Valve Dynamics Research Articles

Quantitative comparison of alternative control schemes for air‐cooled condensers

AbstractAir‐cooled heat exchangers present unique control problems because they are more sensitive to ambient disturbances than water‐cooled heat exchangers. Each of the several alternative methods for manipulating heat removal has some disadvantages. The obvious choice of manipulating the flow rate of the air is mechanically difficult. This investigation compares three alternative process and control structures for controlling pressure in a distillation column with an air‐cooled condenser: manipulating air flow (fan speed, louver opening, or blade pitch), throttling a control valve in the overhead vapor line, or bypassing vapor around the condenser. Results show that direct manipulation of air flow rate is slower than other methods, and the deviations in column pressure are larger. Overhead vapor throttling gives good control of column pressure, provided the nonlinearity of the butterfly valve is taken into account and the dynamics of the large valve are not too slow. However, reflux‐drum pressure and temperature vary drastically. This necessitates the use of a high‐head reflux pump and results in varying degrees of subcooling, which affect the separation in the column. Hot‐vapor bypassing, using two control valves (one in a vapor bypass line controlling reflux drum pressure and one in the vapor line to the condenser controlling column pressure), provides effective pressure control and lessens the variability in reflux‐drum temperature. However, more condenser area is required and the plumbing must be correct so that hydraulic problems do not occur. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Numerical simulation of cardiovascular dynamics with healthy and diseased heart valves

This paper presents a new concentrated parameter model for cardiovascular dynamics that includes an innovative model of heart valve dynamics, which is embedded in the overall model of the four chambers of the heart and the systemic and pulmonary circulation loops. The heart chambers are described with a variable elastance model, and the systemic and pulmonary loops are described with modified Windkessel models. In modelling the heart valve dynamics, the various factors that influence the valve motion are examined, and the governing differential equation for valve motion is derived. The heart valve model includes the influence of the blood pressure effect, the friction effect from the tissue, and from blood motion. As improvement from previous works, the contribution of the blood vortex effect in the vicinity of the valve leaflets to valve motion is specially considered. The proposed model is then used in simulation of healthy and certain pathological conditions such as mitral valve stenosis and aortic regurgitation. The predicted results agree well with results illustrated in cardiology textbooks.

Modelling of a Switching Control Hydraulic System

Modelling of a hydraulic system featuring a specific type of switching control is presented. Despite conventional hydraulic drive technology where rather smooth changes of pressure and flow rate are intended and where oscillations constitute undesired phenomena, switching control provokes oscillations as an indispensable element to achieve high energetic efficiency with valve control. The system under study is one which comprises a novel switching valve, a long line with considerable wave propagation dynamics, a hydraulic cylinder, and the valve's dynamics.

Macrospin models of spin transfer dynamics

The current-induced magnetization dynamics of a spin valve are studied using a macrospin (single-domain) approximation and numerical solutions of a generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative comparison to experiment [S. I. Kiselev, J. C. Sankey, I. N. Krivortov, N. C. Emley, R. J. Schoelkopf, R. A. Buhrman, and D. C. Ralph, Nature 425, 380 (2003)], we calculate the resistance and microwave power as a function of current and external field, including the effects of anisotropies, damping, spin-transfer torque, thermal fluctuations, spin-pumping, and incomplete absorption of transverse spin current. Although many features of experiment appear in the simulations, there are two significant discrepancies: the current dependence of the precession frequency and the presence and/or absence of a microwave quiet magnetic phase with a distinct magnetoresistance signature. Comparison is made to micromagnetic simulations designed to model the same experiment.

Modelling and Dynamic Response of a Damper with Relief Valve

This paper outlines several possible methods of modelling a passive hydraulic damper with a bypass tube that is opened by a precompressed relief valve. Initially a simple algebraic model is derived which is developed into a more computationally complex model incorporating the dynamics of the internal spring valve and fluid compressibility. Numerical simulations indicate realistic dynamical phenomena and suggest key design parameters.

APPROXIMATE REALIZATION OF VALVE DYNAMICS WITH TIME DELAY

This paper describes the model identification of control valve dynamics with a time delay. An experimental set-up is used to perform both frequency response and step response measurements on the control valve. An approximate realization algorithm and a new time delay estimation method are applied on the experimental data. The results illustrate that the suggested approach provides a good time delay estimate and a reasonable state space description for the valve dynamics.

Modelling of a hydraulic power steering system

Consideration is given to the modelling of a commercial hydraulic power steering system. The modelling philosophy with a comprehensive understanding of the control edge geometry is essential to an optimal system and the steady-state characteristics of a non-faceted valve and a two-facet valve are compared to understand the steering feeling. A test-rig is designed which incorporates the power steering assembly with a pneumatic loading system. The dynamics of the control valve and cylinder, pneumatic loading system, a non-linear friction model and the pump characteristics are then considered for dynamic performance and simulation results are compared with experimental results. The friction model contributes to the understanding of the pressure peak at the beginning of steering and the existence of a final steering angle error in practice.

Automatic velocity control of a self-propelled windrower

A velocity control methodology is presented for an agricultural vehicle with a hydrostatic drive-train. A cascade control design incorporating a current control, cylinder position control and velocity control is proposed. The cylinder position controller uses a set of gain scheduled lag compensators in series with a sequence of segmented deadzone inversions. The deadzone inversions compensate for nonlinear valve dynamics. The velocity controller consists of a digital feedback control. A stability analysis of the closed loop system is performed. Experimental results from a self-propelled windrower are used to demonstrate the effectiveness of the control system.

Initial Opening of a Compressor Valve

AbstractThe initial opening of a compressor valve is investigated. The motion of the valve plate is essentially determined by the pressure distribution in the valve gap. Since the initial gap is extremly narrow a conventional CFD solution fails. Therefore the initial stages of the opening process are modelled by using an asymptotic expansion with respect to a small asepct ratio of the valve gap. The purpose of the analysis is to provide an initial condition for a CFD simulation of the valve dynamics. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Feasibility of Applying Active Lubrication to Reduce Vibration in Industrial Compressors

In this paper the complete set of modified Reynolds’ equations for the active lubrication is presented. The solution of such a set of equations allows the determination of stiffness and damping coefficients of actively lubricated bearings. These coefficients are not just dependent on Sommerfeld number, as it would be the case of conventional hydrodynamic bearings, but they are also dependent on the excitation frequencies and gains of the control loop. Stiffness as well as damping coefficients can be strongly influenced by the choice of the control strategy, servo valve dynamics and geometry of the orifices distributed over the sliding surface. The dynamic coefficients of tilting-pad bearings with and without active lubrication and their influence on an industrial compressor of 391 Kg, which operates with a maximum speed of 10,200 rpm, are analyzed. In the original compressor design, the bearing housings are mounted on squeeze-film dampers in order to ensure reasonable stability margins during full load condition (high maximum continuous speed). Instead of having a combination of tilting-pad bearings and squeeze-film dampers, another design solution is proposed and theoretically investigated in the present paper, i.e., using actively lubricated bearings. By choosing a suitable set of control gains, it is possible not only to increase the stability of the rotor-bearing system, but also enlarge its operational frequency range.

Design optimization of a rolling piston compressor for refrigerators

In this paper, the performance of a rolling piston compressor has been optimized under preset operational conditions and design constraints by employing a multi-variable, direct search, constrained optimization technique. A mathematical model for the compressor was first formulated. The model accounts for geometrical configuration, thermodynamics effects, valve dynamics, flow and mechanical considerations. The accuracy of the model was verified by comparing its prediction with measured results. The model was then linked with an optimization algorithm to search for a combination of some six design dimensions and seven sets of design constraints, for an optimum compressor performance with minimum mechanical losses. The results of the study suggested that for a given compressor swept volume, a proper combination of the compressor design dimensions can lead to significant compressor performance. The theoretical study predicted that a 50% reduction in mechanical loss is possible and this brings about an improvement in the coefficient of performance of the compressor of more than 14%.

The Influence of a Nonlinear Resistance Element Upon In Vitro Aortic Pressure Tracings and Aortic Valve Motions

In vitro testing of biological heart valves requires pressure and flow waveforms closely simulating natural conditions, which are mainly influenced by the characteristics of the vascular system. Simulation of the arterial function in artificial circulations was mostly performed by the useful Windkessel model but sometimes failed by generating inadequate systolic pressures. The integration of a novel nonlinear resistance element may improve the Windkessel function. Native porcine aortic valves were studied in a mock circulation with a novel nonlinear resistance element combined with the Windkessel compared with an aperture plate resistance. Pressure and flow measurements were performed at varying heart rates and stroke volumes and analyzed in the time and frequency domain. Aortic valve motions were evaluated using high speed video recording. With the classical afterload configuration including an aperture plate resistance, the pressure tracings showed a nonphysiologic decrease of pressure during systole after early peak pressure. By integration of the novel nonlinear resistance, peak systolic pressure occured later, peak pressure was higher, and the pressure waveform was more physiologically shaped. Leaflet motions of the aortic valves were less oscillatory and compared well with in vivo characteristics. In conclusion, a novel nonlinear resistance element in a mock circulation has the potential to provide more physiologic aortic pressure waveforms as influencing aortic valve dynamics and thus may be a helpful tool for investigation of biological heart valves.

Evaluation of a fictitious domain method for predicting dynamic response of mechanical heart valves

Flow phenomena around heart valves are important for the motion of the valve leaflets, hence the dynamics of the valve. This work presents an evaluation of a two-dimensional moving rigid heart valve, in which a fictitious domain method is used to describe fluid–structure interaction. Valve motion and fluid flow around the valve were computed for several Reynolds and Strouhal numbers. Particle Image Velocimetry measurements in an in vitro experimental set-up were performed to validate the computational results. The influences of variations of the flow-pulse, expressed in Reynolds and Strouhal number, are well predicted by the computational method. As the fictitious domain method can readily be applied to fully three-dimensional fluid–structure interaction problems, this study indicates that this method is well suited for the analysis of valve dynamics and ventricular flow in physiologically realistic geometries.

Switching-mode-dependent magnetic interlayer coupling strength in spin valves and magnetic tunnel junctions

We have studied the magnetization reversal dynamics of spin valves and magnetic tunnel junctions deposited on step bunched silicon substrates with a strong topological modulation. Our measurements show that the magnetization reversal is dominated by domain wall propagation at low field sweep rates and nucleation processes at high sweep rates. The magnetostatic orange peel coupling present in quasi-static conditions between the magnetic layers disappears when switching by nucleation becomes dominant. Micromagnetic simulations show that this phenomenon can be explained taking into account the modulated topology of the substrate.

1035-216 Mitral valve dynamics with different pacing sites

1035-216 Mitral valve dynamics with different pacing sites

The Velocity Synchronizing Control on the Electro-Hydraulic Load Simulator

This paper builds up an accurate nonlinear mathematical model of an electro hydraulic force/torque servo control system, and provides a thorough theoretical analysis on the feedforward compensation for extraneous force/torque, whose limitation is analyzed and revealed. The nonlinear factors and the servo valve dynamics have much influence on the system characteristics. Subsequently a velocity synchronizing compensation method by using the control signal of the control actuator is proposed, which can reduce the lagging effects for the better performance. For the reason of similarity between the model of control actuator and that of the load simulator, the proposed method performs well against the influence of nonlinear factors. The simulations and the experiments confirm that this control scheme results in a quick response, robustness, and excellent ability against disturbance.

Minimal haemodynamic system model including ventricular interaction and valve dynamics

Characterising circulatory dysfunction and choosing a suitable treatment is often difficult and time consuming, and can result in a deterioration in patient condition, or unsuitable therapy choices. A stable minimal model of the human cardiovascular system (CVS) is developed with the ultimate specific aim of assisting medical staff for rapid, on site modelling to assist in diagnosis and treatment. Models found in the literature simulate specific areas of the CVS with limited direct usefulness to medical staff. Others model the full CVS as a closed loop system, but they were found to be very complex, difficult to solve, or unstable. This paper develops a model that uses a minimal number of governing equations with the primary goal of accurately capturing trends in the CVS dynamics in a simple, easily solved, robust model. The model is shown to have long term stability and consistency with non-specific initial conditions as a result. An “open on pressure close on flow” valve law is created to capture the effects of inertia and the resulting dynamics of blood flow through the cardiac valves. An accurate, stable solution is performed using a method that varies the number of states in the model depending on the specific phase of the cardiac cycle, better matching the real physiological conditions. Examples of results include a 9% drop in cardiac output when increasing the thoracic pressure from −4 to 0 mmHg, and an increase in blood pressure from 120/80 to 165/130 mmHg when the systemic resistance is doubled. These results show that the model adequately provides appropriate magnitudes and trends that are in agreement with existing data for a variety of physiologically verified test cases simulating human CVS function.

Finite-element-based computational methods for cardiovascular fluid-structure interaction

In this paper a combined arbitrary Lagrange-Euler fictitious domain (ALE-FD) method for fluid-structure interaction problems in cardiovascular biomechanics is derived in terms of a weighted residual finite-element formulation. For both fluid flow of blood and solid mechanics of vascular tissue, the performance of tetrahedral and hexahedral Crouzeix-Raviart elements are evaluated. Comparable convergence results are found, although for the test cases considered the hexahedral elements are more accurate. The possibilities that are offered by the ALE-FD method are illustrated by means of a simulation of valve dynamics in a simplified left ventricular flow model.

912 A new 3D-echocardiographic system using digital radio frequency data — visualization and quantitative analysis of aortic valve dynamics with high resolution

912 A new 3D-echocardiographic system using digital radio frequency data — visualization and quantitative analysis of aortic valve dynamics with high resolution

Stability analysis of the servohydraulic equations with linear state feedback and harmonic reference signal

AbstractThe servohydraulic equations describe the dynamic behaviour of a hydrostatic drive featuring a servo‐valve and a hydraulic cylinder. Under the assumptions of constant supply pressures, a rigid support of the cylinder, and a rigid mass attached to the piston, the evolution of cylinder pressures as well as piston position and velocity is governed by a 4th order non‐linear ordinary differential equation. The servo valve is assumed to be much faster than the dynamics of the mass‐cylinder system. Thus, the valve dynamics is neglected. Adding a feedback control law makes up the servohydraulic equations.These equations are partially singularly perturbed. The perturbation parameter is associated with fluid compressibility and the reduced system corresponds to the case of an incompressible fluid. This paper deals with the stability analysis of periodic orbits arising in the case of harmonic reference position trajectories. Geometric singular perturbation theory is used to generate stability charts. Numerical computations verify the analytical results. For a given set of hydraulic system parameters, the stability of periodic orbits depends on controller gains as well as on the amplitude and frequency of the reference signal.