in order to maintain the sparse structure of the system matrices.

Journal of Fluids Engineering-transactions of The Asme, Oct 27, 2008

This paper deals with the utilization of the dynamic characteristics of laminar flow in circular ... more This paper deals with the utilization of the dynamic characteristics of laminar flow in circular pipes for the indirect measurement of flow rates. A discrete-time state space realization of the transmission line dynamics is computed via inverse Laplace transform and an identification and model reduction method based on the singular value decomposition. This dynamic system is used for the computation of the flow rate at one end of a pipe section. Special attention is paid to the identification of the speed of sound and the dimensionless dissipation number of the pipe section, since exact knowledge of these parameters is crucial for the reliability of the measurement results. First, experimental validation results are given in a limited range of operating frequencies between 100 Hz and 2000 Hz. Flow rate variations within ±1.2 l/min have been measured with an uncertainty of ±0.07 l/min at the 95% confidence level. The test fluid was mineral oil.

Axial and radial piston pumps are the work horse of the fluid power industry in the medium to hig... more Axial and radial piston pumps are the work horse of the fluid power industry in the medium to high power range. During the maturation of the technology in the last five decades, both the pressure levels and the maximum rotational speeds have been increased significantly to meet the market demands for an increased power to weight or size ratio. The maximum speed of operation is often limited by cavitation occuring in the suction duct of pumps. A well known but expensive solution to the problem is the use of booster pumps to raise the suction pressure at the piston pump inlet. The rationale behind this solution is very simple: The pressure oscillations inevitably caused by the nonuniform operation of the piston pump will occur around an increased mean pressure level, thereby raising also the pressure minima and avoiding cavitation. The present paper looks into the dynamics of the suction process in more detail. A simulation model of an axial piston pump with a detailed model of the wave propagation in the suction line is analysed for the potential of mitigating the pressure oscillations locally at the pump inlet by actuating the pipe wall for instance with piezo ring actuators. In a first simulation study, the power electronics of the actuation system is idealized and a mathematical optimization of the actuation signals for a certain operating point of the pump is set up. A theoretical proof of concept can be achieved in this simulation. A second, and more detailed simulation includes the computation of power budgets for the power electronics operating the piezo actuation. An experimental proof of concept is left to future work at this point.

The International Fluid Power Conference is a two day event, intended for all those professionall... more The International Fluid Power Conference is a two day event, intended for all those professionally-involved with hydraulic or pneumatic power devices and for all those, wishing to be informed about the 'state of the art', new discoveries and innovations within the field of hydraulics and pneumatics. The gathering of experts at this conference in Maribor has been a tradition since 1995, and is organised by the Faculty of Mechanical Engineering at the University of Maribor, in Slovenia. Fluid Power conferences are organised every second year and cover those principal technical events within the field of fluid power technologies in Slovenia, and throughout this region of Europe. This year's conference is taking place on the 14th and 15th September in Maribor. We wish all participants at the International Conference-Fluid Power 2017 continued successful professional work, and hope that we have yet again added another small piece within the mosaic of fluid power.

ABSTRACT Fluid power systems consist of components like pumps, valves and actuators and of the li... more ABSTRACT Fluid power systems consist of components like pumps, valves and actuators and of the lines and hoses interconnecting these systems. Simple interconnections without branch points can be modelled as hydraulic two-port networks. This paper demonstrates the identification of linear state space models describing the input-output behaviour of hydraulic two-port networks in terms of pressure and flow-rate. The linear modelling approach restricts the applicability to the case of laminar flow with negligible influence of convective terms. Special attention is paid to a priori knowledge of certain model properties: The numerical optimization procedure used in the proposed identifcation method guarantees the passivity of the models and allows for instantaneous coupling of collocated pressure and flow variables according to the Joukowsky relation. The method takes experimental frequency response data as an input and generates a series of state-space approximations with increasing system order starting at order one. A hydraulic hose is presented as an example.

IFAC-PapersOnLine, 2015

Due to rising importance of wave propagation effects in fluid power systems, CFD (computational f... more Due to rising importance of wave propagation effects in fluid power systems, CFD (computational fluid dynamics) codes take also on greater significance. These highly sophisticated codes have the capability to calculate the full resolution of a pressure field as well as of a velocity field in arbitrarily complex geometries. But this brilliant development brings also a huge disadvantage, namely enormous computational costs. Because of this reason CFD software often disqualifies itself as first choice application. However, considering a whole computational domain and it's physics the idea comes up, that there are regions which require a higher spatial resolution than others. Mesh refinement is a common practice to get a higher resolution in regions of greater interest while reducing the overall number of cells and hence the simulation time. This paper presents results of simulations with OpenFOAM, where mesh refinement has been done with polyhedral elements. As a simple benchmark system a pipe with a cross-sectional jump has been used. First the optimal CFD result for this system has been determined by doubeling the number of elements in each direction for each additional simulation until the variations in the outcome has vanished. This simuation result has been compared to a set of simulations, where mesh refinement has only adapted to the region after the cross-sectional jump, where the higher number of cells has been considered as a benefit.

ABSTRACT Mineral-oil based hydraulic fluids are used for power transmission in a number of wide-s... more ABSTRACT Mineral-oil based hydraulic fluids are used for power transmission in a number of wide-spread technologies such as hydraulic drives in heavy industrial equipment and construction vehicles as well as automotive systems like power steering and fuel injection. The mathematical modeling of these systems traditionally focuses on the medium to high pressure range where the effects of increased compressiblity due to free air are assumed to be negligible. The goal of the present paper is the precise measurement of the behaviour of a sample of a standard mineral oil based hydraulic fluid under controlled periodic volume change. The fluid is contained in a sealed test chamber equipped with sensors for pressure and temperature as well as a servo-hydraulic actuation system for the controlled volume change. Preliminary results show the expected hysteresis behavior according to the different time scales of the processes of air release and dissolution. Fifty consecutive cycles of compression and decompression are measured in steady state. This collection of measurement data is used for the identification of a dynamic model for the prediction of the amount of free air from the history of pressure change in the system. The fluid used in the tests is an HLP type hydraulic oil with a nominal viscosity of 32 cSt (32· 10−6 m2/s). Absolute pressure values between 0.3 bar and 32 bar (3 · 104 Pa to 3.2 · 106 Pa) are covered by the published experimental results.

ABSTRACT Liquid transmission lines are used in a large number of technologies from water distribu... more ABSTRACT Liquid transmission lines are used in a large number of technologies from water distribution and oil pipelines to the high pressure applications in hydraulic drives and fuel injection. Due to various physical effects the idealization of a weakly compressible fluid with a constant bulk modulus of compressibility resulting in a constant wave speed becomes insufficient in a number of interesting applications. The present paper presents simulation results for a simple case where a single transmission line is excited by a sinusoidal flow rate at one end while the other end is blocked. The flow equations are discretized both in time and space resulting in a huge equation system to be solved for the periodic solution.

Transmission line modeling has played a crucial role in understanding the dynamics of fluid power... more Transmission line modeling has played a crucial role in understanding the dynamics of fluid power systems. A vast body of literature exists from simple lumped parameter approaches to fully coupled three-dimensional fluid structure interaction models. When it comes to computationally efficient, yet physically sound low order models needed for fast computations iteratively called by optimization codes or for the purpose of model based control design, there is still room for improvement. Modal approximations of the input-output behaviour of liquid transmission lines have been around for decades. The basic idea of tuning the parameters of a canonical linear time invariant state space model to fit the transfer functions of a transmission line model in the H2-optimal sense under passivity constraints has been published by the author of the present paper in the past. However, the method so far was barely usable due to numerical difficulties in the underlying optimization process. A new implementation of the method employing quadruple-precision floating point numbers has recently been found to resolve the convergence problems and is reported in the present paper. The new version of the method is based on analytic computation of the cost and constraint functions as well as their gradients in the computer algebra package Maple and automatic code generation for compilation in FORTRAN. Results are very promising because both the entire low frequency behaviour and the first three eigenmodes of a transmission line model can be accurately covered by a model of order eight only.

Figure 1: Typical ILs anions (top) and cations (bottom) it allows a numerous combinations. When c... more Figure 1: Typical ILs anions (top) and cations (bottom) it allows a numerous combinations. When combined with certain specified cation anions you will obtain another saltwhich, if it does not work by trial and error but intentionally, can be synthesised into a completely new material with entirely new properties. However the cations and anions present in ionic liquids are so formulated that the resulting salts hardly crystallise. Therefore the ionic liquid is liquid within a wide temperature range. An important feature of ionic liquids is the possibility of adapting these physical-chemical properties through changing the natures of the anions and cations. The number of possible combinations is extremely high, that is why the best ionic liquid is supposed to be adapted for different usage: 1•10 18. The possibilities for their usages within different areas of technology have been researched because of the numerous good properties of ionic liquids and their advantages over conventional liquids. Within different spheres of industry the ILs they are already used in practice. They are used e.g. during chemical synthesis and separation, cellulose production, crude oil processing, paint production, nuclear and solar energy, hydrogen storage, waste processing and battery production. Also their applicability's regarding heat transfer and storage is at a stage of research and development. The scope of applications is constantly expanding. In addition the arena of functional liquids including lubricants and therefore hydraulic fluids as well are within a the phase of research and development. E.g. [2], [3], [4], [5]. 2 Physico chemical Properties of ILs Due to their excellent properties ILs catches the attention of a broad professional public. The most outstanding properties of ionic liquids are outlined below. ILs have virtually no vapour pressure; pure and degassed ionic liquids show no cavitations. They also do not have boiling point and are non-flammable below the high thermal decomposition point, while their melting points can go down to-60 °C. Due to their excellent thermo-oxidative long term stability up to 300 °C and more they are not prone to ageing. ILs usually have small friction coefficients and good lubrication properties: quite often pure ionic liquids without any EP and AW additives can be found, which show friction coefficients better (and sometimes even much better) than a conventional fully synthetic high performance fluid under identical conditions.

ASME/BATH 2019 Symposium on Fluid Power and Motion Control, 2019

The hydraulic binary counter requires switching valves with a hysteretic response. In this paper ... more The hydraulic binary counter requires switching valves with a hysteretic response. In this paper an elastic snap through element is studied as means for that. The concept is based on a buckling beam which is elastically supported in axial direction in order to adjust its buckling properties with moderate manufacturing precision and to assure a well defined snap through behavior. The elastic support is provided by a cantilever beam. A rigorous optimization is performed heading for a most compact and fatigue durable design which exhibits the required lateral force displacement characteristics. A genetic algorithm is used to find the global design optimum. The stress/displacement properties of each design variant are computed by a compact model of the snap through system. It is derived by a Ritz method to obtain approximate solutions of the nonlinear buckling beam behavior. Its validity is checked by a Finite Element model. A compact design is possible if high strength spring steel is ...

Models for the dynamic behaviour of transmission lines are essential for the understanding of wav... more Models for the dynamic behaviour of transmission lines are essential for the understanding of wave propagation phenomena in hydraulic pipelines and hoses. If the Reynolds numbers are low enough to justify the assumption of laminar flow and if convective terms are negligible, the governing equations are linear and a very compact description of the input-output behaviour of a transmission line exists in the frequency domain. For a coupled simulation of networks of transmission lines interacting with other, possibly nonlinear components such as valves, there are numerous approaches for the approximation of the transcendental transfer functions arising from the transmission line modelling by finite dimensional models in the time domain. Discrete-time approaches such as the method of characteristics with a fixed grid are known to be very accurate but computationally ineffective due to the high number of state variables involved. This paper shows a method for the derivation of reduced order models with a trade-off between the degree of accuracy and the system order and with the additional feature that important properties like the passivity of the transmission line model is guaranteed.